Quantum Physics
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Showing new listings for Thursday, 14 November 2024
- [1] arXiv:2411.08110 [pdf, html, other]
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Title: Characterising memory in quantum channel discrimination via constrained separability problemsComments: 32 pages, comments are welcome!Subjects: Quantum Physics (quant-ph); Mathematical Physics (math-ph)
Quantum memories are a crucial precondition in many protocols for processing quantum information. A fundamental problem that illustrates this statement is given by the task of channel discrimination, in which an unknown channel drawn from a known random ensemble should be determined by applying it for a single time. In this paper, we characterise the quality of channel discrimination protocols when the quantum memory, quantified by the auxiliary dimension, is limited. This is achieved by formulating the problem in terms of separable quantum states with additional affine constraints that all of their factors in each separable decomposition obey. We discuss the computation of upper and lower bounds to the solutions of such problems which allow for new insights into the role of memory in channel discrimination. In addition to the single-copy scenario, this methodological insight allows to systematically characterise quantum and classical memories in adaptive channel discrimination protocols. Especially, our methods enabled us to identify channel discrimination scenarios where classical or quantum memory is required, and to identify the hierarchical and non-hierarchical relationships within adaptive channel discrimination protocols.
- [2] arXiv:2411.08119 [pdf, html, other]
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Title: Genuine Multipartite Entanglement in Quantum OptimizationComments: 8+5 pages, 7+7 figuresSubjects: Quantum Physics (quant-ph)
The ability to generate bipartite entanglement in quantum computing technologies is widely regarded as pivotal. However, the role of genuinely multipartite entanglement is much less understood than bipartite entanglement, particularly in the context of solving complicated optimization problems using quantum devices. It is thus crucial from both the algorithmic and hardware standpoints to understand whether multipartite entanglement contributes to achieving a good solution. Here, we tackle this challenge by analyzing genuine multipartite entanglement -- quantified by the generalized geometric measure -- generated in Trotterized quantum annealing and the quantum approximate optimization algorithm. Using numerical benchmarks, we analyze its occurrence in the annealing schedule in detail. We observe a multipartite-entanglement barrier, and we explore how it correlates to the algorithm's success. We also prove how multipartite entanglement provides an upper bound to the overlap of the instantaneous state with an exact solution. Vice versa, the overlaps to the initial and final product states, which can be easily measured experimentally, offer upper bounds for the multipartite entanglement during the entire schedule. Our results help to shed light on how complex quantum correlations come to bear as a resource in quantum optimization.
- [3] arXiv:2411.08131 [pdf, html, other]
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Title: On some states minimizing uncertainty relationsComments: 9 pages, comments are welcomeSubjects: Quantum Physics (quant-ph); Mathematical Physics (math-ph)
Analyzing Heisenberg-Robertson (HR) and Schrödinger uncertainty relations we found, that there can exist large set of states of the quantum system under considerations, for which the lower bound of product of the standard deviations of a pair of non-commuting observables, $A$ and $B$, is zero. These states are not eigenstates of either the observable $A$ or $B$. We have also shown that the so-called "sum uncertainty relations" also do not provide any information about lower bounds on the standard deviations calculated for these states.
- [4] arXiv:2411.08132 [pdf, html, other]
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Title: A cat qubit stabilization scheme using a voltage biased Josephson junctionComments: 30 pages, 7 figuresSubjects: Quantum Physics (quant-ph)
DC-voltage-biased Josephson junctions have been recently employed in superconducting circuits for Hamiltonian engineering, demonstrating microwave amplification, single photon sources and entangled photon generation. Compared to more conventional approaches based on parametric pumps, this solution typically enables larger interaction strengths. In the context of quantum information, a two-to-one photon interaction can stabilize cat qubits, where bit-flip errors are exponentially suppressed, promising significant resource savings for quantum error correction. This work investigates how the DC bias approach to Hamiltonian engineering can benefit cat qubits. We find a simple circuit design that is predicted to showcase a two-to-one photon exchange rate larger than that of the parametric pump-based implementation while dynamically averaging typically resonant parasitic terms such as Kerr and cross Kerr. In addition to addressing qubit stabilization, we propose to use injection locking with a cat-qubit adapted frequency filter to prevent long-term drifts of the cat qubit angle associated to DC voltage noise. The whole scheme is simulated without rotating-wave approximations, highlighting for the first time the amplitude of related oscillatory effects in cat-qubit stabilization schemes. This study lays the groundwork for the experimental realization of such a circuit.
- [5] arXiv:2411.08186 [pdf, html, other]
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Title: Two types of quantum chaos: testing the limits of the Bohigas-Giannoni-Schmit conjectureComments: 20 pages, 6 FiguresSubjects: Quantum Physics (quant-ph); Statistical Mechanics (cond-mat.stat-mech); High Energy Physics - Theory (hep-th); Chaotic Dynamics (nlin.CD)
There are two types of quantum chaos: eigenbasis chaos and spectral chaos. The first type controls the early-time physics, e.g. the thermal relaxation and the sensitivity of the system to initial conditions. It can be traced back to the Eigenstate Thermalization Hypothesis (ETH), a statistical hypothesis about the eigenvectors of the Hamiltonian. The second type concerns very late-time physics, e.g. the ramp of the Spectral Form Factor. It can be traced back to Random Matrix Universality (RMU), a statistical hypothesis about the eigenvalues of the Hamiltonian. The Bohigas-Giannoni-Schmit (BGS) conjecture asserts a direct relationship between the two types of chaos for quantum systems with a chaotic semiclassical limit. The BGS conjecture is challenged by the Poissonian Hamiltonian ensembles, which can be used to model any quantum system displaying RMU. In this paper, we start by analyzing further aspects of such ensembles. On general and numerical grounds, we argue that these ensembles can have chaotic semiclassical limits. We then study the Poissonian ensemble associated with the Sachdev-Ye-Kitaev (SYK) model. While the distribution of couplings peaks around the original SYK model, the Poissonian ensemble is not $k$-local. This suggests that the link between ETH and RMU requires of physical $k$-locality as an assumption. We test this hypothesis by modifying the couplings of the SYK Hamiltonian via the Metropolis algorithm, rewarding directions in the space of couplings that do not display RMU. The numerics converge to a $k$-local Hamiltonian with eigenbasis chaos but without spectral chaos. We finally comment on ways out and corollaries of our results.
- [6] arXiv:2411.08214 [pdf, html, other]
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Title: The role of correlations in a sequence of quantum observations on empirical measuresComments: 19 pages, 10 figuresSubjects: Quantum Physics (quant-ph); Statistical Mechanics (cond-mat.stat-mech)
The outcome of continuously measuring a quantum system is a string of data whose intricate correlation properties reflect the underlying quantum dynamics. In this paper we study the role of these correlation in reconstructing the probabilities of finite sequences of outcomes, the so-called empirical distributions. Our approach is cast in terms of generic quantum instruments, and therefore encompass all types of sequential and continuous quantum measurements. We also show how this specializes to important cases, such as quantum jumps. To quantify the precise role of correlations, we introduce a relative-entropy based measure that quantifies the range of correlations in the string, and the influence that these correlations have in reconstructing finite sequences.
- [7] arXiv:2411.08230 [pdf, html, other]
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Title: A Geometric Substructure for Quantum DynamicsSubjects: Quantum Physics (quant-ph)
The description of a closed quantum system is extended with the identification of an underlying substructure in which an expanded formulation of dynamics in the Heisenberg picture is given. Between measurements a ``state point" moves in an underlying multi-dimensional complex, projective space with constant velocity determined by the quantum state vector. Following a measurement, the point changes direction and moves with new constant velocity along one of several possible new orthogonal paths, with probabilities determined by Born's Rule. A possible generalization to a Riemannian substructure is speculated upon, suggesting an unexpected interaction with the background gravitational field.
- [8] arXiv:2411.08238 [pdf, html, other]
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Title: Neural-network Generated Quantum State Can Alleviate the Barren Plateau in Variational Quantum CircuitsSubjects: Quantum Physics (quant-ph)
We find that using neural networks to generate quantum states can effectively alleviate the barren plateau phenomenon present in random variational quantum circuits.
- [9] arXiv:2411.08266 [pdf, other]
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Title: A decompositional framework for process theories in spacetimeComments: 24+5 pages, many figures and diagramsSubjects: Quantum Physics (quant-ph)
There has been a recent surge in interest in quantum foundations coming from incorporating ideas from general relativity and quantum gravity. In particular, the field of indefinite causal order has emerged and is now an important research topic in its own right. Many of the tools that we use in quantum foundations and information, are, however, totally agnostic as to the underlying spacetime in which the quantum systems live. To give a practical example, whenever we draw a quantum circuit we are not taking into account the connectivity of the physical qubits which will realize this circuit. In this work, we aim to address this limitation. In particular, we show how to extend the formalism of process theories (a framework to study both quantum and post-quantum theories) to incorporate a background causal structure arising from a fixed spacetime. We discuss when processes are embeddable in spacetime under certain constraints. To this end, we introduce the concept of implementations of a process, which are decompositions of the process. A process is then embeddable if one of its implementations can be embedded in such a way that all the processes are localized and all wires follow time-like paths. The set of all implementations of a process is a rather unwieldy object but we show that there exists a subset with useful properties which tells us everything we need to know about the remaining implementations and the embeddability of a process. We call this subset the set of minimal representatives. Future directions include defining and analysing the compositional structure of the framework more rigorously, extending the framework to indefinite causal structures, studying exotic causal influence, and using the minimal representatives to probe the decompositional structure of quantum theory and beyond.
- [10] arXiv:2411.08288 [pdf, html, other]
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Title: Microscopic Theory of Polariton Group Velocity RenormalizationSubjects: Quantum Physics (quant-ph); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)
Cavity exciton-polaritons exhibit ballistic transport and can achieve a distance of 100 $\mu $m in one picosecond. This ballistic transport significantly enhances mobility compared to that of bare excitons, which often move diffusively and become the bottleneck for energy conversion and transfer devices. Despite being robustly reproduced in experiments and simulations, there is no comprehensive microscopic theory addressing the group velocity of polariton transport, and its renormalization due to phonon scattering while still preserving this ballistic behavior. In this work, we develop a microscopic theory to describe the group velocity renormalization using a finite-temperature Green's function approach. Utilizing the generalized Holstein-Tavis-Cummings Hamiltonian, we analytically derive an expression for the group velocity renormalization and find that it is caused by phonon-mediated transitions from the lower polariton states to the dark states. The theory predicts that the magnitude of group velocity renormalization scales linearly with the phonon bath reorganization energy under weak coupling conditions and also linearly depends on the temperature in the high-temperature regime. These predictions are numerically verified using quantum dynamics simulations via the mean-field Ehrenfest method, demonstrating quantitative agreement. Our findings provide theoretical insights and a predictive analytical framework that advance the understanding and design of cavity-modified semiconductors and molecular ensembles, opening new avenues for engineered polaritonic devices.
- [11] arXiv:2411.08356 [pdf, html, other]
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Title: Proposal for a Bell Test with Entangled Atoms of Different MassSubjects: Quantum Physics (quant-ph); Quantum Gases (cond-mat.quant-gas); Atomic Physics (physics.atom-ph)
We propose an experimental scheme for performing a Bell test using momentum-entangled pairs of atoms with different masses. This proposal uses the metastable isotopes of helium 3He* and 4He* for their single-atom detection capabilities, though the approach could be generalised to other atoms. The entangled state is generated by colliding these species and manipulated via two independent atom interferometers. This method enables precise control of the relative phases of each species, overcoming previous challenges in single-species atomic momentum Bell tests. Numerical simulations of the process show that it should be possible to achieve a significant violation of Bell's inequality under realistic experimental conditions and within a quantum mechanics framework. This type of superposition is interesting from a general relativity perspective as it introduces ambiguity in determining spacetime curvature of the quantum state. Such experiments are necessary to better understand the intersection between quantum mechanics and gravity.
- [12] arXiv:2411.08358 [pdf, html, other]
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Title: 10 GHz Robust polarization modulation towards high-speed satellite-based quantum communicationZexu Wang, Huaxing Xu, Ju Li, Jinquan Huang, Hui Han, Changlei Wang, Ping Zhang, Feifei Yin, Kun Xu, Bo Liu, Yitang DaiComments: 16 pages, 10 figuresSubjects: Quantum Physics (quant-ph)
In practical satellite-based quantum key distribution (QKD) systems, the preparation and transmission of polarization-encoding photons suffer from complex environmental effects and high channel-loss. Consequently, the hinge to enhancing the secure key rate (SKR) lies in achieving robust, low-error and high-speed polarization modulation. Although the schemes that realize self-compensation exhibit remarkable robustness. Their modulation speed is constrained to approximately 2 GHz to avoid the interaction between the electrical signal and the reverse optical pulses. Here we utilize the non-reciprocity of the lithium niobate modulators and eliminate the modulation on the reverse optical pulses. As this characteristic is widely available in the radio-frequency band, the modulation speed is no longer limited by the self-compensating optics and can be further increased. The measured average intrinsic QBER of the different polarization states at 10 GHz system repetition frequency is as low as 0.53% over 10 min without any compensation. And the experiment simulation shows that the proposed scheme extends the transmission distance to more than 350 km. Our work can be be efficient performed to the high-speed and high-loss satellite-based quantum communication scenario.
- [13] arXiv:2411.08365 [pdf, html, other]
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Title: Non-Hermitian Effects in Dicke modelsSubjects: Quantum Physics (quant-ph); Optics (physics.optics)
The Dicke model, which describes the collective interaction between an ensemble of atoms and a single-mode photon field, serves as a fundamental framework for studying light-matter interactions and quantum electrodynamic phenomena. In this work, we investigate the manifestation of non-Hermitian effects in a generalized Dicke model, where two dissipative atom ensembles interact with a single-mode photon field. By applying the Holstein-Primakoff transformation, we explore the system in the semiclassical limit as a non-Hermitian Dicke model, revealing rich exceptional points (EPs) and diabolic points in such a system. We find that, by introducing the nonlinear saturation gain into an atomic ensemble, higher-order EP can be induced, leading to intriguing properties. Furthermore, if the system is extended to a one-dimensional chain, then the band topology will interplay with the non-Hermitian effect. In the quantum regime, we explore the quantum signature of EPs, noting that the conditions for their emergence are influenced by discrete photon numbers. We further study the transition from photon anti-bunching to bunching at a steady state, driven by non-Hermitian dynamics. Our findings deepen the understanding of non-Hermitian physics in light-matter interaction which is instructive for the design of advanced photonic and quantum systems.
