Quantum Physics
Papers
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Monday 12 January
High-dimensional entanglement is considered to hold great potential for quantum key distribution (QKD) in high-loss and -noise scenarios. To harness its robustness, we construct a source for high-dimensional time-bin entangled photons optimized for high brightness, low complexity, and long-term stability. We certify the generated high-dimensional entanglement with a new witness employing nested Franson interferometry. Finally, we obtain key rates using a novel, noise-resilient QKD protocol. Our flexible evaluation method, centered around discretizations of the time stream, enables the same dataset to be processed while varying parameters such as state dimensionality and time bin length, allowing optimization of performance under given environmental conditions. Our results indicate regions within the accessible parameter space where high key rates per time are achievable for dimensionalities larger than two.
Quantum Physics
On the Lifshitz formula of dispersion interaction
The Lifshitz formula and methods of its preparation in the literature are considered. It is shown that in Lifshitz's work itself, this formula is given without a consistent conclusion. Moreover, the approach to the conclusion proposed in this work does not allow us to obtain it. The most general conclusion of this formula can be the method proposed by Levin and Rytov, the variation method of Schwinger and the method proposed by Van Kampen and co-authors. The Levin and Rytov approach is applicable in principle to bodies of arbitrary shape if the diffraction loss fields for electric and magnetic dipoles are determined, while the Van Kampen approach is applicable to any plane-layered structure and is quite simple. It is enough to write down the dispersion equations of the plasmon-polaritone structure. The specific dispersion force for a number of structures is calculated based on the Van Kampen method. It is shown that at small gaps, the force (pressure) density changes the inverse fourth-degree dependence on the distance and practically ceases to depend on it at distances less than 1 nm. For thin identical plates, this density is proportional to the square of their thickness at such distances, but the dependence quickly becomes saturated and already at thicknesses of the order of 10 nm practically ceases to depend on it.
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Quantum Physics · Cross-listed from cond-mat.quant-gas
From coherent to fermionized microwave photons in a superconducting transmission line
We investigate superconducting transmission lines as a novel platform for realizing a quantum fluid of microwave photons in a propagating geometry. We predict that the strong photon-photon interactions provided by the intrinsic nonlinearity of Josephson junctions are sufficient to enter a regime of strongly interacting photons for realistic parameters. A suitable tapering of the transmission line parameters allows for the adiabatic conversion of an incident coherent field into a Tonks-Girardeau gas of fermionized photons close to its ground state. Signatures of the strong correlations are anticipated in the correlation properties of the transmitted light.
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Quantum Physics · Cross-listed from physics.chem-ph
Dynamics of hyperpolarized nuclear spins at Zeeman-insensitive points
Avoided crossing (LAC) transitions are central to spin-based precision measurements since they provide strong observability and field-perturbation insensitivity, but while observed commonly in isolated-atom or engineered solid-state platforms, such transitions are unexploited in molecules in liquids. Here we report a solution-state, nuclear-spin LAC transition in hyperpolarized [1-$^{13}$C]-fumarate. The singlet-triplet LAC, occurring near 400 nT produces pronounced magnetization oscillations at approximately 2 Hz with a coherence time of 25 s, three times longer than the longest single-spin $T_2^*$. In the regime of high spin concentration and high polarization the sample's internal dipolar field couples measurably to the transition, providing a direct probe of nonlinear spin dynamics. These results establish nuclear-spin LACs as a platform for quantum sensing in liquids.
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Quantum Physics
Quasi-optimal quantum Markov chain spectral gap estimation
This paper proposes a quantum algorithm for Markov chain spectral gap estimation that is quasi-optimal (i.e., optimal up to a polylogarithmic factor) in the number of vertices for all parameters, and additionally quasi-optimal in the reciprocal of the spectral gap itself, if the permitted relative error is above some critical value. In particular, these results constitute an almost quadratic advantage over the best-possible classical algorithm. Our algorithm also improves on the quantum state of the art, and we contend that this is not just theoretically interesting but also potentially practically impactful in real-world applications: knowing a Markov chain's spectral gap can speed-up sampling in Markov chain Monte Carlo. Our approach uses the quantum singular value transformation, and as a result we also develop some theory around block-encoding Markov chain transition matrices, which is potentially of independent interest. In particular, we introduce explicit block-encoding methods for the transition matrices of two algebraically-defined classes of Markov chains.