- [14] arXiv:2411.08368 [pdf, html, other]
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Title: Quantum partial coherence measures constructed from Fisher informationComments: 17 pages, 1 figureJournal-ref: Phys. Scr. 99(2024),125110Subjects: Quantum Physics (quant-ph)
Quantum mechanics gives a new breakthrough to the field of parameter estimation. In the realm of quantum metrology, the precision of parameter estimation is limited by the quantum Fisher information. We introduce the measures of partial coherence based on (quantum) Fisher information by taking into account the post-selective non-unitary parametrization process. These partial coherence measures present a clear operational interpretation by directly linking the coherence to the parameter estimation accuracy. Furthermore, we explore the distinctions between our partial coherence measure and the quantum Fisher information within the context of unitary parametrization. We provide an analytical expression for the partial coherence measure of two-qubit states. We elucidate the operational significance of the partial coherence measures by establishing the connections between the partial coherence measures and quantum state discrimination.
- [15] arXiv:2411.08415 [pdf, html, other]
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Title: Advances in quantum imagingHugo Defienne, Warwick P. Bowen, Maria Chekhova, Gabriela Barreto Lemos, Dan Oron, Sven Ramelow, Nicolas Treps, Daniele FaccioJournal-ref: Nature Photonics 18, 1024-1036 (2024)Subjects: Quantum Physics (quant-ph)
Modern imaging technologies are widely based on classical principles of light or electromagnetic wave propagation. They can be remarkably sophisticated, with recent successes ranging from single molecule microscopy to imaging far-distant galaxies. However, new imaging technologies based on quantum principles are gradually emerging. They can either surpass classical approaches or provide novel imaging capabilities that would not otherwise be possible. {Here }we provide an overview {of the most recently developed quantum imaging systems, highlighting the non-classical properties of sources such as bright squeezed light, entangled photons, and single-photon emitters that enable their functionality.} We outline potential upcoming trends and the associated challenges, all driven by a central inquiry, which is to understand whether quantum light can make visible the invisible.
- [16] arXiv:2411.08441 [pdf, other]
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Title: One-Sided Device-Independent Random Number Generation Through Fiber ChannelsJinfang Zhang, Yi Li, Mengyu Zhao, Dongmei Han, Jun Liu, Meihong Wang, Qihuang Gong, Yu Xiang, Qiongyi He, Xiaolong SuSubjects: Quantum Physics (quant-ph); Optics (physics.optics)
Randomness is an essential resource and plays important roles in various applications ranging from cryptography to simulation of complex systems. Certified randomness from quantum process is ensured to have the element of privacy but usually relies on the device's behavior. To certify randomness without the characterization for device, it is crucial to realize the one-sided device-independent random number generation based on quantum steering, which guarantees security of randomness and relaxes the demands of one party's device. Here, we distribute quantum steering between two distant users through a 2 km fiber channel and generate quantum random numbers at the remote station with untrustworthy device. We certify the steering-based randomness by reconstructing covariance matrix of the Gaussian entangled state shared between distant parties. Then, the quantum random numbers with a generation rate of 7.06 Mbits/s are extracted from the measured amplitude quadrature fluctuation of the state owned by the remote party. Our results demonstrate the first realization of steering-based random numbers extraction in a practical fiber channel, which paves the way to the quantum random numbers generation in asymmetric networks.
- [17] arXiv:2411.08502 [pdf, html, other]
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Title: A fiber array architecture for atom quantum computingXiao Li, Jia-Yi Hou, Jia-Chao Wang, Guang-Wei Wang, Xiao-Dong He, Feng Zhou, Yi-Bo Wang, Min Liu, Jin Wang, Peng Xu, Ming-Sheng ZhanComments: 12 pagesSubjects: Quantum Physics (quant-ph)
Arrays of single atoms trapped in optical tweezers are increasingly recognized as a promising platform for scalable quantum computing. In both the fault-tolerant and NISQ eras, the ability to individually control qubits is essential for the efficient execution of quantum circuits. Time-division multiplexed control schemes based on atom shuttling or beam scanning have been employed to build programmable neutral atom quantum processors, but achieving high-rate, highly parallel gate operations remains a challenge. Here, we propose a fiber array architecture for atom quantum computing capable of fully independent control of individual atoms. The trapping and addressing lasers for each individual atom are emitted from the same optical waveguide, enabling robust control through common-mode suppression of beam pointing noise. Using a fiber array, we experimentally demonstrate the trapping and independent control of ten single atoms in two-dimensional optical tweezers, achieving individually addressed single-qubit gate with an average fidelity of 0.9966(3). Moreover, we perform simultaneous arbitrary single-qubit gate on four randomly selected qubits, resulting in an average fidelity of 0.9961(4). Our work paves the way for time-efficient execution of quantum algorithms on neutral atom quantum computers.
- [18] arXiv:2411.08532 [pdf, html, other]
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Title: Conditional expectations in Quantum Mechanics and causal interpretations: the Bohm momentum as a best predictorSubjects: Quantum Physics (quant-ph); Mathematical Physics (math-ph)
Given a normalized state-vector $\psi $, we define the conditional expectation $\mathbb{E }_{\psi } (A | B ) $ of a Hermitian operator $A $ with respect to a strongly commuting family of self-adjoint operators $B $ as the best approximation, in the operator mean square norm associated to $\psi $, of $A $ by a real-valued function of $B . $ A fundamental example is the conditional expectation of the momentum operator $P $ given the position operator $X $, which is found to be the Bohm momentum. After developing the Bohm theory from this point of view we treat conditional expectations with respect to general $B $, which we apply to non-relativistic spin 1/2-particles. We derive the dynamics of the conditional expectations of momentum and spin with respect to position and the third spin component. These dynamics can be interpreted in terms of classical continuum mechanics as a two-component fluid whose components carry intrinsic angular momentum. Interpreting the joint spectrum of the conditioning operators as a space of beables, we can introduce a classical-stochastic particle dynamics on this space which is compatible with the time-evolution of the Born probability, by combining the de Broglie-Bohm guidance condition with a Markov jump process, following an idea of J. Bell. This results in a new Bohm-type model for particles with spin. A basic problem is that such auxiliary particle dynamics are far from unique.
We finally examine the relation of our conditional expectations with the conditional expectations of the theory of $C^* $-algebras and, as an application, derive a general evolution equation for conditional expectations for operators acting on finite dimensional Hilbert spaces. Two appendices re-interpret the classical Bohm model as an integrable constrained Hamiltonian system, and provide the details of the two-fluid interpretation. - [19] arXiv:2411.08543 [pdf, html, other]
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Title: Quantum Time Travel Revisited: Noncommutative M\"{o}bius Transformations and Time LoopsComments: 9 pages, 11 FiguresSubjects: Quantum Physics (quant-ph)
We extend the theory of quantum time loops introduced by Greenberger and Szovil [1] from the scalar situation (where paths have just an associated complex amplitude) to the general situation where the time traveling system has multi-dimensional underlying Hilbert space. The main mathematical tool which emerges is the noncommutative M\{o}bius Transformation and this affords a formalism similar to the modular structure well known to feedback control problems. We argue that a sum-over-all-paths approach may be carried out in the scalar case, but quickly becomes unwieldy in the general case. It is natural to replace the beamsplitters of [1] with more general components having their own quantum structure, in which case the theory starts to resemble the quantum feedback networks theory for open quantum optical models and indeed we exploit this to look at more realistic physical models of time loops. We analyze some Grandfather paradoxes in the new setting
- [20] arXiv:2411.08589 [pdf, html, other]
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Title: Uncertainty Relations Relative to Phase Space Quantum Reference FramesSubjects: Quantum Physics (quant-ph)
Heisenberg's uncertainty relation does not take into account that position and momentum are defined relative to a quantum reference frame (QRF). We introduce such a QRF as a covariant phase space observable to derive novel, frame-relative uncertainty relations and show that relative to such a frame, position and momentum appear compatible. We investigate the classical limit of the QRF, and demonstrate that in a Galilei-invariant setting, large frame mass corresponds to classical frame preparation. Finally, we give further conditions under which the standard uncertainty relations are recovered.
- [21] arXiv:2411.08594 [pdf, other]
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Title: Encodings of the weighted MAX k-CUT on qubit systemsSubjects: Quantum Physics (quant-ph)
The weighted MAX k-CUT problem involves partitioning a weighted undirected graph into k subsets to maximize the sum of the weights of edges between vertices in different subsets. This problem has significant applications across multiple domains. This paper explores encoding methods for MAX k-CUT on qubit systems, utilizing quantum approximate optimization algorithms (QAOA) and addressing the challenge of encoding integer values on quantum devices with binary variables. We examine various encoding schemes and evaluate the efficiency of these approaches. The paper presents a systematic and resource efficient method to implement phase separation for diagonal square binary matrices. When encoding the problem into the full Hilbert space, we show the importance of balancing the "bin sizes". We also explore the option to encode the problem into a suitable subspace, by designing suitable state preparations and constrained mixers (LX- and Grover-mixer). Numerical simulations on weighted and unweighted graph instances demonstrate the effectiveness of these encoding schemes, particularly in optimizing circuit depth, approximation ratios, and computational efficiency.
- [22] arXiv:2411.08609 [pdf, html, other]
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Title: Expressibility, entangling power and quantum average causal effect for causally indefinite circuitsPedro C. Azado, Guilherme I. Correr, Alexandre Drinko, Ivan Medina, Askery Canabarro, Diogo O. Soares-PintoComments: 10 pages, 5 figuresSubjects: Quantum Physics (quant-ph)
Parameterized quantum circuits are the core of new technologies such as variational quantum algorithms and quantum machine learning, which makes studying its properties a valuable task. We implement parameterized circuits with definite and indefinite causal order and compare their performance under particular descriptors. One of these is the expressibility, which measures how uniformly a given quantum circuit can reach the whole Hilbert space. Another property that we focus on this work is the entanglement capability, more specifically the concurrence and the entangling power. We also find the causal relation between the qubits of our system with the quantum average causal effect measure. We have found that indefinite circuits offer expressibility advantages over definite ones while maintaining the level of entanglement generation. Our results also point to the existence of a correlation between the quantum average causal effect and the entangling power.
- [23] arXiv:2411.08616 [pdf, html, other]
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Title: Multiplexed bi-layered realization of fault-tolerant quantum computation over optically networked trapped-ion modulesComments: 20 pages, 19 figuresSubjects: Quantum Physics (quant-ph); Distributed, Parallel, and Cluster Computing (cs.DC)
We study an architecture for fault-tolerant measurement-based quantum computation (FT-MBQC) over optically-networked trapped-ion modules. The architecture is implemented with a finite number of modules and ions per module, and leverages photonic interactions for generating remote entanglement between modules and local Coulomb interactions for intra-modular entangling gates. We focus on generating the topologically protected Raussendorf-Harrington-Goyal (RHG) lattice cluster state, which is known to be robust against lattice bond failures and qubit noise, with the modules acting as lattice sites. To ensure that the remote entanglement generation rates surpass the bond-failure tolerance threshold of the RHG lattice, we employ spatial and temporal multiplexing. For realistic system timing parameters, we estimate the code cycle time of the RHG lattice and the ion resources required in a bi-layered implementation, where the number of modules matches the number of sites in two lattice layers, and qubits are reinitialized after measurement. For large distances between modules, we incorporate quantum repeaters between sites and analyze the benefits in terms of cumulative resource requirements. Finally, we derive and analyze a qubit noise-tolerance threshold inequality for the RHG lattice generation in the proposed architecture that accounts for noise from various sources. This includes the depolarizing noise arising from the photonically-mediated remote entanglement generation between modules due to finite optical detection efficiency, limited visibility, and the presence of dark clicks, in addition to the noise from imperfect gates and measurements, and memory decoherence with time. Our work thus underscores the hardware and channel threshold requirements to realize distributed FT-MBQC in a leading qubit platform today -- trapped ions.
- [24] arXiv:2411.08670 [pdf, html, other]
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Title: RSFQ All-Digital Programmable Multi-Tone Generator For Quantum ApplicationsJoão Barbosa, Jack C. Brennan, Alessandro Casaburi, M. D. Hutchings, Alex Kirichenko, Oleg Mukhanov, Martin WeidesComments: Submitted to IEEE Transactions on Quantum EngineeringSubjects: Quantum Physics (quant-ph); Applied Physics (physics.app-ph)
One of the most important and topical challenges of quantum circuits is their scalability. Rapid Single Flux Quantum (RSFQ) technology is at the forefront of replacing current standard CMOS-based control architectures for a number of applications, including quantum computing and quantum sensor arrays. By condensing the control and readout to SFQ-based on-chip devices that are directly connected to the quantum systems, it is possible to minimise the total system overhead, improving scalability and integration. In this work, we present a novel RSFQ device that generates multi tone digital signals, based on complex pulse train sequences using a Circular Shift Register (CSR) and a comb filter stage. We show that the frequency spectrum of the pulse trains is dependent on a preloaded pattern on the CSR, as well as on the delay line of the comb filter stage. By carefully selecting both the pattern and delay, the desired tones can be isolated and amplified as required. Finally, we propose architectures where this device can be implemented to control and readout arrays of quantum devices, such as qubits and single photon detectors.
- [25] arXiv:2411.08689 [pdf, html, other]
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Title: Quantum Measurement Induced Radiative Processes in Continuously Monitored Optical EnvironmentsComments: 10 pages, 4 figuresSubjects: Quantum Physics (quant-ph)
We investigate the emission characteristics of a measurement-driven quantum emitter in a continuously monitored optical environment. The quantum emitter is stimulated by observing the Pauli spin along its transition dipole that maximally non-commutes with the Hamiltonian of the emitter. It also exchanges energy resonantly with the optical environment, observable as quantum jumps corresponding to the absorption or emission of a photon and the null events where the quantum emitter did not make a jump. We characterize the finite-time statistics of quantum jumps and estimate their covariance and precision using the large deviation principle. We also generalize our considerations to coarse-grained measurements of the optical field and compute the finite-time statistics of the sum of absorption and emission events, which we refer to as the negation of null events in our problem. While the statistics of absorption and emission events are generically sub-Poissonian, our analysis reveals a spin-measurement-induced transition from super-Poissonian to sub-Poissonian in their sum. Our findings suggest that quantum measurement-induced fluctuations can be a useful alternative to coherent drives for stimulating radiative transitions having controllable emission characteristics, with implications extending to atomic and nuclear clocks.
- [26] arXiv:2411.08721 [pdf, other]
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Title: Shortwave DPS-QKD Employing a SiN Micro-Ring Resonator as Compact Quantum State AnalyserFlorian Honz (1), Paul Müllner (1), Michael Hentschel (1), Stefan Nevlacsil (1), Jochen Kraft (2), Martin Sagmeister (2), Philip Walther (3), Rainer Hainberger (1), Bernhard Schrenk (1) ((1) AIT Austrian Institute of Technology, (2) ams-OSRAM AG, (3) University of Vienna, Faculty of Physics)Subjects: Quantum Physics (quant-ph)
We show simplified DPS-QKD using a SiN micro-ring resonator operated at 852 nm. A raw-key rate of up to 25.3 kb/s is reached at a QBER suitable for secure-key extraction. Short-reach QKD operation is maintained for zero-touch link layouts with C-band telecom fiber.
- [27] arXiv:2411.08736 [pdf, html, other]
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Title: Topological phase transitions in a constrained two-qubit quantum control landscapeComments: 11 pages, 9 figuresSubjects: Quantum Physics (quant-ph); Quantum Gases (cond-mat.quant-gas); Statistical Mechanics (cond-mat.stat-mech)
In optimal quantum control, control landscape phase transitions (CLPTs) indicate sharp changes occurring in the set of optimal protocols, as a physical model parameter is varied. Here, we demonstrate the existence of a new class of CLPTs, associated with changes in the topological properties of the optimal level set in a two-qubit state-preparation problem. In particular, the distance distribution of control protocols sampled through stochastic homotopic dynamics reveals discontinuous changes in the number of connected components in the optimal level set, as a function of the protocol duration. We demonstrate how topological CLPTs can be detected in modern-day experiments.
- [28] arXiv:2411.08741 [pdf, html, other]
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Title: Unified analysis of non-Markovian open quantum systems in Gaussian environment using superoperator formalismComments: 46 pagesSubjects: Quantum Physics (quant-ph); Mathematical Physics (math-ph)
We present perturbative error bounds for the non-Markovian dynamics of observables in open quantum systems interacting with Gaussian environments, governed by general Liouville dynamics. This extends the work of [Mascherpa et al., Phys. Rev. Lett. 118, 100401, 2017], which demonstrated qualitatively tighter bounds over the standard Grönwall-type analysis, where the joint system-environment evolution is unitary. Our results apply to systems with both bosonic and fermionic environments. Our approach utilizes a superoperator formalism, which avoids the need for formal coherent state path integral calculations, or the dilation of Lindblad dynamics into an equivalent unitary framework with infinitely many degrees of freedom. This enables a unified treatment of a wide range of open quantum systems. These findings provide a solid theoretical basis for various recently developed pseudomode methods in simulating open quantum system dynamics.
- [29] arXiv:2411.08765 [pdf, html, other]
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Title: Tolerant Testing of Stabilizer States with Mixed State InputsComments: 15 pagesSubjects: Quantum Physics (quant-ph); Data Structures and Algorithms (cs.DS)
We study the problem of tolerant testing of stabilizer states. In particular, we give the first such algorithm that accepts mixed state inputs. Formally, given a mixed state $\rho$ that either has fidelity at least $\varepsilon_1$ with some stabilizer pure state or fidelity at most $\varepsilon_2$ with all such states, where $\varepsilon_2 \leq \varepsilon_1^{O(1)}$, our algorithm distinguishes the two cases with sample complexity $\text{poly}(1/\varepsilon_1)$ and time complexity $O(n \cdot \text{poly}(1/\varepsilon_1))$.
- [30] arXiv:2411.08824 [pdf, html, other]
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Title: Reducing QAOA Circuit Depth by Factoring out Semi-SymmetriesJonas Nüßlein, Leo Sünkel, Jonas Stein, Tobias Rohe, Daniëlle Schuman, Claudia Linnhoff-Popien, Sebastian FeldSubjects: Quantum Physics (quant-ph)
QAOA is a quantum algorithm for solving combinatorial optimization problems. It is capable of searching for the minimizing solution vector $x$ of a QUBO problem $x^TQx$. The number of two-qubit CNOT gates in the QAOA circuit scales linearly in the number of non-zero couplings of $Q$ and the depth of the circuit scales accordingly. Since CNOT operations have high error rates it is crucial to develop algorithms for reducing their number. We, therefore, present the concept of \textit{semi-symmetries} in QUBO matrices and an algorithm for identifying and factoring them out into ancilla qubits. \textit{Semi-symmetries} are prevalent in QUBO matrices of many well-known optimization problems like \textit{Maximum Clique}, \textit{Hamilton Cycles}, \textit{Graph Coloring}, \textit{Vertex Cover} and \textit{Graph Isomorphism}, among others. We theoretically show that our modified QUBO matrix $Q_{mod}$ describes the same energy spectrum as the original $Q$. Experiments conducted on the five optimization problems mentioned above demonstrate that our algorithm achieved reductions in the number of couplings by up to $49\%$ and in circuit depth by up to $41\%$.
- [31] arXiv:2411.08855 [pdf, other]
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Title: Ultrafast pump-probe phase-randomized tomographyFilippo Glerean, Enrico Maria Rigoni, Giacomo Jarc, Shahla Yasmin Mathengattil, Angela Montanaro, Francesca Giusti, Matteo Mitrano, Fabio Benatti, Daniele FaustiComments: 21 pages, 10 figures (including Supplementary Information)Subjects: Quantum Physics (quant-ph); Optics (physics.optics)
Measuring fluctuations in matter's low energy excitations is the key to unveil the nature of the nonequilibrium response of materials. A promising outlook in this respect is offered by spectroscopic methods that address matter fluctuations by exploiting the statistical nature of light-matter interactions with weak few-photon probes. Here we report the first implementation of ultrafast phase randomized tomography, combining pump-probe experiments with quantum optical state tomography, to measure the ultrafast non-equilibrium dynamics in complex materials. Our approach utilizes a time-resolved multimode heterodyne detection scheme with phase-randomized coherent ultrashort laser pulses, overcoming the limitations of phase-stable configurations and enabling a robust reconstruction of the statistical distribution of phase-averaged optical observables. This methodology is validated by measuring the coherent phonon response in $\alpha$-quartz. By tracking the dynamics of the shot-noise limited photon number distribution of few-photon probes with ultrafast resolution, our results set an upper limit to the non-classical features of phononic state in $\alpha$-quartz and provide a pathway to access nonequilibrium quantum fluctuations in more complex quantum materials.
- [32] arXiv:2411.08857 [pdf, html, other]
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Title: A Classical Analogue of Entanglement for a Kicked TopSubjects: Quantum Physics (quant-ph)
The kicked top is one of the most extensively studied paradigms of quantum chaos. In this model, an intricate connection has been observed between entanglement entropy and classical dynamics. This connection appears surprising since both chaos and entanglement are understood to be exclusive to classical and quantum mechanics respectively. In this paper, we have argued that from an alternative standpoint on classical physics, this connection becomes completely natural. According to this view, classical states are more accurately represented by distributions instead of infinitely precise points in phase space. Many properties that have traditionally been held to be exclusively quantum, such as non-separability of states, appear in classical physics too in this picture. Looking at the kicked top from this paradigm of classical physics provides a completely fresh outlook to the chaos-entanglement discussion. This finding opens new avenues of understanding in quantum chaos and the more general problem of classical-quantum correspondence.
- [33] arXiv:2411.08860 [pdf, html, other]
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Title: How NOT to Fool the Masses When Giving Performance Results for Quantum ComputersComments: 14 pages; 17 pages including referencesSubjects: Quantum Physics (quant-ph); Emerging Technologies (cs.ET)
In 1991, David Bailey wrote an article describing techniques for overstating the performance of massively parallel computers. Intended as a lighthearted protest against the practice of inflating benchmark results in order to ``fool the masses" and boost sales, the paper sparked development of procedural standards that help benchmarkers avoid methodological errors leading to unjustified and misleading conclusions.
Now that quantum computers are starting to realize their potential as viable alternatives to classical computers, we can see the mistakes of three decades ago being repeated by a new batch of researchers who are unfamiliar with this history and these standards.
Inspired by Bailey's model, this paper presents four suggestions for newcomers to quantum performance benchmarking, about how not to do it. They are: (1) Don't claim superior performance without mentioning runtimes; (2) Don't report optimized results without mentioning the tuning time needed to optimize those results; (3) Don't claim faster runtimes for (or in comparison to) solvers running on imaginary platforms; and (4) No cherry-picking (without justification and qualification). Suggestions for improving current practice appear in the last section. - [34] arXiv:2411.08869 [pdf, other]
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Title: Equivalence between the second order steady state for spin-Boson model and its quantum mean force Gibbs stateComments: Comments are welcomeSubjects: Quantum Physics (quant-ph)
When the coupling of a quantum system to its environment is non-negligible, its steady state is known to deviate from the textbook Gibbs state. The Bloch-Redfield quantum master equation, one of the most widely adopted equations to solve the open quantum dynamics, cannot predict all the deviations of the steady state of a quantum system from the Gibbs state. In this paper, for a generic spin-boson model, we use a higher-order quantum master equation (in system environment coupling strength) to analytically calculate all the deviations of the steady state of the quantum system up to second order in the coupling strength. We also show that this steady state is exactly identical to the corresponding generalized Gibbs state, the so-called quantum mean force Gibbs state, at arbitrary temperature. All these calculations are highly general, making them immediately applicable to a wide class of systems well modeled by the spin-Boson model, ranging from various condensed phase processes to quantum thermodynamics. As an example, we use our results to study the dynamics and the steady state of a double quantum dot system under physically relevant choices of parameters.
- [35] arXiv:2411.08877 [pdf, html, other]
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Title: Quantum cryptography beyond key distribution: theory and experimentComments: Submitted to Reviews of Modern Physics. Comments are welcomeSubjects: Quantum Physics (quant-ph)
Due to its fundamental principles, quantum theory holds the promise to enhance the security of modern cryptography, from message encryption to anonymous communication, digital signatures, online banking, leader election, one-time passwords and delegated computation. While quantum key distribution (QKD) has already enabled secure key exchange over hundreds of kilometers, a myriad of other quantum-cryptographic primitives are being developed to secure future applications against quantum adversaries. This article surveys the theoretical and experimental developments in quantum cryptography beyond QKD over the past decades, along with advances in secure quantum computation. It provides an intuitive classification of the main quantum primitives and their security levels, summarizes their possibilities and limits, and discusses their implementation with current photonic technology.
New submissions (showing 35 of 35 entries)
- [36] arXiv:2411.08046 (cross-list from physics.soc-ph) [pdf, html, other]
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Title: Mathematical representation of bias and nudges centered on intangible goods using quantum information theorySubjects: Physics and Society (physics.soc-ph); Quantum Physics (quant-ph)
The purpose of this study is to explore whether the relationship between bias and nudges can be mathematically expressed in terms of quantum information theory, particularly by means of individually customized nudges. Based on the value function of customer satisfaction, which is subject to uncertainty due to the subjectivity of customer evaluations, a model of bias and nudges is proposed that takes into account the environment for intangible goods. Then, by defining an index of nudges from the mathematical properties of the value function obtained from this economic model, a model could be expressed in this study that has a mathematical structure of the same nature as nudges expressed in standard economics, where welfare is impaired by bias from the mathematical structure of the gross social surplus derived as a social welfare function. Moreover, the mathematical structure of the gross social surplus can be made larger than that in standard economics, adding knowledge about the mathematical design of nudges as individually customized customer experiences. This increases the feasibility of the economic model based on quantum information theory and the mathematical design of customized nudges.
- [37] arXiv:2411.08121 (cross-list from cond-mat.str-el) [pdf, other]
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Title: Cavity-Vacuum-Induced Chiral Spin Liquids in Kagome Lattices: Tuning and Probing Topological Quantum Phases via Cavity Quantum ElectrodynamicsComments: 13 pages, 4 figuresSubjects: Strongly Correlated Electrons (cond-mat.str-el); Optics (physics.optics); Quantum Physics (quant-ph)
Topological phases in frustrated quantum magnetic systems have captivated researchers for decades, with the chiral spin liquid (CSL) standing out as one of the most compelling examples. Featured by long-range entanglement, topological order, and exotic fractional excitations, the CSL has inspired extensive exploration for practical realizations. In this work, we demonstrate that CSLs can emerge in a kagome lattice driven by vacuum quantum fluctuations within a single-mode chiral cavity. The chiral cavity imprints quantum fluctuations with time-reversal symmetry breaking, fostering chiral interactions among electrons and stabilizing a robust CSL phase without external laser excitation. Moreover, we identify experimentally accessible observables -- such as average photon number and transport properties -- that reveal connections between photon dynamics and the emergent chiral order. Our findings establish a novel pathway for creating, controlling, and probing topological and symmetry-breaking quantum phases in strongly correlated systems.
- [38] arXiv:2411.08162 (cross-list from hep-ph) [pdf, html, other]
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Title: Impact of background field localization on vacuum polarization effectsComments: 18 pagesSubjects: High Energy Physics - Phenomenology (hep-ph); High Energy Physics - Theory (hep-th); Quantum Physics (quant-ph)
We aim at insights about how localization of the background field impacts nonlinear quantum vacuum signatures probed by photons in purely magnetic, electric and crossed fields. The starting point of our study are the one-loop results for the Heisenberg-Euler effective Lagrangian and the photon polarization tensor in quantum electrodynamics (QED) evaluated in a uniform constant electromagnetic field. As is well known and often employed, especially in the weak-field limit, within certain restrictions these results also allow for the reliable analysis of vacuum polarization effects in slowly varying background fields. Here, our main interest is in manifestly non-perturbative effects. To this end, we make use of the fact that for the particular case of background field inhomogeneities of Lorentzian shape with $0\leq d\leq3$ inhomogeneous directions analytical insights are possible. We study the scaling of conventional nonlinear QED signatures, such as probe-photon polarization flip and probe-photon induced electron-positron pair production, with relevant parameters. Special attention is put on the $d$ dependence of the considered effects.
- [39] arXiv:2411.08202 (cross-list from cond-mat.mes-hall) [pdf, html, other]
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Title: Latent Haldane ModelsComments: 10 pages, 12 figures, regular article to be submitted to Physical Review BSubjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Strongly Correlated Electrons (cond-mat.str-el); Quantum Physics (quant-ph)
Latent symmetries, which materialize after performing isospectral reductions, have recently been shown to be instrumental in revealing novel topological phases in one-dimensional systems, among many other applications. In this work, we explore how to construct a family of seemingly complicated two-dimensional models that result in energy-dependent Haldane models upon performing an isospectral reduction. In these models, we find energy-dependent latent Semenoff masses without introducing a staggered on-site potential. In addition, energy-dependent latent Haldane masses also emerge in decorated lattices with nearest-neighbor complex hoppings. Using the Haldane model's properties, we then predict the location of the topological gaps in the aforementioned family of models and construct phase diagrams to determine where the topological phases lie in parameter space. This idea yielded, for instance, useful insights in the case of a modified version of $\alpha$-graphyne and hexagonal plaquettes with additional decorations, where the gap-closing energies can be calculated using the ISR to predict topological phase transitions.