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Quantum Physics · Cross-listed from cond-mat.mtrl-sci
Anisotropic anomalous Hall effect in distorted kagome GdTi3Bi4
Topological kagome magnets offer a rich landscape for exploring the intricate interplay of quantum interactions among geometry, topology, spin, and correlation. GdTi3Bi4 crystallizes in layered Ti based kagome nets intertwined with zigzag Gd chains along the a axis and orders antiferromagnetically below 15 K. Here, we present the temperature and field dependent electrical transport of GdTi3Bi4 in different directions. The material exhibits anomalous Hall conductivity (AHC) of 410 S cm-1 at 2 K for B parallel c and it is completely absent for B parallel a, despite the similar magnetization observed in both orientations. This behavior is quite contradictory, as anomalous Hall effect (AHE) typically scales with the magnetization. Through first principles calculations, it is demonstrated that in the presence of time reversal symmetry broken by the Gd 4f sublattice and spin orbit coupling, the magnetization direction controls the orbital mixing in the Ti t2g bands, relocating Berry curvature hot spots and producing the observed orientation selective AHC. The results establish GdTi3Bi4 as platform for investigating new avenues of AHE, such as directional AHE, and thus shed new light on the intricate coupling between magnetic and electronic structures, paving the way for exploring novel quantum phenomena.
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Quantum Physics · Cross-listed from cond-mat.quant-gas
Excitation spectrum of a bright solitary wave in a Bose-Einstein condensate and its connection with the Higgs and the Goldstone modes
We consider the problem of Bose-Einstein condensed atoms, which are confined in a (quasi) one-dimensional toroidal potential. We focus on the case of an effective attractive interaction between the atoms. The formation of a localized blob (i.e., a “bright" solitary wave) for sufficiently strong interactions provides an example of spontaneous symmetry breaking. We evaluate analytically and numerically the excitation spectrum for both cases of a homogeneous and of a localized density distribution. We identify in the excitation spectrum the emergence of the analogous to the Goldstone and the Higgs modes, evaluating various relevant observables, gaining insight into these two fundamental modes of excitation.
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Quantum Physics
Coupling a discrete state to a quasi-continuum: A model quantum mechanical system that interpolates between Rabi oscillations and decay-revival dynamics
We formulate a quantum mechanical system consisting of a single discrete state coupled to an infinite ladder of equally-spaced states, the coupling between the two being given by a Lorentzian profile. Various limits of this system correspond to well-known models from quantum optics, namely, the narrow resonance limit gives the Rabi system, the wide resonance limit gives the Bixon-Jortner system, the wide resonance, true continuum limit gives the Wigner-Weisskopf system, and the fixed resonance, true continuum limit gives a system that is typically studied by methods developed by Fano. We give a semi-analytical solution of the eigenvalue problem by reducing it to a transcendental equation, and demonstrate the aforementioned limiting behaviors. We then study the dynamics of the initial discrete state numerically, and show that it gives a wide range of behaviors in various limiting cases as predicted by our asymptotic theory including exponential decay, revivals, Rabi oscillations, and damped oscillations. The ability of this system to interpolate between such a rich set of behaviors and existing model systems, and the accessibility of a semi-analytical solution, make it a useful model system in quantum optics and related fields.
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Quantum Physics
Hardware-Economic Manipulation of Dual-Type ${}^{171}$Yb$^+$ Qubits
The dual-type qubit scheme is an emerging method to suppress crosstalk errors in scalable trapped-ion quantum computation and quantum network. Here we report a hardware-economic way to control dual-type $^{171}\mathrm{Yb}^+$ qubits using a single $355\,$nm mode-locked pulsed laser. Utilizing its broad frequency comb structure, we drive the Raman transitions of both qubit types encoded in the $S_{1/2}$ and the $F_{7/2}$ hyperfine levels, and probe their carrier transitions and the motional sidebands. We further demonstrate a direct entangling gate between the two qubit types. Our work can simplify the manipulation of the $^{171}\mathrm{Yb}^+$ qubits both at the hardware and the software level.