- [40] arXiv:2411.08236 (cross-list from physics.atom-ph) [pdf, html, other]
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Title: Enhancement of Rydberg Blockade via Microwave DressingDeniz Kurdak, Patrick R. Banner, Yaxin Li, Sean R. Muleady, Alexey V. Gorshkov, S. L. Rolston, J. V. PortoComments: 7 pages, 3 figures and supplementary 9 pages, 7 figuresSubjects: Atomic Physics (physics.atom-ph); Quantum Physics (quant-ph)
Experimental control over the strength and angular dependence of interactions between atoms is a key capability for advancing quantum technologies. Here, we use microwave dressing to manipulate and enhance Rydberg-Rydberg interactions in an atomic ensemble. By varying the cloud length relative to the blockade radius and measuring the statistics of the light retrieved from the ensemble, we demonstrate a clear enhancement of the interaction strength due to microwave dressing. These results are successfully captured by a theoretical model that accounts for the excitations dynamics, atomic density distribution, and the phase-matched retrieval efficiency. Our approach offers a versatile platform for further engineering interactions by exploiting additional features of the microwave fields, such as polarization and detuning, opening pathways for new quantum control strategies.
- [41] arXiv:2411.08276 (cross-list from hep-th) [pdf, html, other]
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Title: Embedded structure in quantum theory, functional operator and multiverseComments: 27 pagesSubjects: High Energy Physics - Theory (hep-th); General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Phenomenology (hep-ph); Quantum Physics (quant-ph)
We explore a wider theoretical framework that has quantum field theory built-in, taking the fact that quantum mechanics is reconstructed from quantum field theory as a hint. We formulate a quantum theory with an embedded structure by introducing functional operators, and we find that it could describe the level II multiverse. Topics related to a beginning of the universe such as inflation, the third quantization and the landscape are discussed in our formulation.
- [42] arXiv:2411.08391 (cross-list from cond-mat.supr-con) [pdf, html, other]
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Title: A Simple Model of Superconductors: Insights from Free Fermion and Boson GasesComments: 8 pges, 2 pdf figuresSubjects: Superconductivity (cond-mat.supr-con); High Energy Physics - Theory (hep-th); Quantum Physics (quant-ph)
Superconductors at temperatures below the critical temperature $T_c$ can be modeled as a mixture of Fermi and Bose gases, where the Fermi gas consists of conduction electrons and the Bose gas comprises Cooper pairs. This simple model enables the computation of the temperature dependence of $2 r(T) / N$, where $N$ is the total number of conduction electrons and $r(T)$ is the number of Cooper pairs at temperature $T$. Analyzing $2 r(T) / N$ across various superconductors may provide significant insights into the mechanisms behind high-temperature superconductivity, especially regarding coherence in Cooper pairs.
- [43] arXiv:2411.08442 (cross-list from physics.optics) [pdf, other]
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Title: Making Silicon Emit Light Using Third Harmonic GenerationComments: 6 pages, 3 figures, International Workshop on Information and Electronics Engineering (IWIEE) - 2012Journal-ref: Procedia Engineering 2012Subjects: Optics (physics.optics); Materials Science (cond-mat.mtrl-sci); Quantum Physics (quant-ph)
Despite its excellent performance in microelectronic industry, silicon was not able to perform well in photonic devices arena. This is because the silicon has never been a good optical source mainly due to its indirect band gap structure. Many of the device functionalities in silicon have been reported, with an exception of, until recently, a reliable optical source. Silicon is a nonlinear material which makes use of its nonlinearities to realize various functionalities. This paper presents a theoretical treatment of generating and enhancing third-harmonic field which may be used as optical source, crystal state monitoring and all-optical signal processing applications.
- [44] arXiv:2411.08552 (cross-list from cs.LG) [pdf, html, other]
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Title: Leveraging Pre-Trained Neural Networks to Enhance Machine Learning with Variational Quantum CircuitsComments: In submissionSubjects: Machine Learning (cs.LG); Artificial Intelligence (cs.AI); Quantum Physics (quant-ph)
Quantum Machine Learning (QML) offers tremendous potential but is currently limited by the availability of qubits. We introduce an innovative approach that utilizes pre-trained neural networks to enhance Variational Quantum Circuits (VQC). This technique effectively separates approximation error from qubit count and removes the need for restrictive conditions, making QML more viable for real-world applications. Our method significantly improves parameter optimization for VQC while delivering notable gains in representation and generalization capabilities, as evidenced by rigorous theoretical analysis and extensive empirical testing on quantum dot classification tasks. Moreover, our results extend to applications such as human genome analysis, demonstrating the broad applicability of our approach. By addressing the constraints of current quantum hardware, our work paves the way for a new era of advanced QML applications, unlocking the full potential of quantum computing in fields such as machine learning, materials science, medicine, mimetics, and various interdisciplinary areas.
- [45] arXiv:2411.08598 (cross-list from physics.chem-ph) [pdf, html, other]
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Title: Space-local memory in generalized master equations: Reaching the thermodynamic limit for the cost of a small lattice simulationSubjects: Chemical Physics (physics.chem-ph); Materials Science (cond-mat.mtrl-sci); Computational Physics (physics.comp-ph); Quantum Physics (quant-ph)
The exact quantum dynamics of lattice models can be computationally intensive, especially when aiming for large system sizes and extended simulation times necessary to converge transport coefficients. By leveraging finite memory times to access long-time dynamics using only short-time data, generalized master equations (GMEs) can offer a route to simulating the dynamics of lattice problems efficiently. However, such simulations are limited to small lattices whose dynamics exhibit finite-size artifacts that contaminate transport coefficient predictions. To address this problem, we introduce a novel approach that exploits finite memory in time \textit{and} space to efficiently predict the many-body dynamics of dissipative lattice problems involving short-range interactions. This advance enables one to leverage the short-time dynamics of small lattices to simulate arbitrarily large systems over long times. We demonstrate the strengths of this method by focusing on nonequilibrium polaron relaxation and transport in the dispersive Holstein model, successfully simulating lattice dynamics in one and two dimensions free from finite-size effects, reducing the computational expense of such simulations by multiple orders of magnitude. Our method is broadly applicable and provides an accurate and efficient means to investigate nonequilibrium relaxation with microscopic resolution over mesoscopic length and time scales that are relevant to experiment.
- [46] arXiv:2411.08644 (cross-list from cond-mat.mes-hall) [pdf, html, other]
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Title: Optimizing state transfer in a three-qubit array via quantum brachistochrone methodSubjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Optics (physics.optics); Quantum Physics (quant-ph)
Quantum brachistochrone method has recently emerged as a technique allowing one to implement the desired unitary evolution operator in a physical system within the minimal time. Here, we apply this approach to the problem of time-optimal quantum state transfer in the array of three qubits with time-varying nearest-neighbor couplings and analytically derive the fastest protocol.
- [47] arXiv:2411.08776 (cross-list from cond-mat.quant-gas) [pdf, html, other]
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Title: Quantum Gas Microscopy of Fermions in the ContinuumComments: 9 pages, 5 figuresSubjects: Quantum Gases (cond-mat.quant-gas); Atomic Physics (physics.atom-ph); Quantum Physics (quant-ph)
Microscopically probing quantum many-body systems by resolving their constituent particles is essential for understanding quantum matter. In most physical systems, distinguishing individual particles, such as electrons in solids, or neutrons and quarks in neutron stars, is impossible. Atom-based quantum simulators offer a unique platform that enables the imaging of each particle in a many-body system. Until now, however, this capability has been limited to quantum systems in discretized space such as optical lattices and tweezers, where spatial degrees of freedom are quantized. Here, we introduce a novel method for imaging atomic quantum many-body systems in the continuum, allowing for in situ resolution of every particle. We demonstrate the capabilities of our approach on a two-dimensional atomic Fermi gas. We probe the density correlation functions, resolving their full spatial functional form, and reveal the shape of the Fermi hole arising from Pauli exclusion as a function of temperature. Our method opens the door to probing strongly-correlated quantum gases in the continuum with unprecedented spatial resolution, providing in situ access to spatially resolved correlation functions of arbitrarily high order across the entire system.
- [48] arXiv:2411.08779 (cross-list from cond-mat.quant-gas) [pdf, html, other]
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Title: In situ imaging of the thermal de Broglie wavelength in an ultracold Bose gasJinggang Xiang, Enid Cruz-Colón, Candice C. Chua, William R. Milner, Julius de Hond, Jacob F. Fricke, Wolfgang KetterleSubjects: Quantum Gases (cond-mat.quant-gas); Atomic Physics (physics.atom-ph); Quantum Physics (quant-ph)
We report the first in situ observation of density fluctuations on the scale of the thermal de Broglie wavelength in an ultracold gas of bosons. Bunching of $^{87}$Rb atoms in a quasi two-dimensional system is observed by single-atom imaging using a quantum gas microscope. Compared to a classical ensemble, we observe a 30 percent enhancement of the second-order correlation function. We show the spatial and thermal dependence of these correlations. The reported method of detecting in situ correlations can be applied to interacting many-body systems and to the study of critical phenomena near phase transitions.
- [49] arXiv:2411.08780 (cross-list from cond-mat.quant-gas) [pdf, html, other]
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Title: Measuring pair correlations in Bose and Fermi gases via atom-resolved microscopyComments: 7 pages, 6 figuresSubjects: Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el); Atomic Physics (physics.atom-ph); Quantum Physics (quant-ph)
We demonstrate atom-resolved detection of itinerant bosonic $^{23}$Na and fermionic $^6$Li quantum gases, enabling the direct in situ measurement of interparticle correlations. In contrast to prior work on lattice-trapped gases, here we realize microscopy of quantum gases in the continuum. We reveal Bose-Einstein condensation with single-atom resolution, measure the enhancement of two-particle $g^{(2)}$ correlations of thermal bosons, and observe the suppression of $g^{(2)}$ for fermions; the Fermi or exchange hole. For strongly interacting Fermi gases confined to two dimensions, we directly observe non-local fermion pairs in the BEC-BCS crossover. We obtain the pairing gap, the pair size, and the short-range contact directly from the pair correlations. In situ thermometry is enabled via the fluctuation-dissipation theorem. Our technique opens the door to the atom-resolved study of strongly correlated quantum gases of bosons, fermions, and their mixtures.
- [50] arXiv:2411.08844 (cross-list from physics.atom-ph) [pdf, html, other]
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Title: Trapped-ion laser cooling in structured light fieldsSubjects: Atomic Physics (physics.atom-ph); Optics (physics.optics); Quantum Physics (quant-ph)
Laser cooling is fundamental to quantum computation and metrology with trapped ions, and can occupy a majority of runtime in current systems. A key limitation to cooling arises from unwanted carrier excitation, which in typically used running wave (RW) fields invariably accompanies the sideband transitions effecting cooling. We consider laser cooling in structured light profiles enabling selective sideband excitation with nulled carrier drive; motivated by integrated photonic approaches' passive phase and amplitude stability, we propose simple configurations realizable with waveguide addressing using either standing wave (SW) or first-order Hermite-Gauss (HG) modes. We quantify performance of Doppler cooling from beyond the Lamb-Dicke regime (LDR), and ground-state (GS) cooling using electromagnetically induced transparency (EIT) leveraging these field profiles. Carrier-free EIT offers significant benefits simultaneously in cooling rate, motional frequency bandwidth, and final phonon number. Carrier-free Doppler cooling's advantage is significantly compromised beyond the LDR but continues to hold, indicating such configurations are applicable for highly excited ions. Our simulations focus on level structure relevant to $^{40}$Ca$^+$, though the carrier-free configurations can be generally applied to other species. We also quantify performance limitations due to polarization and modal impurities relevant to experimental implementation. Our results indicate potential for simple structured light profiles to alleviate bottlenecks in laser cooling, and for scalable photonic devices to improve basic operation quality in trapped-ion systems.
- [51] arXiv:2411.08850 (cross-list from cond-mat.stat-mech) [pdf, html, other]
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Title: Typical Positivity of Nonequilibrium Entropy Production for Pure StatesComments: 4.5 pages main text with 1 figure, comments welcomeSubjects: Statistical Mechanics (cond-mat.stat-mech); Quantum Physics (quant-ph)
We establish that the nonequilibrium dynamics of most pure states gives rise to the same entropy production as that of the corresponding ensemble, provided the effective dimension of the ensemble is large enough. This establishes the positivity of entropy production under a wide variety of nonequilibrium situations. Our results follow from dynamical typicality and suitable continuity properties alone, without relying on non-integrability, and they complement other recent efforts to establish "pure state second laws". An explicit comparison with the distinctively different two-point measurement scheme is also provided.
Cross submissions (showing 16 of 16 entries)
- [52] arXiv:2105.07021 (replaced) [pdf, html, other]
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Title: Quantum dot-based high-fidelity universal quantum gates in noisy environmentSubjects: Quantum Physics (quant-ph); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)
Quantum dot-based spin qubit realization is one of the most promising quantum computing systems owing to its integrability with classical computation hardware and its versatility in realizing qubits and quantum gates. In this work, we investigate a quantum dot-based universal set of quantum gates (single qubit gates and the Toffoli gate) in the presence of hyperfine fluctuation noise and phononic charge noise. We model the spin dynamics and noise processes in the NOT gate, Hadamard gate and the Toffoli gate using the Lindblad master equation formalism to estimate the operating ranges of the external static and ac magnetic fields to achieve high fidelity operation of these gates in a noisy environment. In addition, the generality of the framework proposed in this paper enables modeling of larger quantum processors based on spin qubits in realistic conditions.
- [53] arXiv:2209.00832 (replaced) [pdf, other]
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Title: Efficiency of estimators for locally asymptotically normal quantum statistical modelsJournal-ref: Ann. Statist. 51 (3) 1159 - 1182, June 2023Subjects: Quantum Physics (quant-ph); Statistics Theory (math.ST)
We herein establish an asymptotic representation theorem for locally asymptotically normal quantum statistical models. This theorem enables us to study the asymptotic efficiency of quantum estimators such as quantum regular estimators and quantum minimax estimators, leading to a universal tight lower bound beyond the i.i.d. assumption. This formulation complements the theory of quantum contiguity developed in the previous paper [Fujiwara and Yamagata, Bernoulli 26 (2020) 2105-2141], providing a solid foundation of the theory of weak quantum local asymptotic normality.
- [54] arXiv:2209.11322 (replaced) [pdf, other]
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Title: On proving the robustness of algorithms for early fault-tolerant quantum computersComments: 27 pages, 3 figures, 1 table, 1 algorithm. Submitted to QIP 2023, APS March Meetings 2023, QuantumSubjects: Quantum Physics (quant-ph)
The hope of the quantum computing field is that quantum architectures are able to scale up and realize fault-tolerant quantum computing. Due to engineering challenges, such ''cheap'' error correction may be decades away. In the meantime, we anticipate an era of ''costly'' error correction, or early fault-tolerant quantum computing. Costly error correction might warrant settling for error-prone quantum computations. This motivates the development of quantum algorithms which are robust to some degree of error as well as methods to analyze their performance in the presence of error. Several such algorithms have recently been developed; what is missing is a methodology to analyze their robustness. To this end, we introduce a randomized algorithm for the task of phase estimation and give an analysis of its performance under two simple noise models. In both cases the analysis leads to a noise threshold, below which arbitrarily high accuracy can be achieved by increasing the number of samples used in the algorithm. As an application of this general analysis, we compute the maximum ratio of the largest circuit depth and the dephasing scale such that performance guarantees hold. We calculate that the randomized algorithm can succeed with arbitrarily high probability as long as the required circuit depth is less than 0.916 times the dephasing scale.
- [55] arXiv:2303.11317 (replaced) [pdf, html, other]
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Title: Opening the Black Box Inside Grover's AlgorithmComments: 22 pages, 10 figuresJournal-ref: Phys. Rev. X 14, 041029 (2024)Subjects: Quantum Physics (quant-ph); Strongly Correlated Electrons (cond-mat.str-el)
Grover's algorithm is a primary algorithm offered as evidence that quantum computers can provide an advantage over classical computers. It involves an "oracle" specified for a given application whose structure is not part of the formal scaling of the quadratic speedup guaranteed by the algorithm. Grover's algorithm also requires exponentially many calls to the quantum oracle to succeed (about $\sqrt{2^n}$ calls for $n$ qubits), raising the question of its implementation on both noisy and error-corrected quantum computers. In this work, we construct a quantum-inspired algorithm, executable on a classical computer, that performs Grover's task in a linear number of calls to (simulations of) the oracle - an exponentially smaller number than Grover's algorithm - and demonstrate this algorithm explicitly for Boolean satisfiability problems. The complexity of our algorithm depends on the cost to simulate the oracle once which may or may not be exponential. Indeed, Grover's algorithm does not have an a priori quantum speedup as soon as one is given access to the "source code" of the oracle. Our findings illustrate this point explicitly as our algorithm exploits the structure of the quantum circuit used to program the quantum computer to speed up the search. There remain problems where Grover's algorithm would provide an asymptotic speedup if it could be run accurately for large enough sizes. Our quantum-inspired algorithm provides lower bounds, in terms of circuit complexity, for quantum hardware to beat classical approaches for these problems. These estimates, combined with the unfavorable scaling of the success probability of Grover's algorithm - which in the presence of noise decays as a double exponential in the number of qubits - makes a practical speedup unrealistic even under extremely optimistic assumptions of the evolution of both hardware quality and availability.
- [56] arXiv:2304.00214 (replaced) [pdf, html, other]
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Title: Pulsed Vector Atomic Magnetometer Using an Alternating Fast-Rotating FieldComments: 10 pagesSubjects: Quantum Physics (quant-ph); Applied Physics (physics.app-ph)
We present a vector atomic magnetometer based on applying a fast-rotating magnetic field to a pulsed $^{87}$Rb scalar atomic magnetometer. This method enables simultaneous measurements of the total magnetic field and two polar angles relative to the plane of magnetic field rotation. Using two channels in a gradiometer mode, it provides simultaneous measurements of the total field gradient with a sensitivity of 35 $\mathrm{fT/\sqrt{Hz}}$ (0.7 part per billion), as well as two polar angles with resolutions of 6 $\mathrm{nrad/\sqrt{Hz}}$ at 50 $\mu$T Earth field strength. The noise spectrums of these measurements are flat down to 1 Hz and 0.1 Hz, respectively. Crucially, this approach avoids several metrological difficulties associated with vector magnetometers and gradiometers. We detail the fundamental, systematic, and practical limits of such vector magnetometers. Notably, we provide a comprehensive study of the systematic effects of vector atomic magnetometers. We introduce a new concept of dynamic heading error and investigate several other systematic effects. A unique peak-altering fast rotating field modulation is proposed to cancel out these systematics. Additionally, we derive fundamental limits on the sensitivity of such sensors and demonstrate that the vector sensitivity of the sensor can approach its scalar sensitivity while retaining the accuracy and metrological advantages of scalar sensors. This high-dynamic-range vector magnetometer, with ultrahigh resolution and inherent calibration, is suitable for a wide array of applications.
- [57] arXiv:2304.10258 (replaced) [pdf, html, other]
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Title: First Principles Numerical Demonstration of Emergent Decoherent HistoriesComments: 13.5 pages with 12 figures plus references as accepted by PRX. Compared to v2, revised presentation of discussion and conclusions and one new figure quantifying the structure of the Multiverse. And a new title. :(Journal-ref: Phys. Rev. X 14, 041027 (2024)Subjects: Quantum Physics (quant-ph); Statistical Mechanics (cond-mat.stat-mech); General Relativity and Quantum Cosmology (gr-qc)
Within the histories formalism the decoherence functional is a formal tool to investigate the emergence of classicality in isolated quantum systems, yet an explicit evaluation of it from first principles has not been reported. We provide such an evaluation for up to five-time histories based on exact numerical diagonalization of the Schrödinger equation. We find a robust emergence of decoherence for slow and coarse observables of a generic random matrix model and extract a finite size scaling law by varying the Hilbert space dimension over four orders of magnitude. Specifically, we conjecture and observe an exponential suppression of coherent effects as a function of the particle number of the system. This suggests a solution to the preferred basis problem of the many worlds interpretation (or the set selection problem of the histories formalism) within a minimal theoretical framework -- without relying on environmentally induced decoherence, quantum Darwinism, Markov approximations, low-entropy initial states or ensemble averages.
- [58] arXiv:2305.05262 (replaced) [pdf, html, other]
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Title: Two-qubit operations for finite-energy Gottesman-Kitaev-Preskill encodingsIvan Rojkov, Paul Moser Röggla, Martin Wagener, Moritz Fontboté-Schmidt, Stephan Welte, Jonathan Home, Florentin ReiterComments: 7 + 40 pages, 3 + 25 figuresSubjects: Quantum Physics (quant-ph)
We present techniques for performing two-qubit gates on Gottesman-Kitaev-Preskill (GKP) codes with finite energy, and find that operations designed for ideal infinite-energy codes create undesired entanglement when applied to physically realistic states. We demonstrate that this can be mitigated using recently developed local error-correction protocols, and evaluate the resulting performance. We also propose energy-conserving finite-energy gate implementations which largely avoid the need for further correction.
- [59] arXiv:2306.16028 (replaced) [pdf, html, other]
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Title: Exponential separations between classical and quantum learnersComments: this article supersedes arXiv:2208.06339Subjects: Quantum Physics (quant-ph); Machine Learning (cs.LG)
Despite significant effort, the quantum machine learning community has only demonstrated quantum learning advantages for artificial cryptography-inspired datasets when dealing with classical data. In this paper we address the challenge of finding learning problems where quantum learning algorithms can achieve a provable exponential speedup over classical learning algorithms. We reflect on computational learning theory concepts related to this question and discuss how subtle differences in definitions can result in significantly different requirements and tasks for the learner to meet and solve. We examine existing learning problems with provable quantum speedups and find that they largely rely on the classical hardness of evaluating the function that generates the data, rather than identifying it. To address this, we present two new learning separations where the classical difficulty primarily lies in identifying the function generating the data. Furthermore, we explore computational hardness assumptions that can be leveraged to prove quantum speedups in scenarios where data is quantum-generated, which implies likely quantum advantages in a plethora of more natural settings (e.g., in condensed matter and high energy physics). We also discuss the limitations of the classical shadow paradigm in the context of learning separations, and how physically-motivated settings such as characterizing phases of matter and Hamiltonian learning fit in the computational learning framework.
- [60] arXiv:2310.08405 (replaced) [pdf, html, other]
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Title: Emergence of noise-induced barren plateaus in arbitrary layered noise modelsComments: 46 pages, 11 Figures, We give additional numerical results for the decay of the average purity in QAOA circuits under amplitude damping noiseJournal-ref: Quantum Sci. Technol., Volume 9, Number 4, (2024) 045019Subjects: Quantum Physics (quant-ph)
In variational quantum algorithms the parameters of a parameterized quantum circuit are optimized in order to minimize a cost function that encodes the solution of the problem. The barren plateau phenomenon manifests as an exponentially vanishing dependence of the cost function with respect to the variational parameters, and thus hampers the optimization process. We discuss how, and in which sense, the phenomenon of noise-induced barren plateaus emerges in parameterized quantum circuits with a layered noise model. Previous results have shown the existence of noise-induced barren plateaus in the presence of local Pauli noise [arXiv:2007.14384]. We extend these results analytically to arbitrary completely-positive trace preserving maps in two cases: 1) when a parameter-shift rule holds, 2) when the parameterized quantum circuit at each layer forms a unitary $2$-design. The second example shows how highly expressive unitaries give rise not only to standard barren plateaus [arXiv:1803.11173], but also to noise-induced ones. In the second part of the paper, we study numerically the emergence of noise-induced barren plateaus in QAOA circuits focusing on the case of MaxCut problems on $d$-regular graphs and amplitude damping noise.
- [61] arXiv:2311.13945 (replaced) [pdf, html, other]
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Title: Quantum network-entanglement measuresComments: 6+20 pages, 8 figuresSubjects: Quantum Physics (quant-ph)
Quantum networks are of high interest nowadays and a quantum internet has been long envisioned. Network-entanglement adapts the notion of entanglement to the network scenario and network-entangled states are considered to be a resource to overcome the limitations of a given network structure. In this work, we introduce measures of quantum network-entanglement that are well-defined within the general framework of quantum resource theories, which at the same time have a clear operational interpretation characterizing the extra resources necessary to prepare a targeted quantum state within a given network. In particular, we define the network communication cost and the network round complexity, which turn out to be intimately related to graph-theoretic parameters. We also provide methods to estimate these measures by introducing novel witnesses of network-entanglement.
- [62] arXiv:2312.14151 (replaced) [pdf, html, other]
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Title: Variational Quantum Multi-Objective OptimizationComments: revision of text, added shot noise analysisSubjects: Quantum Physics (quant-ph)
Solving combinatorial optimization problems on near-term quantum devices has gained a lot of attraction in recent years. Currently, most works have focused on single-objective problems, whereas many real-world applications need to consider multiple, mostly conflicting objectives, such as cost and quality. We present a variational quantum optimization algorithm to solve discrete multi-objective optimization problems on quantum computers. The proposed quantum multi-objective optimization (QMOO) algorithm incorporates all cost Hamiltonians representing the classical objective functions in the quantum circuit and produces a quantum state consisting of Pareto-optimal solutions in superposition. From this state we retrieve a set of solutions and utilize the widely applied hypervolume indicator to determine its quality as an approximation to the Pareto-front. The variational parameters of the QMOO circuit are tuned by maximizing the hypervolume indicator in a quantum-classical hybrid fashion. We show the effectiveness of the proposed algorithm on several benchmark problems with up to five objectives. We investigate the influence of the classical optimizer, the circuit depth and compare to results from classical optimization algorithms. We find that the algorithm is robust to shot noise and produces good results with as low as 128 measurement shots in each iteration. These promising result open the perspective to run the algorithm on near-term quantum hardware.
- [63] arXiv:2312.15065 (replaced) [pdf, other]
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Title: Exact finite-time correlation functions for multi-terminal setups: Connecting theoretical frameworks for quantum transport and thermodynamicsJournal-ref: Phys. Rev. Research 6, 043091 (2024)Subjects: Quantum Physics (quant-ph); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)
Transport in open quantum systems can be explored through various theoretical frameworks, including the quantum master equation, scattering matrix, and Heisenberg equation of motion. The choice of framework depends on factors such as the presence of interactions, the coupling strength between the system and environment, and whether the focus is on steady-state or transient regimes. Existing literature treats these frameworks independently, lacking a unified perspective. Our work addresses this gap by clarifying the role and status of these approaches using a minimal single-level quantum dot model in a two-terminal setup under voltage and temperature biases. We derive analytical expressions for particle and energy currents and their fluctuations in both steady-state and transient regimes. Exact results from the Heisenberg equation are shown to align with scattering matrix and master equation approaches within their respective validity regimes. Crucially, we establish a protocol for the weak-coupling limit, bridging the applicability of master equations at weak-coupling with Heisenberg or scattering matrix approaches at arbitrary coupling strength.
- [64] arXiv:2401.02251 (replaced) [pdf, html, other]
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Title: Nonreciprocal Unconventional Photon Blockade with Kerr MagnonsComments: 9 pages,8 figuresJournal-ref: Advanced Quantum Technologies 7 (8), 2400043 (2024)Subjects: Quantum Physics (quant-ph)
Nonreciprocal devices, allowing to manipulate one-way signals, are crucial to quantum information processing and quantum network. Here we propose a nonlinear cavity-magnon system, consisting of a microwave cavity coupled to one or two yttrium-iron-garnet (YIG) spheres supporting magnons with Kerr nonlinearity, to investigate nonreciprocal unconventional photon blockade. The nonreciprocity originates from the direction-dependent Kerr effect, distinctly different from previous proposals with spinning cavities and dissipative couplings. For a single sphere case, nonreciprocal unconventional photon blockade can be realized by manipulating the nonreciprocal destructive interference between two active paths, via vary the Kerr coefficient from positive to negative, or vice versa. By optimizing the system parameters, the perfect and well-tuned nonreciprocal unconventional photon blockade can be predicted. For the case of two spheres with opposite Kerr effects, only reciprocal unconventional photon blockade can be observed when two cavity-magnon coupling strengths Kerr strengths are symmetric. However, when coupling strengths or Kerr strengths become asymmetric, nonreciprocal unconventional photon blockade appears. This implies that two-sphere nonlinear cavity-magnon systems can be used to switch the transition between reciprocal and nonreciprocal unconventional photon blockades. Our study offers a potential platform for investigating nonreciprocal photon blockade effect in nonlinear cavity magnonics.