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Quantum Physics
Thermodynamic Recycling of Algorithmic Failure Branches: Quantum-Computer Demonstration with Quantum Error Correction
Thermodynamic trade-off relations dictate fundamental limits on the performance of thermodynamic tasks through costs such as heat dissipation. Here, we propose a framework called thermodynamic recycling to circumvent these limits in quantum processors by exploiting failure branches of quantum algorithms, which are usually discarded. The key component is an athermal bath naturally generated during the resetting of a failure branch. By coupling this bath to a target system prior to relaxation, thermodynamic tasks can be performed beyond conventional thermodynamic limits. We apply this framework to information erasure and derive the reduction in heat dissipation analytically. As a demonstration, we implement our framework on IBM's superconducting quantum processor by combining the Harrow–Hassidim–Lloyd algorithm with three-qubit quantum error correction, thereby reducing the heat dissipated in erasing syndrome information. Despite substantial noise and errors in current hardware, our method achieves erasure with heat dissipation below the Landauer limit. This work establishes an operational connection between quantum computing and quantum thermodynamics for resource-efficient quantum computation.
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Quantum Physics
Einstein's Worries and Actual Physics: Beyond Pilot Waves
Tim Maudlin has argued that the standard formulation of quantum mechanics fails to provide a clear ontology and dynamics and that the de Broglie–Bohm pilot-wave theory offers a better completion of the formalism, more in line with Einstein's concerns. I suggest that while Bohmian mechanics improves on textbook quantum theory, it does not go far enough. In particular, it relies on the “quantum equilibrium hypothesis” and accepts explicit nonlocality as fundamental. A deeper completion is available in stochastic mechanics, where the wavefunction and the Born rule emerge from an underlying diffusion process, and in a contextual, category-theoretic semantics in which measurement and EPR–Bell correlations are reinterpreted as features of contextual truth rather than of mysterious dynamics. In this framework, the measurement problem and “spooky action-at-a-distance” are dissolved rather than solved. Finally, a dynamics based on Rosen's “classical Schrödinger equation” provides a continuous passage between quantum and classical regimes, eliminating any sharp Heisenberg cut.
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Quantum Physics
Quantum model for black holes and clocks
We consider a stationary quantum system consisting of two non-interacting yet entangled subsystems, $\Xi$ and $\Gamma$. We identify a quantum theory characterizing $\Xi$ such that, in the quantum-to-classical crossover of the composite system, $\Gamma$ behaves as a test particle within the gravitational field of a Schwarzschild Black Hole (SBH) near its event horizon. We then show that this same quantum theory naturally provides a representation of $\Xi$ in terms of bosonic modes, whose features match those of the Hawking radiation; this facilitates the establishment of precise relations between the phenomenological parameters of the SBH and the microscopic details of the quantum model for $\Xi$. Finally, we recognize that the conditions used to characterize $\Gamma$ and $\Xi$ coincide with those required by the Page and Wootters mechanism for identifying an evolving system and an associated clock. This leads us to discuss how the quantum model for $\Xi$ endows the SBH with all the characteristics of a "perfect" clock.
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Quantum Physics
Scalable Certification of Entanglement in Quantum Networks
Quantum networks form the backbone of long-distance quantum information processing. Genuine multipartite entanglement (GME) serves as a key indicator of network performance and overall state quality. However, the widely used methods for certifying GME suffer from a major drawback that they either detect only a limited range of states or are applicable only to systems with a small number of parties. To overcome these limitations, we propose a family of sub-symmetric witnesses (SSWs), which are tractable both theoretically and experimentally. Analytically, we establish a connection between SSWs and the cut space of graph theory, enabling several powerful detection criteria tailored to practical quantum networks. Numerically, we show that the optimal detection can be formulated as a linear program, offering a significant efficiency advantage over the semidefinite programs commonly employed in quantum certification. Experimentally, SSWs can be evaluated via local measurements, with resource requirements independent of the local dimension in general, and even independent of the overall network size in many practical networks.
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Quantum Physics
Impact of Boundary Conditions on the Double-Kicked Quantum Rotor
We study the on-resonance Spin-1/2 Double Kicked Rotor, a periodically driven quantum system that hosts topological phases. Motivated by experimental constraints, we analyze the effects of open and periodic boundary conditions in contrast to the idealized case of infinite momentum space. As a bulk probe for topological invariants, we focus on the Mean Chiral Displacement (MCD) and show that it exhibits a pronounced sensitivity to boundary conditions, which can be traced to the dynamics in momentum space. Under open boundaries, states that would otherwise extend freely become localized at the edges of the finite momentum space, forming quasienergy edge states. While the bulk response measured by the MCD is strongly affected once the evolving wave packet reaches the boundaries, the persistence of these edge states still reflects the bulk-edge correspondence and provides reliable signatures of topological transitions.
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Quantum Physics
Fault-tolerant modular quantum computing with surface codes using single-shot emission-based hardware
Fault-tolerant modular quantum computing requires stabilizer measurements across the modules in a quantum network. For this, entangled states of high quality and rate must be distributed. Currently, two main types of entanglement distribution protocols exist, namely emission-based and scattering-based, each with its own advantages and drawbacks. On the one hand, scattering-based protocols with cavities or waveguides are fast but demand stringent hardware such as high-efficiency integrated circulators or strong waveguide coupling. On the other hand, emission-based platforms are experimentally feasible but so far rely on Bell-pair fusion with extensive use of slow two-qubit memory gates, limiting thresholds to $\approx 0.16\%$. Here, we consider a fully distributed surface code using emission-based entanglement schemes that generate GHZ states in a single shot, i.e., without the need for Bell-pair fusions. We show that our optical setup produces Bell pairs, W states, and GHZ states, enabling both memory-based and optical protocols for distilling high-fidelity GHZ states with significantly improved success rates. Furthermore, we introduce protocols that completely eliminate the need for memory-based two-qubit gates, achieving thresholds of $\approx 0.19\%$ with modest hardware enhancements, increasing to above $\approx 0.24\%$ with photon-number-resolving detectors. These results show the feasibility of emission-based architectures for scalable fault-tolerant operation.
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Quantum Physics · Cross-listed from cs.IT
Bias-Aware BP Decoding of Quantum Codes via Directional Degeneracy
We study directionally informed belief propagation (BP) decoding for quantum CSS codes, where anisotropic Tanner-graph structure and biased noise concentrate degeneracy along preferred directions. We formalize this by placing orientation weights on Tanner-graph edges, aggregating them into per-qubit directional weights, and defining a directional degeneracy enumerator that summarizes how degeneracy concentrates along those directions. A single bias parameter~$\beta$ maps these weights into site-dependent log-likelihood ratios (LLRs), yielding anisotropic priors that plug directly into standard BP$\rightarrow$OSD decoders without changing the code construction. We derive bounds relating directional and Hamming distances, upper bound the number of degenerate error classes per syndrome as a function of distance, rate, and directional bias, and give a MacWilliams-type expression for the directional enumerator. Finite-length simulations under code-capacity noise show significant logical error-rate reductions – often an order of magnitude at moderate physical error rates – confirming that modest anisotropy is a simple and effective route to hardware-aware decoding gains.
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Quantum Physics
Reply to Comment on "Properties and dynamics of generalized squeezed states"
In our paper [1], our numerical simulations showed that, unlike displacement and conventional squeezing, higher-order squeezing exhibits oscillatory dynamics. Subsequently, Gordillo and Puebla pointed out that simulation results depend on whether the size of the state space in the simulations is even or odd [2]. Using additional derivations, they argued that the oscillatory dynamics is unphysical and that the photon number must increase monotonically as a function of the squeezing parameter $r$. We agree with the observation of an even-odd parity dependence in the simulations. We independently noticed the same feature in our simulations after the publication of Ref. [1]. This observation led us to perform a more detailed investigation of the numerical simulation and mathematical aspects of the generalized squeezing problem. Our new findings were reported in Ref. [3]. Further analysis was reported in Ref. [4]. Our conclusion is that the generalized squeezing operator is physically not well defined but can be made well defined when combined with additional information about the physical system under study. We demonstrated this point in the case where we include an additional nonlinear interaction term in the Hamiltonian. We disagree with the claim that the photon number must be a monotonically increasing function of $r$. This claim contradicts the mathematically rigorous results of Ref. [4]. Furthermore, we show that the oscillatory behaviour persists in two closely related, well-behaved models.