- [65] arXiv:2401.15951 (replaced) [pdf, html, other]
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Title: Observation of quantum strong Mpemba effectJie Zhang (1,2 and 3), Gang Xia (1), Chun-Wang Wu (1,2 and 3), Ting Chen (1,2 and 3), Qian Zhang (4), Yi Xie (1,2 and 3), Wen-Bo Su (1), Wei Wu (1,2 and 3), Cheng-Wei Qiu (5), Ping-xing Chen (1,2 and 3), Weibin Li (6), Hui Jing (4), Yan-Li Zhou (1,2 and 3) ((1) Institute for Quantum Science and Technology, College of Science, NUDT, Changsha, China,(2) Hunan Key Laboratory of Mechanism and technology of Quantum Information, Changsha, China,(3) Hefei National Laboratory, Hefei, Anhui, China,(4) Key Laboratory of Low-Dimensional Quantum Structures and Quantum Control of Ministry of Education, Department of Physics and Synergetic Innovation Center for Quantum Effects and Applications, Hunan Normal University, Changsha, China,(5) Department of Electrical and Computer Engineering, National University of Singapore, Singapore, Singapore,(6) School of Physics and Astronomy, and Centre for the Mathematics and Theoretical Physics of Quantum Non-equilibrium Systems, University of Nottingham, Nottingham, United Kingdom)Comments: Accepted by Nature CommunicationsSubjects: Quantum Physics (quant-ph)
An ancient and counterintuitive phenomenon know as the Mpemba effect (water can cool faster when initially heated up) showcases the critical role of initial conditions in relaxation processes. How to realize and utilize this effect for speeding up relaxation is an important but challenging task in purely quantum system till now. Here, we report the first experiment, as far as we know,about the strong Mpemba effect in a single trapped ion system in which an exponentially expedited relaxation in time is observed by preparing an optimal initial state with no excitation of the slowest decaying mode. Also, we find that the condition of realizing such effect coincides with the Liouvillian exceptional point, featuring the coalescence of both the eigenvalues and the eigenmodes of the system. Our work provides an efficient strategy to exponentially accelerate relaxations of quantum system to their stationary state, and suggests a link unexplored yet between the Mpemba effect and the non-Hermitian physics. It could open up the door to engineer a wide range of dissipative quantum systems by utilizing the anomalous Mpemba effect, for applications in quantum simulation and quantum information processing.
- [66] arXiv:2402.13134 (replaced) [pdf, html, other]
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Title: An architecture for two-qubit encoding in neutral ytterbium-171 atomsZhubing Jia, William Huie, Lintao Li, Won Kyu Calvin Sun, Xiye Hu, Aakash, Healey Kogan, Abhishek Karve, Jong Yeon Lee, Jacob P. CoveyComments: 22 pages, 14 figuresJournal-ref: npj Quantum Information 10, 106 (2024)Subjects: Quantum Physics (quant-ph); Atomic Physics (physics.atom-ph)
We present an architecture for encoding two qubits within the optical "clock" transition and nuclear spin-1/2 degree of freedom of neutral ytterbium-171 atoms. Inspired by recent high-fidelity control of all pairs of states within this four-dimensional ququart space, we present a toolbox for intra-ququart (single atom) one- and two-qubit gates, inter-ququart (two atom) Rydberg-based two- and four-qubit gates, and quantum nondemolition (QND) readout. We then use this toolbox to demonstrate the advantages of the ququart encoding for entanglement distillation and quantum error correction which exhibit superior hardware efficiency and better performance in some cases since fewer two-atom (Rydberg-based) operations are required. Finally, leveraging single-state QND readout in our ququart encoding, we present a unique approach to studying interactive circuits as well as to realizing a symmetry protected topological phase of a spin-1 chain with a shallow, constant-depth circuit. These applications are all within reach of recent experiments with neutral ytterbium-171 atom arrays or with several trapped ion species.
- [67] arXiv:2405.12598 (replaced) [pdf, html, other]
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Title: Machine learning of quantum channels on NISQ devicesGiovanni Cemin, Marcel Cech, Erik Weiss, Stanislaw Soltan, Daniel Braun, Igor Lesanovsky, Federico CarolloComments: 6+8 pages, 5+5 figures, comments welcomeJournal-ref: Phys. Rev. A 110, 052418 (2024)Subjects: Quantum Physics (quant-ph); Disordered Systems and Neural Networks (cond-mat.dis-nn); Statistical Mechanics (cond-mat.stat-mech)
World-wide efforts aim at the realization of advanced quantum simulators and processors. However, despite the development of intricate hardware and pulse control systems, it may still not be generally known which effective quantum dynamics, or channels, are implemented on these devices. To systematically infer those, we propose a neural-network algorithm approximating generic discrete-time dynamics through the repeated action of an effective quantum channel. We test our approach considering time-periodic Lindblad dynamics as well as non-unitary subsystem dynamics in many-body unitary circuits. Moreover, we exploit it to investigate cross-talk effects on the ibmq_ehningen quantum processor, which showcases our method as a practically applicable tool for inferring quantum channels when the exact nature of the underlying dynamics on the physical device is not known a priori. While the present approach is tailored for learning Markovian dynamics, we discuss how it can be adapted to also capture generic non-Markovian discrete-time evolutions.
- [68] arXiv:2405.18746 (replaced) [pdf, html, other]
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Title: STIQ: Safeguarding Training and Inferencing of Quantum Neural Networks from Untrusted CloudSubjects: Quantum Physics (quant-ph); Machine Learning (cs.LG)
The high expenses imposed by current quantum cloud providers, coupled with the escalating need for quantum resources, may incentivize the emergence of cheaper cloud-based quantum services from potentially untrusted providers. Deploying or hosting quantum models, such as Quantum Neural Networks (QNNs), on these untrusted platforms introduces a myriad of security concerns, with the most critical one being model theft. This vulnerability stems from the cloud provider's full access to these circuits during training and/or inference. In this work, we introduce STIQ, a novel ensemble-based strategy designed to safeguard QNNs against such cloud-based adversaries. Our method innovatively trains two distinct QNNs concurrently, hosting them on same or different platforms, in a manner that each network yields obfuscated outputs rendering the individual QNNs ineffective for adversaries operating within cloud environments. However, when these outputs are combined locally (using an aggregate function), they reveal the correct result. Through extensive experiments across various QNNs and datasets, our technique has proven to effectively masks the accuracy and losses of the individually hosted models by upto $76\%$, albeit at the expense of $\leq 2\times$ increase in the total computational overhead. This trade-off, however, is a small price to pay for the enhanced security and integrity of QNNs in a cloud-based environment prone to untrusted adversaries. We also demonstrated STIQ's practical application by evaluating it on multiple real quantum hardwares, showing that STIQ achieves up to $\approx 70\%$ obfuscation, with combined performance similar to an unobfuscated model.
- [69] arXiv:2405.19577 (replaced) [pdf, html, other]
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Title: Non-equilibrium quantum Monte Carlo algorithm for stabilizer Renyi entropy in spin systemsComments: 6 pages, 4 figures + 7 pages, 5 figuresSubjects: Quantum Physics (quant-ph); Statistical Mechanics (cond-mat.stat-mech); Computational Physics (physics.comp-ph)
Quantum magic, or nonstabilizerness, provides a crucial characterization of quantum systems, regarding the classical simulability with stabilizer states. In this work, we propose a novel and efficient algorithm for computing stabilizer Rényi entropy, one of the measures for quantum magic, in spin systems with sign-problem free Hamiltonians. This algorithm is based on the quantum Monte Carlo simulation of the path integral of the work between two partition function ensembles and it applies to all spatial dimensions and temperatures. We demonstrate this algorithm on the one and two dimensional transverse field Ising model at both finite and zero temperatures and show the quantitative agreements with tensor-network based algorithms. Furthermore, we analyze the computational cost and provide both analytical and numerical evidences for it to be polynomial in system size.
- [70] arXiv:2406.01060 (replaced) [pdf, html, other]
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Title: Mechanical dynamics around higher-order exceptional point in magno-optomechanicsComments: 7 pages, 6 figuresJournal-ref: Advanced Quantum Technologies 2400275 (2024)Subjects: Quantum Physics (quant-ph)
We theoretically study diverse exceptional points (EPs) in an experimentally feasible magno-optomechanics consisting of an optomechanical subsystem coupled to a magnomechanical subsystem via physically direct contact. By adiabatically eliminating both the cavity and the Kittel mode, dissipative and parity-time symmetric exceptional points can be observed. When only the cavity mode is eliminated, a second (third) -order pseudo-Hermitian EP emerges for nondegenerate (degenerate) mechanical modes. The distinct dynamical behavior of two mechanical modes around these EPs are further studied. Our proposal provides a promising way to engineer diverse EPs and quantify non-Hermitian phase transition with exceptional dynamical behavior in magno-optomechanics.
- [71] arXiv:2407.00450 (replaced) [pdf, html, other]
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Title: Hybrid Quantum-Classical Clustering for Preparing a Prior Distribution of EigenspectrumComments: add fig.3Subjects: Quantum Physics (quant-ph)
Determining the energy gap in a quantum many-body system is critical to understanding its behavior and is important in quantum chemistry and condensed matter physics. The challenge of determining the energy gap requires identifying both the excited and ground states of a system. In this work, we consider preparing the prior distribution and circuits for the eigenspectrum of time-independent Hamiltonians, which can benefit both classical and quantum algorithms for solving eigenvalue problems. The proposed algorithm unfolds in three strategic steps: Hamiltonian transformation, parameter representation, and classical clustering. These steps are underpinned by two key insights: the use of quantum circuits to approximate the ground state of transformed Hamiltonians and the analysis of parameter representation to distinguish between eigenvectors. The algorithm is showcased through applications to the 1D Heisenberg system and the LiH molecular system, highlighting its potential for both near-term quantum devices and fault-tolerant quantum devices. The paper also explores the scalability of the method and its performance across various settings, setting the stage for more resource-efficient quantum computations that are both accurate and fast. The findings presented here mark a new insight into hybrid algorithms, offering a pathway to overcoming current computational challenges.
- [72] arXiv:2407.02617 (replaced) [pdf, html, other]
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Title: Open quantum dynamics with variational non-Gaussian states and the truncated Wigner approximationLiam J. Bond, Bas Gerritsen, Jiří Minář, Jeremy T. Young, Johannes Schachenmayer, Arghavan Safavi-NainiComments: 21+3 pages, 12+2 figures. Final versionJournal-ref: J. Chem. Phys. 161, 184113 (2024)Subjects: Quantum Physics (quant-ph); Quantum Gases (cond-mat.quant-gas); Chemical Physics (physics.chem-ph)
We present a framework for simulating the open dynamics of spin-boson systems by combining variational non-Gaussian states with a quantum trajectories approach. We apply this method to a generic spin-boson Hamiltonian that has both Tavis-Cummings and Holstein type couplings, and which has broad applications to a variety of quantum simulation platforms, polaritonic physics, and quantum chemistry. Additionally, we discuss how the recently developed truncated Wigner approximation for open quantum systems can be applied to the same Hamiltonian. We benchmark the performance of both methods and identify the regimes where each method is best suited to. Finally we discuss strategies to improve each technique.
- [73] arXiv:2407.06599 (replaced) [pdf, html, other]
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Title: Quantum coherence measures for generalized Gaussian wave packets under a Lorentz boostComments: 16 pages LATEXJournal-ref: Phys. Rev. A 110 (2024) 052413Subjects: Quantum Physics (quant-ph); High Energy Physics - Theory (hep-th)
In this paper we consider a single particle, spin-momentum entangled state and measure the effect of relativistic boost on quantum coherence. The effect of the relativistic boost on single-particle generalized Gaussian wave packets is studied. The coherence of the wave function as measured by the boosted observer is studied as a function of the momentum and the boost parameter. Using various formulations of coherence, it is shown that in general the coherence decays with the increase in momentum of the state, as well as the boost applied to it. A more prominent loss of coherence due to relativistic boost is observed for a single particle electron than that of a neutron. The analysis is carried out with generalized Gaussian wave packet of the form $\mathcal{N} p^n \exp(-\frac{p^2}{\sigma^2})$ with $n$ being the ``generalization parameter" and $\mathcal{N}$ denoting the appropriate normalization constant. We also obtain a range for parameter $n$ appearing in the wave packet. The upper bound is found to have a dependence on the mass of the particle and the width of the Gaussian wave packet. We have obtained the Frobenius-norm measure of coherence, $l_1$ and $l_2$ norm measure of coherence, and relative entropy of coherence for a (1+1) and (3+1)-dimensional analysis. Corresponding to each of the cases, we observe that the $l_1$ norm measure of coherence is equal to the Frobenius norm measure of coherence. We have analyzed the scenario for which such a beautiful coincidence can occur. Finally, we have plotted different measures of coherence for the electron as well as the neutron for different values of the width of the wave-function $\sigma$, boost parameter $\beta$, and generalization parameter $n$.
- [74] arXiv:2407.16849 (replaced) [pdf, html, other]
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Title: Automated modal analysis of entanglement with bipartite self-configuring opticsSubjects: Quantum Physics (quant-ph); Optics (physics.optics)
Entanglement is a unique feature of quantum mechanics. In coupled systems of light and matter, entanglement manifests itself in the linear superposition of multipartite quantum states (e.g., parametrized by the multiple spatial, spectral, or temporal degrees of freedom of a light field). In bipartite systems, the Schmidt decomposition provides a modal decomposition of the entanglement structure over independent, separable states. Although ubiquitous as a mathematical tool to describe and measure entanglement, there exists no general efficient experimental method to decompose a bipartite quantum state onto its Schmidt modes. Here, we propose a method that relies on bipartite self-configuring optics that automatically ``learns'' the Schmidt decomposition of an arbitrary pure quantum state. Our method is agnostic to the degrees of freedom over which quantum entanglement is distributed and can reconstruct the Schmidt modes and values by variational optimization of the network's output powers or coincidences. We illustrate our method with numerical examples of spectral entanglement analysis for biphotons generated via spontaneous parametric down conversion and provide experimental guidelines for its realization, including the influence of losses and impurities. Our method provides a versatile and scalable way of analyzing entanglement in bipartite integrated quantum photonic systems.