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Quantum Physics
Quantum Error Correction and Detection for Quantum Machine Learning
At the intersection of quantum computing and machine learning, quantum machine learning (QML) is poised to revolutionize artificial intelligence. However, the vulnerability of the current generation of quantum computers to noise and computational error poses a significant barrier to this vision. Whilst quantum error correction (QEC) offers a promising solution for almost any type of hardware noise, its application requires millions of qubits to encode even a simple logical algorithm, rendering it impractical in the near term. In this chapter, we examine strategies for integrating QEC and quantum error detection (QED) into QML under realistic resource constraints. We first quantify the resource demands of fully error-corrected QML and propose a partial QEC approach that reduces overhead while enabling error correction. We then demonstrate the application of a simple QED method, evaluating its impact on QML performance and highlighting challenges we have yet to overcome before we achieve fully fault-tolerant QML.
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Quantum Physics
Quantum-Compatible Dictionary Learning via Doubly Sparse Models
Dictionary learning (DL) is a core tool in signal processing and machine learning for discovering sparse representations of data. In contrast with classical successes, there is currently no practical quantum dictionary learning algorithm. We argue that this absence stems from structural mismatches between classical DL formulations and the operational constraints of quantum computing. We identify the fundamental bottlenecks that prevent efficient quantum realization of classical DL and show how a structurally restricted model, doubly sparse dictionary learning (DSDL), naturally avoids these problems. We present a simple, hybrid quantum-classical algorithm based on projection-based randomized Kaczmarz iterations with Qiskit-compatible quantum inner products. We outline practical considerations and share an open-source implementation at https://github.com/AngshulMajumdar/quantum-dsdl-kaczmarz. The goal is not to claim exponential speedups, but to realign dictionary learning with the realities of near-term quantum devices.
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Quantum Physics
Direct temperature readout in nonequilibrium quantum thermometry
Quantum thermometry aims to measure temperature in nanoscale quantum systems, paralleling classical thermometry. However, temperature is not a quantum observable, and most theoretical studies have therefore concentrated on analyzing fundamental precision limits set by the quantum Fisher information through the quantum Cramer-Rao bound. In contrast, whether a direct temperature readout can be achieved in quantum thermometry remains largely unexplored, particularly under the nonequilibrium conditions prevalent in real-world applications. To address this, we develop a direct temperature readout scheme based on a thermodynamic inference strategy. The scheme integrates two conceptual developments: (i) By applying the maximum entropy principle with the thermometer's mean energy as a constraint, we assign a reference temperature to the nonequilibrium thermometer. We demonstrate that this reference temperature outperforms a commonly used effective temperature defined through equilibrium analogy. (ii) We obtain positive semi-definite error functions that lower-bound the deviation of the reference temperature from the true temperature-in analogy to the quantum Cramer-Rao bound for the mean squared error-and vanish upon thermalization with the sample. Combining the reference temperature with these error functions, we introduce a notion of corrected dynamical temperature which furnishes a postprocessed temperature readout under nonequilibrium conditions. This corrected dynamical temperature can be evaluated adaptively without prior knowledge of the actual temperature. We validate the corrected dynamical temperature in a qubit-based thermometer under a range of nonequilibrium initial states, confirming its capability to estimate the true temperature. Importantly, we find that increasing quantum coherence can enhance the precision of this readout.
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Quantum Physics · Cross-listed from gr-qc
Chiroptical effect induced by gravitational waves
We propose the gravitational analog of the chiroptical effect for the first time, demonstrating that gravitational waves (GWs) can induce a reversal of photon chirality through the exchange of angular momentum, namely the spin-2-gravitation chiroptical effect. By analyzing the interaction between photon spin angular momentum (SAM) and GWs, we derive the selection rules governing this exchange, which are strictly dictated by the spin-1 and spin-2 nature of the electromagnetic and gravitational fields, respectively. We find that the gravitational chiroptical effect reflects the local nature of SAM which prevents the accumulation of gravitational perturbations over spatial phase windings, and offers a theoretically rigorous tool to probe the chiral structure of GWs. This mechanism provides a novel observational pathway to constrain modified gravity theories, measure the asymmetric properties of compact binaries, and explore parity-violating physics in the early universe.
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Quantum Physics
Strong coupling of virtual negative states in the Kapitza-Dirac effect
Negative states are an intrinsic property of relativistic quantum theory and related to anti-particles in the context of the Dirac sea concept. We show that negative states can dominantly contribute to the diffraction amplitude in the quantum dynamics of the two-photon Kapitza-Dirac effect. We draw our conclusion by investigating solutions from time-dependent perturbation theory, where the perturbative solutions are in match with numeric solutions of the relativistic quantum system and also with the numeric and analytic solutions from the relativistic equations of motion of a classical point-like electron in an external standing wave light field. While our numeric solutions assume a strong laser field, the analytic solutions indicate that negative state coupling remains dominant for arbitrary low field amplitudes, where in the single-photon case (Compton scattering) negative state coupling can be mathematically associated with the interaction of a virtual electron-positron pair in the context of a quantized theory in old-fashioned perturbation theory.
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Quantum Physics
Composable Verification in the Circuit-Model via Magic-Blindness
As quantum computing machines move towards the utility regime, it is essential that users are able to verify their delegated quantum computations with security guarantees that are (i) robust to noise (ii) composable with other secure protocols and (iii) exponentially stronger as the number of resources dedicated to security increases. Previous works that achieve these guarantees are expressed in the Measurement-Based Quantum Computation (MBQC) model and benefit from a modular framework of verification protocols. This leaves architectures based on the circuit-model – in particular those using the Magic State Injection (MSI) – with fewer options to verify their computations or with the need to compile their circuits in MBQC which leads to overheads. This paper introduces a family of noise robust, composable and efficient verification protocols for Clifford + MSI circuits that are secure against arbitrary malicious behavior. This family contains the verification protocol of Broadbent (2018, ToC), extends its security guarantees while also bridging the modularity gap between protocols for MBQC and those for the circuit-model, and reducing quantum communication costs. As a result, it opens the prospect of rapid implementation tailored to near-term quantum devices. Our technique is based on a refined notion of blindness, called magic-blindness, which hides only the injected magic states – the sole source of non-Clifford computational power. This enables verification by randomly interleaving computation rounds with classically simulable, magic-free test rounds, leading to a trap-based framework for circuit verification. As a result, circuit-based quantum verification attains the same level of security and robustness previously known only in MBQC. It also reduces the quantum communication cost as transmitted qubits are required only at the locations of state injection.
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Sunday 11 January
Quantum Physics
Tackling Heterogeneity in Quantum Federated Learning: An Integrated Sporadic-Personalized Approach
Quantum federated learning (QFL) emerges as a powerful technique that combines quantum computing with federated learning to efficiently process complex data across distributed quantum devices while ensuring data privacy in quantum networks. Despite recent research efforts, existing QFL frameworks struggle to achieve optimal model training performance primarily due to inherent heterogeneity in terms of (i) quantum noise where current quantum devices are subject to varying levels of noise due to varying device quality and susceptibility to quantum decoherence, and (ii) heterogeneous data distributions where data across participating quantum devices are naturally non-independent and identically distributed (non-IID). To address these challenges, we propose a novel integrated sporadic-personalized approach called SPQFL that simultaneously handles quantum noise and data heterogeneity in a single QFL framework. It is featured in two key aspects: (i) for quantum noise heterogeneity, we introduce a notion of sporadic learning to tackle quantum noise heterogeneity across quantum devices, and (ii) for quantum data heterogeneity, we implement personalized learning through model regularization to mitigate overfitting during local training on non-IID quantum data distributions, thereby enhancing the convergence of the global model. Moreover, we conduct a rigorous convergence analysis for the proposed SPQFL framework, with both sporadic and personalized learning considerations. Theoretical findings reveal that the upper bound of the SPQFL algorithm is strongly influenced by both the number of quantum devices and the number of quantum noise measurements. Extensive simulation results in real-world datasets also illustrate that the proposed SPQFL approach yields significant improvements in terms of training performance and convergence stability compared to the state-of-the-art methods.