- [75] arXiv:2408.08768 (replaced) [pdf, html, other]
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Title: Low temperature decoherence dynamics in molecular spin systems using the Lindblad master equationSubjects: Quantum Physics (quant-ph)
Understanding the spin dynamics in low-temperature settings is crucial to designing and optimizing molecular spin systems for use in emerging quantum technologies. At low temperatures, irreversible loss occurs due to ensemble dynamics facilitated by electronic-nuclear spin interactions. We develop a combined open quantum systems and electronic structure theory capable of predicting trends in relaxation rates in molecular spin ensembles. We use the Gorini-Kossakowski-Sudarshan-Lindblad master equation and explicitly include electronic structure information in the decoherence channels. We apply this theory to several molecular systems pertinent to contemporary quantum technologies. Our theory provides a framework to describe irreversible relaxation effects in molecular spin systems with applications in quantum information science, quantum sensing, molecular spintronics, and other spin systems dominated by spin-spin relaxation.
- [76] arXiv:2409.13691 (replaced) [pdf, html, other]
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Title: Efficient Measurement-Driven Eigenenergy Estimation with Classical ShadowsYizhi Shen, Alex Buzali, Hong-Ye Hu, Katherine Klymko, Daan Camps, Susanne F. Yelin, Roel Van BeeumenComments: 32 pages (main text 15 pages), 7 figures (main text 5 figures)Subjects: Quantum Physics (quant-ph)
Quantum algorithms exploiting real-time evolution under a target Hamiltonian have demonstrated remarkable efficiency in extracting key spectral information. However, the broader potential of these methods, particularly beyond ground state calculations, is underexplored. In this work, we introduce the framework of multi-observable dynamic mode decomposition (MODMD), which combines the observable dynamic mode decomposition, a measurement-driven eigensolver tailored for near-term implementation, with classical shadow tomography. MODMD leverages random scrambling in the classical shadow technique to construct, with exponentially reduced resource requirements, a signal subspace that encodes rich spectral information. Notably, we replace typical Hadamard-test circuits with a protocol designed to predict low-rank observables, thus marking a new application of classical shadow tomography for predicting many low-rank observables. We establish theoretical guarantees on the spectral approximation from MODMD, taking into account distinct sources of error. In the ideal case, we prove that the spectral error scales as $\exp(- \Delta E t_{\rm max})$, where $\Delta E$ is the Hamiltonian spectral gap and $t_{\rm max}$ is the maximal simulation time. This analysis provides a rigorous justification of the rapid convergence observed across simulations. To demonstrate the utility of our framework, we consider its application to fundamental tasks, such as determining the low-lying, i.e. ground or excited, energies of representative many-body systems. Our work paves the path for efficient designs of measurement-driven algorithms on near-term and early fault-tolerant quantum devices.
- [77] arXiv:2410.14233 (replaced) [pdf, html, other]
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Title: Quantum-annealing-inspired algorithms for multijet clusteringComments: Submitted to Physics Letters BSubjects: Quantum Physics (quant-ph); High Energy Physics - Experiment (hep-ex); High Energy Physics - Phenomenology (hep-ph)
Jet clustering or reconstruction is a crucial component at high energy colliders, a procedure to identify sprays of collimated particles originating from the fragmentation and hadronization of quarks and gluons. It is a complicated combinatorial optimization problem and requires intensive computing resources. In this study, we formulate jet reconstruction as a quadratic unconstrained binary optimization (QUBO) problem and introduce novel quantum-annealing-inspired algorithms for clustering multiple jets in electron-positron collision events. One of these quantum-annealing-inspired algorithms, ballistic simulated bifurcation, overcomes problems previously observed in multijet clustering with quantum-annealing approaches. We find that both the distance defined in the QUBO matrix and the prediction power of the QUBO solvers have crucial impacts on the multijet clustering performance. This study opens up a new approach to globally reconstructing multijet beyond dijet in one go, in contrast to the traditional iterative method.
- [78] arXiv:2411.02339 (replaced) [pdf, html, other]
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Title: Quantum detailed balance via elementary transitionsComments: v1: 34 pages. v2: Minor corrections, changes and additions. Further references. 35 pagesSubjects: Quantum Physics (quant-ph)
Quantum detailed balance is formulated in terms of elementary transitions, in close analogy to detailed balance in a classical Markov chain on a finite set of points. An elementary transition is taken to be a pure state of two copies of the quantum system, as a quantum analogue of an ordered pair of classical points representing a classical transition from the first to the second point. This form of quantum detailed balance is shown to be equivalent to standard quantum detailed balance with respect to a reversing operation, thus providing a new conceptual foundation for the latter. Aspects of parity in quantum detailed balance are clarified in the process. The connection with the Accardi-Cecchini dual and the KMS dual (or Petz recovery map) is also elucidated.
- [79] arXiv:2411.03132 (replaced) [pdf, html, other]
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Title: All three-angle variants of Tsirelson's precession protocol, and improved bounds for wedge integrals of Wigner functionsComments: 13+8 pages, 15+2 figures. Fixed typos and formatting errorsSubjects: Quantum Physics (quant-ph)
Tsirelson's precession protocol is a nonclassicality witness that can be defined for both discrete and continuous variable systems. Its original version involves measuring a precessing observable, like the quadrature of a harmonic oscillator or a component of angular momentum, along three equally-spaced angles. In this work, we characterise all three-angle variants of this protocol. For continuous variables, we show that the maximum score $\mathbf{P}_3^\infty$ achievable by the quantum harmonic oscillator is the same for all such generalised protocols. We also derive markedly tighter bounds for $\mathbf{P}_3^\infty$, both rigorous and conjectured, which translate into improved bounds on the amount of negativity a Wigner function can have in certain wedge-shaped regions of phase space. For discrete variables, we show that changing the angles significantly improves the score for most spin systems. Like the original protocol, these generalised variants can detect non-Gaussian and multipartite entanglement when applied on composite systems. Overall, this work broadens the scope of Tsirelson's original protocol, making it capable to detect the nonclassicality and entanglement of many more states.
- [80] arXiv:2411.03293 (replaced) [pdf, html, other]
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Title: Genuine tripartite entanglement in graviton-matter interactionsComments: 6 pages, 2 figures, typos correctedSubjects: Quantum Physics (quant-ph)
We demonstrate the existence of genuine tripartite non-Gaussian entanglement in a quantum gravitational system formed by a quantum harmonic oscillator coupled to a single frequency of a quantized gravitational wave. For this purpose, we introduce a new entanglement witness, well-suited for the graviton-matter interaction Hamiltonian analysed here. We show that the witness is non-zero for the three-mode states generated by the Hamiltonian when the system starts in the ground state, thus proving the generation of genuine multipartite quantum entanglement at the most fundamental level of theoretical graviton-matter interactions.
- [81] arXiv:2312.00649 (replaced) [pdf, html, other]
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Title: Robustness of quantum chaos and anomalous relaxation in open quantum circuitsComments: 13 pages, 9 figures. v2: expanded discussion and numerical results. v3: additional discussion and numerics, version as publishedJournal-ref: Nat. Commun. 15, 9808 (2024)Subjects: Statistical Mechanics (cond-mat.stat-mech); Disordered Systems and Neural Networks (cond-mat.dis-nn); Strongly Correlated Electrons (cond-mat.str-el); High Energy Physics - Theory (hep-th); Quantum Physics (quant-ph)
Dissipation is a ubiquitous phenomenon that affects the fate of chaotic quantum many-body dynamics. Here, we show that chaos can be robust against dissipation but can also assist and anomalously enhance relaxation. We compute exactly the dissipative form factor of a generic Floquet quantum circuit with arbitrary on-site dissipation modeled by quantum channels and find that, for long enough times, the system always relaxes with two distinctive regimes characterized by the presence or absence of gap-closing. While the system can sustain a robust ramp for a long (but finite) time interval in the gap-closing regime, relaxation is ``assisted'' by quantum chaos in the regime where the gap remains nonzero. In the latter regime, we prove that, if the thermodynamic limit is taken first, the gap does not close even in the dissipationless limit. We complement our analytical findings with numerical results for quantum qubit circuits.
- [82] arXiv:2402.06577 (replaced) [pdf, html, other]
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Title: Determinant- and Derivative-Free Quantum Monte Carlo Within the Stochastic Representation of WavefunctionsJournal-ref: Rep. Prog. Phys. 87, 118001 (2024)Subjects: Strongly Correlated Electrons (cond-mat.str-el); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Quantum Physics (quant-ph)
Describing the ground states of continuous, real-space quantum many-body systems, like atoms and molecules, is a significant computational challenge with applications throughout the physical sciences. Recent progress was made by variational methods based on machine learning (ML) ansatzes. However, since these approaches are based on energy minimization, ansatzes must be twice differentiable. This (a) precludes the use of many powerful classes of ML models; and (b) makes the enforcement of bosonic, fermionic, and other symmetries costly. Furthermore, (c) the optimization procedure is often unstable unless it is done by imaginary time propagation, which is often impractically expensive in modern ML models with many parameters. The stochastic representation of wavefunctions (SRW), introduced in Nat Commun 14, 3601 (2023), is a recent approach to overcoming (c). SRW enables imaginary time propagation at scale, and makes some headway towards the solution of problem (b), but remains limited by problem (a). Here, we argue that combining SRW with path integral techniques leads to a new formulation that overcomes all three problems simultaneously. As a demonstration, we apply the approach to generalized ``Hooke's atoms'': interacting particles in harmonic wells. We benchmark our results against state-of-the-art data where possible, and use it to investigate the crossover between the Fermi liquid and the Wigner molecule within closed-shell systems. Our results shed new light on the competition between interaction-driven symmetry breaking and kinetic-energy-driven delocalization.
- [83] arXiv:2402.07887 (replaced) [pdf, html, other]
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Title: Hybrid acousto-optical swing-up state control in a quantum dotComments: 10 pages, 7 figures + supplement: 3 pages, 3 figuresSubjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Quantum Physics (quant-ph)
State transfer between different quantum systems is key for successful quantum technologies. Over long distances, photons are irreplaceable, but on short ranges in miniaturized complex devices or hybrid systems, coupling via orders of magnitude shorter wavelength acoustic waves has great potential. With interfaces to light, acoustic waves, and more, optically active quantum dots (QDs) are essential for multi-component systems. Here, we propose a hybrid acousto-optical method of non-resonant QD charge state control as an extension of the recent all-optical swing-up state preparation. We show that exciton and biexciton states or other superpositions of charge states can be prepared. Each field can act as a trigger, so an optically gated acoustic control and the opposite scheme, where an optical pulse controls the transition during acoustic modulation, can be implemented. Thus, we introduce acoustic state control into a system that lacks direct acoustic coupling between the states. The method does not rely on pulse shaping and is expected to work with arbitrary pulse shapes. Evaluating the phonon impact, we find an almost decoherence-free exciton preparation even at elevated temperatures with current QD and acoustic technology. This approach may also pave the way for optically controlled entanglement between emitters and acoustic modes and further on-chip state transfer via quantum acoustic busses.
- [84] arXiv:2402.16952 (replaced) [pdf, html, other]
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Title: Orbital selective order and $\mathbb{Z}_3$ Potts nematicity from a non-Fermi liquidComments: 23 pages, 23 figuresSubjects: Strongly Correlated Electrons (cond-mat.str-el); Quantum Physics (quant-ph)
Motivated by systems where a high temperature non-Fermi liquid gives way to low temperature $\mathbb{Z}_3$ Potts nematic order, we studied a three-orbital Sachdev-Ye-Kitaev (SYK) model in the large-$N$ limit. In the single-site limit, this model exhibits a spontaneous orbital-selective transition which preserves average particle-hole symmetry, with two orbitals becoming insulators while the third orbital remains a non-Fermi liquid down to zero temperature. We extend this study to lattice models of three-orbital SYK dots, exploring uniform symmetry broken states on the triangular and cubic lattices. At high temperature, these lattice models exhibit an isotropic non-Fermi liquid metal phase. On the three-dimensional (3D) cubic lattice, the low temperature uniform $\mathbb{Z}_3$ nematic state corresponds to an orbital selective layered state which preserves particle-hole symmetry at small hopping and spontaneously breaks the particle-hole symmetry at large hopping. Over a wide range of temperature, the transport in this layered state shows metallic in-plane resistivity but insulating out-of-plane resistivity. On the 2D triangular lattice, the low temperature state with uniform orbital order is also a correlated $\mathbb{Z}_3$ nematic with orbital-selective transport but it remains metallic in both principal directions. We discuss a Landau theory with $\mathbb{Z}_3$ clock terms which captures salient features of the phase diagram and nematic order in all these models. We also present results on the approximate wavevector dependent orbital susceptibility of the isotropic non-Fermi liquid states.
- [85] arXiv:2404.15940 (replaced) [pdf, html, other]
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Title: Complexity Measure Diagnostics of Ergodic to Many-Body Localization TransitionComments: 7+10 pages, 7+ 17 figures. v2: appendix I added, typos corrected, published versionJournal-ref: Phys. Rev. B 110, L180101 (2024)Subjects: High Energy Physics - Theory (hep-th); Statistical Mechanics (cond-mat.stat-mech); Quantum Physics (quant-ph)
We introduce new diagnostics of the transition between the ergodic and many-body localization phases, which are based on complexity measures defined via the probability distribution function of the Lanczos coefficients of the tri-diagonalized Hamiltonian. We use these complexity measures to analyze the power-law random banded matrix model as a function of the correlation strength and show that the moments and the entropy of the distribution diagnose the ergodic to many-body transition, as well as the distinctive feature of the phases concerning the memory of the initial conditions.
- [86] arXiv:2406.08573 (replaced) [pdf, html, other]
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Title: Refined cyclic renormalization group in Russian Doll modelComments: 15 pages, 3 figures, 34 referencesSubjects: Disordered Systems and Neural Networks (cond-mat.dis-nn); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); High Energy Physics - Theory (hep-th); Quantum Physics (quant-ph)
Focusing on Bethe-ansatz integrable models, robust to both time-reversal symmetry breaking and disorder, we consider the Russian Doll Model (RDM) for finite system sizes and energy levels. Suggested as a time-reversal-symmetry breaking deformation of Richardson's model, the well-known and simplest model of superconductivity, RDM revealed an unusual cyclic renormalization group (RG) over the system size $N$, where the energy levels repeat themselves, shifted by one after a finite period in $\ln N$, supplemented by a hierarchy of superconducting condensates, with the superconducting gaps following the so-called Efimov (exponential) scaling. The equidistant single-particle spectrum of RDM made the above Efimov scaling and cyclic RG to be asymptotically exact in the wideband limit of the diagonal potential. Here, we generalize this observation in various respects. We find that, beyond the wideband limit, when the entire spectrum is considered, the periodicity of the spectrum is not constant, but appears to be energy-dependent. Moreover, we resolve the apparent paradox of shift in the spectrum by a single level after the RG period, despite the disappearance of a finite fraction of energy levels. We also analyze the effects of disorder in the diagonal potential on the above periodicity and show that it survives only for high energies beyond the energy interval of the disorder amplitude. Our analytic analysis is supported with exact diagonalization.