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Quantum Physics
Quantum Computing and Visualization Research Challenges and Opportunities
Quantum computing (QC) has experienced rapid growth in recent years with the advent of robust programming environments, readily accessible software simulators and cloud-based QC hardware platforms, and growing interest in learning how to design useful methods that leverage this emerging technology for practical applications. From the perspective of the field of visualization, this article examines research challenges and opportunities along the path from initial feasibility to practical use of QC platforms applied to meaningful problems.
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Quantum Physics
Nonadiabatic theory for subcycle ionic dynamics in multielectron tunneling ionization
Multielectron tunneling ionization creates ionic coherence crucial for lasing and driving electron motion in molecules. While tunneling is well understood as a single active electron process, less emphasis has been placed on theoretical descriptions of bound electrons during tunneling. This work systematically investigates multielectron tunneling ionization based on the strong field approximation, establishing a theoretical foundation and demonstrating the equivalence of wave function and density matrix approaches for subcycle ionic dynamics. An accurate subcycle nonadiabatic ionization rate is also derived and incorporated into the theory to improve its quantitative accuracy. Applying the theory to N$_{2}$ and CO$_{2}$, this work showcases how an intense laser field can induce ionic coherence in molecules as observed in previous experiments. These findings encourage future investigations into multielectron tunneling ionization and its applications in lasing and in controlling chemical reactions.
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Quantum Physics
Stochastic phase-space simulation of multimode cat states via the positive-P representation
We present a comprehensive study of the transient dynamics of multimode Schrödinger cat states in dissipatively coupled resonator arrays using the positive-P phase-space method. By employing the positive-P representation, we derive the exact stochastic differential equations governing the system's dynamics, enabling the simulation of system sizes significantly larger than those accessible via direct master equation simulation. We demonstrate the utility of this method by simulating transient dynamics for networks up to N=21 sites. Furthermore, we critically examine the method's usefulness and limitations, specifically highlighting the computational instability encountered when estimating the state parity in the systems. Our results provide a pathway for scalable simulations of non-Gaussian states in large open quantum systems.
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Quantum Physics
Subspace Selected Variational Quantum Configuration Interaction with a Partial Walsh Series
Estimating the ground-state energy of a quantum system is one of the most promising applications for quantum algorithms. Here we propose a variational quantum eigensolver (VQE) Ansatz for finding ground state configuration interaction (CI) wavefunctions. We map CI for fermions to a quantum circuit using a subspace superposition, then apply diagonal Walsh operators to encode the wavefunction. The algorithm can be used to solve both full CI and selected CI wavefunctions, resuling in exact and near-exact solutions for electronic ground states. Both the subspace selection and wavefunction Ansatz can be applied to any Hamiltonian that can be written in a qubit basis. The algorithm bypasses costly classical matrix diagonalizations, which is advantageous for large-scale applications. We demonstrate results for several molecules using quantum simulators and hardware.
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Quantum Physics
Irreversibility of decorrelating processes: an experimental assessment in cavity QED
Entropy production quantifies the amount of irreversibility of a physical process, leading to fundamental bounds for thermodynamic quantities. It captures the inability to run a physical system forward and then backward, bringing it to the same initial state. Considerable research has been carried out in the last decades to extend entropy production to non-equilibrium quantum processes. We experimentally investigate the entropy production of such forward-backward cycles affected by genuinely quantum irreversibility. Namely, we consider processes realized to erase different types of correlations between two interacting systems, from obliterating solely quantum coherence to completely decorrelating local states. This makes the measurement of entropy production experimentally challenging. Addressing this challenge is the purpose of this paper.
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Quantum Physics
Graphene-assisted resonant transmission and enhanced Goos-Hänchen shift in a frustrated total internal reflection configuration
Graphene-assisted resonant transmission and enhanced Goos-Hänchen shift are investigated in a two-prism frustrated-total-internal-reflection configuration. Due to the excitation of surface plasmons induced by graphene in low terahertz frequency range, there exist the resonant transmission and anomalous Goos-Hänchen shifts in such optical tunneling configuration. As compared to the case of quantum well, graphene sheet with unique optical properties can enhance the resonant transmission with relatively low loss, and modulate the large negative and positive Goos-Hänchen shifts by adjusting chemical potential or electron relaxation time. These intriguing phenomena may lead to some potential applications in graphene-based electro-optic devices.