- [87] arXiv:2406.15005 (replaced) [pdf, html, other]
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Title: Manipulating Spectral Windings and Skin Modes through Nonconservative CouplingsComments: 12 pages, 6 figuresJournal-ref: Physical Review A 110, 053507 (2024)Subjects: Optics (physics.optics); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Quantum Physics (quant-ph)
The discovery of the non-Hermitian skin effect (NHSE) has revolutionized our understanding of wave propagation in non-Hermitian systems, highlighting unexpected localization effects beyond conventional theories. Here, we discover that NHSE, accompanied by multitype spectral phases, can be induced by manipulating nonconservative couplings. By characterizing the spectra through the windings of the energy bands, we demonstrate that band structures with identical, opposite, and even twisted windings can be achieved. These inequivalent types of spectra originate from the multichannel interference resulting from the interplay between conservative and nonconservative couplings. Associated with the multitype spectra, unipolar and bipolar NHSE with different eigenmode localizations can be observed. Additionally, our findings link the nonreciprocal transmission properties of the system to multiple spectral phases, indicating a connection with the skin modes. This paper paves new pathways for investigating non-Hermitian topological effects and manipulating nonreciprocal energy flow.
- [88] arXiv:2406.17140 (replaced) [pdf, html, other]
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Title: Absorption Spectroscopy of $^{40}$Ca Atomic Beams Produced via Pulsed Laser Ablation: A Quantitative Comparison of Ca and CaTiO$_3$ TargetsJournal-ref: Appl. Phys. B 130, 214 (2024)Subjects: Atomic Physics (physics.atom-ph); Quantum Physics (quant-ph)
Pulsed laser ablation is an increasingly prevalent method for fast ion trap loading of various species, however characteristics of the ablation target source material can affect the ion-loading process. One factor which can reduce the atomic flux from a target is oxidation during atmospheric exposure when preparing or making changes to the ion trap vacuum system. Recent work has shown that perovskite ablation targets produce consistent atomic densities even after exposure to atmosphere when compared to elemental source targets. In this work, we directly compare calcium (Ca) and calcium-titanate (CaTiO$_3$) ablation targets, characterizing the neutral atomic beam flux using resonant, time-resolved absorption spectroscopy of the 423 nm $^{1}S_0 \rightarrow$ $^{1}P_1$ transition in neutral Ca. We measure the ablation plume longitudinal and transverse temperatures, number density, ion production, and spot lifetime for each target. In addition, we compare the ablated atomic beam density for both targets before and after 21 hours of exposure to atmosphere, demonstrating the relative robustness of the CaTiO$_3$ source.
- [89] arXiv:2406.19463 (replaced) [pdf, html, other]
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Title: Many-body Fu-Kane-Mele indexComments: v1 --> v2: Equivalence with non-interacting case refined and extendedSubjects: Mathematical Physics (math-ph); Quantum Physics (quant-ph)
We define a $\mathbb{Z}_2$-valued index for stably short-range entangled states of two-dimensional fermionic lattice systems with charge conservation and time reversal symmetry. The index takes its non-trivial value precisely if the `fluxon', the state obtained by inserting a $\pi$-flux through the system, transforms under time reversal as part of a Kramers pair. This index extends the Fu-Kane-Mele index of free fermionic topological insulators to interacting systems.
- [90] arXiv:2407.01753 (replaced) [pdf, html, other]
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Title: R\'enyi second laws for black holesComments: 51 pages, 12 figures; v2: clarifications and references added, minor typos corrected, published versionSubjects: High Energy Physics - Theory (hep-th); General Relativity and Quantum Cosmology (gr-qc); Quantum Physics (quant-ph)
Hawking's black hole area theorem provides a geometric realization of the second law of thermodynamics and constrains gravitational processes. In this work we explore a one-parameter extension of this constraint formulated in terms of the monotonicity properties of Rényi entropies. We focus on black hole mergers in asymptotically AdS space and determine new restrictions which these Rényi second laws impose on the final state. We evaluate the entropic inequalities starting from the thermodynamic ensembles description of black hole geometries, and find that for many situations they set more stringent bounds than those imposed by the area increase theorem.
- [91] arXiv:2408.04741 (replaced) [pdf, html, other]
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Title: A framework for generalizing toric inequalities for holographic entanglement entropySubjects: High Energy Physics - Theory (hep-th); Quantum Physics (quant-ph)
We conjecture a multi-parameter generalization of the toric inequalities of \cite{Czech:2023xed}. We then extend their proof methods for the generalized toric inequalities in two ways. The first extension constructs the graph corresponding to the toric inequalities and the generalized toric conjectures by tiling the Euclidean space. An entanglement wedge nesting relation then determines the geometric structure of the tiles. In the second extension, we exploit the cyclic nature of the inequalities and conjectures to construct cycle graphs. Then, the graph can be obtained using graph Cartesian products of cycle graphs. In addition, we define a set of knots on the graph by following \cite{Czech:2023xed}. These graphs with knots then imply the validity of their associated inequality. We study the case where the graph can be decomposed into disjoint unions of torii. Under the specific case, we explore and prove the conjectures for some ranges of parameters. We also discuss ways to explore the conjectured inequalities whose corresponding geometries are $d$-dimensional torii $(d>2)$
- [92] arXiv:2408.08391 (replaced) [pdf, html, other]
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Title: Flat-plane based double-counting free and parameter free many-body DFT+UComments: As accepted for publication in Phys. Rev. B. 22 pages, 9 figures, and 4 tablesJournal-ref: Phys. Rev. B. (2024)Subjects: Strongly Correlated Electrons (cond-mat.str-el); Chemical Physics (physics.chem-ph); Computational Physics (physics.comp-ph); Quantum Physics (quant-ph)
Burgess et al. have recently introduced the BLOR corrective exchange-correlation functional that is, by construction, the unique simplified rotationally-invariant DFT+U functional that enforces the flat-plane condition separately on each effective orbital of a localized subspace. Detached from the Hubbard model, functionals of this type are both double-counting correction free and, when optimized in situ using appropriate error quantifiers, effectively parameter free. In this work, the extension of the BLOR functional to address many-body errors (mBLOR) is derived. The mBLOR functional is built to enforce the flat-plane condition on the entire subspace, rather than on each orbital individually. In this way inter-orbital errors are corrected on the same footing as the single-particle ones. Focusing on exact test cases with strong inter-orbital interactions, the BLOR and mBLOR functionals were benchmarked against contemporary DFT+U functionals using the total energy extensivity condition on stretched homo-nuclear p-block dimers that represent various self-interaction and static-correlation error regimes. The BLOR functional outperformed all other DFT+$U$ functionals tested, which often act to increase total-energy errors, yet it still yielded large errors in some systems. mBLOR instead yielded low energy errors across all four strongly-correlated dimers, while being constructed using only semi-local approximation ingredients. As mBLOR would not otherwise introduce a band-gap correction in the manner that is a desirable feature of DFT+U, we developed a cost-free technique to reintroduce it automatically by moving the functional's unusual explicit derivative discontinuity into the potential. With this in place, mBLOR is the only known DFT$+U$ functional that opens the bandgap of stretched neutral homo-nuclear dimers without the aid of unphysical spin-symmetry breaking.
- [93] arXiv:2408.13882 (replaced) [pdf, html, other]
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Title: Phonon-induced modification of polaritonic Rabi oscillations in the presence of the dark excitonic condensateComments: replaced with expanded and revised versionSubjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Optics (physics.optics); Quantum Physics (quant-ph)
Optically inactive (dark) intervalley momentum-forbidden excitons are characterized by relatively long life time, and therefore are desirable candidates for realizing collective excitonic phases. However, testing their coherence by light directly is impossible. Here we propose a method for detecting a dark excitonic condensate. It relies on the interaction between excitons and phonons responsible for the interconversion between bright and dark excitons. As long as the dark condensate forms, the Rabi oscillations between photons and bright excitons can become strongly modified, and can be viewed as the photon-exciton-phonon polaritonic effect. The multi-component nature of the dark condensate consistent with the point-group symmetry is taken into account in the limit of weak phonon-exciton interaction. A perspective for the case of the strong interaction leading to the polaronic effect is discussed.
- [94] arXiv:2410.05932 (replaced) [pdf, html, other]
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Title: Quantum-Inspired Portfolio Optimization In The QUBO FrameworkSubjects: Portfolio Management (q-fin.PM); Quantum Physics (quant-ph)
A quantum-inspired optimization approach is proposed to study the portfolio optimization aimed at selecting an optimal mix of assets based on the risk-return trade-off to achieve the desired goal in investment. By integrating conventional approaches with quantum-inspired methods for penalty coefficient estimation, this approach enables faster and accurate solutions to portfolio optimization which is validated through experiments using a real-world dataset of quarterly financial data spanning over ten-year period. In addition, the proposed preprocessing method of two-stage search further enhances the effectiveness of our approach, showing the ability to improve computational efficiency while maintaining solution accuracy through appropriate setting of parameters. This research contributes to the growing body of literature on quantum-inspired techniques in finance, demonstrating its potential as a useful tool for asset allocation and portfolio management.
- [95] arXiv:2410.15395 (replaced) [pdf, html, other]
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Title: Floquet-Enriched Nontrivial Topology at Quantum CriticalityComments: 7 pages with supplemental materials, 25 figuresSubjects: Statistical Mechanics (cond-mat.stat-mech); Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el); Quantum Physics (quant-ph)
Periodically driven (Floquet) systems have attracted growing attention due to the emergence of intriguing phenomena that are absent in equilibrium physics. In this letter, we identify a new class of Floquet criticality characterized by nontrivial topology. For generic driven Majorana fermion chains with chiral symmetry, we analytically demonstrate that Floquet driving can enrich the transition point, resulting in topologically distinct quantum critical lines that are absent in undriven systems. Furthermore, we provide an intuitive physical explanation for the underlying mechanism of the nontrivial topology at Floquet criticality and generalize our results to higher dimensions. This work not only extends the scope of topological physics in Floquet systems but also deepens our understanding of gapless topological phases of matter.
- [96] arXiv:2411.04033 (replaced) [pdf, html, other]
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Title: Energy transport in a free Euler-Bernoulli beam in terms of Schr\"odinger's wave functionComments: 5 pagesSubjects: Mathematical Physics (math-ph); Classical Physics (physics.class-ph); Quantum Physics (quant-ph)
The Schrödinger equation is not frequently used in the framework of the classical mechanics, though historically this equation was derived as a simplified equation, which is equivalent to the classical Germain-Lagrange dynamic plate equation. The question concerning the exact meaning of this equivalence is still discussed in modern literature. In this note, we consider the one-dimensional case, where the Germain-Lagrange equation reduces to the Euler-Bernoulli equation, which is used in the classical theory of a beam. We establish a one-to-one correspondence between the set of all solutions (i.e., wave functions $\psi$) of the 1D time-dependent Schrödinger equation for a free particle with arbitrary complex valued initial data and the set of ordered pairs of quantities (the beam strain and the particle velocity), which characterize solutions $u$ of the beam equation with arbitrary real valued initial data. Thus, the dynamics of a free infinite Euler-Bernoulli beam can be described by the Schrödinger equation for a free particle and vice versa. Finally, we show that for two corresponding solutions $u$ and $\psi$ the mechanical energy density calculated for $u$ propagates in the beam exactly in the same way as the probability density calculated for $\psi$.
- [97] arXiv:2411.05737 (replaced) [pdf, html, other]
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Title: Accurate Unsupervised Photon Counting from Transition Edge Sensor SignalsNicolas Dalbec-Constant, Guillaume Thekkadath, Duncan England, Benjamin Sussman, Thomas Gerrits, Nicolás QuesadaComments: Code is publicly available at this https URL. Data is publicly available at this https URL and this https URLSubjects: Instrumentation and Detectors (physics.ins-det); Optics (physics.optics); Quantum Physics (quant-ph)
We compare methods for signal classification applied to voltage traces from transition edge sensors (TES) which are photon-number resolving detectors fundamental for accessing quantum advantages in information processing, communication and metrology. We quantify the impact of numerical analysis on the distinction of such signals. Furthermore, we explore dimensionality reduction techniques to create interpretable and precise photon number embeddings. We demonstrate that the preservation of local data structures of some nonlinear methods is an accurate way to achieve unsupervised classification of TES traces. We do so by considering a confidence metric that quantifies the overlap of the photon number clusters inside a latent space. Furthermore, we demonstrate that for our dataset previous methods such as the signal's area and principal component analysis can resolve up to 16 photons with confidence above $90\%$ while nonlinear techniques can resolve up to 21 with the same confidence threshold. Also, we showcase implementations of neural networks to leverage information within local structures, aiming to increase confidence in assigning photon numbers. Finally, we demonstrate the advantage of some nonlinear methods to detect and remove outlier signals.
- [98] arXiv:2411.06585 (replaced) [pdf, html, other]
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Title: Double-target BEC atomtronic rotation sensorOluwatobi Adeniji, Charles Henry, Stephen Thomas, Robert Colson Sapp, Anish Goyal, Charles W. Clark, Mark EdwardsComments: 13 pages, 10 figuresSubjects: Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)
We present a proof-of-concept design for an atomtronic rotation sensor consisting of an array of ``double-target'' Bose-Einstein condensates (BECs). A ``target'' BEC is a disk-shaped condensate surrounded by a concentric ring-shaped condensate. A ``double-target'' BEC is two adjacent target BECs whose ring condensates partially overlap. The sensor consists of an $n\times m$ array of these double-target BECs. The measurement of the frame rotation speed, $\Omega_{R}$, is carried out by creating the array of double-target BECs (setup step), inducing one unit of quantized flow in the top ring of each member of the array (initialization step), applying potential barriers in the overlap region of each member (measurement step), and observing whether the induced flow is transferred from the top to the bottom ring in each member (readout step). We describe a set of simulations showing that a single instance of a double-target BEC behaves in a way that enables the efficient operation of an $n\times m$ array for measuring $\Omega_{R}$. As an example of sensor operation we present a simulation showing that a 2$\times$2 array can be designed to measure $\Omega_{R}$ in a user-specified range.