Quantum Physics
Dynamics of quantum entanglement in two time-dependent coupled harmonic oscillators
Ayoub Ghaba, Radouan Hab-arrih, Elhoussine Atmani et al. (+1 more)
We investigate the
quantum entanglement dynamics of two coupled harmonic oscillators with a time-dependent interaction. Using the Lewis-Riesenfeld invariant method, we derive the exact analytical wave functions without any perturbative or adiabatic approximations and combine this with a phase-space analysis using the Wigner function to provide a complete description of the system's
quantum state
evolution. We obtain general expression form for the purity and the linear
entropy $S_L=1-\mathcal{P}$ for arbitrary excitation numbers $(n,m)$, allows a systematic study of
entanglement for a large class of
quantum states. We show that the
entanglement dynamics is very sensitive to the interplay between the detuning parameters $\theta$ and $\vartheta_2$, the frequency parameter $\beta_0$ and the coupling strength $\epsilon$: the increase of detuning takes the system from slow irregular oscillations to fast and regular periodic behavior, and the stronger coupling systematically enhances both the amplitude and the average value of the linear
entropy. Most importantly, for the resonance case $\omega_1=\omega_2=1$ and strong couplings $\epsilon \approx 0.99$, the system shows robust undamped synchronized periodic oscillations of the linear
entropy for all
quantum states considered, indicating preserved
quantum coherence without saturation. Our findings demonstrate that linear
entropy is a sensitive and practical
entanglement witness, and we establish explicit analytical relations between the coupling parameters of the system and its
entanglement properties, which are directly relevant to
quantum information processing and the control of
quantum correlations in continuous-variable systems.
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All authors Ayoub Ghaba, Radouan Hab-arrih, Elhoussine Atmani, Abdallah Slaoui
Quantum Physics
Gaussian Quantum Metrology with Realistic Linear Sensors
Jacques Ding, James W. Gardner, Tuvia Gefen et al. (+1 more)
Quantum sensing promises enhanced precision, but the usual
quantum Cramer Rao bound can be too optimistic for realistic linear sensors, where squeezing, filtering, and loss reshape
quantum noise. We derive the tight Holevo Cramer Rao bound and show that realistic degradation yields a hierarchy with the usual bound and homodyne readout. This hierarchy already exists in gravitational-wave detectors. We propose a hardware-efficient readout that reaches the Holevo bound without extra signal loss, increasing compact-binary detection rates by up to 25% over the present LIGO homodyne readout.
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All authors Jacques Ding, James W. Gardner, Tuvia Gefen, Yanbei Chen
Quantum Physics
Physics-Informed Neural Quantum Control for Rovibrational Photoassociation in a Morse Molecular System
Murilo D. Forlevesi, Emanuel Fernandes de Lima and Edson Denis Leonel
We present a Physics-Informed Neural
Quantum Control (PINQC) framework for rovibrational photoassociation in a Morse molecular system. The proposed method combines neural-network-based
laser-field generation with differentiable
quantum propagation, allowing optimized
laser pulses to be obtained directly from the underlying
quantum dynamics without requiring external training data. The optimized control fields efficiently transfer an initially continuum-like Gaussian wave packet into the vibrational ground-state level, promoting continuum-to-bound population transfer through coherent rovibrational dynamics. The resulting photoassociation process involves both vibrational stabilization and rotational redistribution arising naturally from dipole-induced couplings between neighboring rotational channels. A central result of the present work is the successful application of the PINQC framework to extended rovibrational models containing larger rotational levels than those previously accessible in our conventional photoassociation calculations. The optimization remains numerically stable despite the increased complexity of the molecular system, demonstrating that differentiable optimization provides an effective strategy for treating rovibrational models of increased dimensionality. These results establish the PINQC framework as a promising computational tool for molecular photoassociation and motivate future investigations of increasingly complex rovibrational
quantum-control problems.
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Quantum Physics · Cross-listed from cs.LG
STEMGym: Benchmarking Sequential Decision-Making under Dose Budgets in Autonomous Electron Microscopy
Can Polat, Erchin Serpedin, Mustafa Kurban et al. (+1 more)
A central premise of autonomous scientific imaging is that smarter
navigation, whether
Bayesian, RL-based, or otherwise adaptive, is the principal lever for sample-efficient acquisition. We present evidence to the contrary in scanning transmission electron microscopy (STEM), an atomic-resolution imaging modality whose every measurement deposits damaging electron dose. We introduce STEMGym, an open-source Gymnasium
benchmark of 15 physics-simulated STEM worlds spanning five materials, three difficulty levels, and four characterisation tasks, scored by the Dose-Efficiency Curve area (DEC-AUC), a single scalar capturing the information-vs-dose Pareto frontier. Across 33
agent configurations under realistic dose budgets, the dominant determinant of dose efficiency is the analyst (perception) pipeline, not the navigator: pairing a trained CNN analyst with naïve raster scanning raises DEC-AUC by 5.5x over a CNN-free raster baseline (0.287 vs.\ 0.052), while substituting
Bayesian or adaptive finite-state-machine
navigation for raster yields no statistically significant further gain. Production-tier
vision-
language models further underperform task-specific CNNs by {\sim}13x on crystallographic defect analysis. By decoupling perception,
navigation, and planning under a unified dose budget, STEMGym reframes where ML effort should be invested in autonomous electron microscopy and provides the measurement infrastructure to test it.
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All authors Can Polat, Erchin Serpedin, Mustafa Kurban, Hasan Kurban
Quantum Physics
The QAOA on the ring of disagrees
Kunal Marwaha
We study the performance of symmetric local
algorithms finding large cuts on the cycle graph. Such
algorithms that cannot see the whole graph at depth p cut at most a (2p+1)/(2p+2) fraction of edges in expectation. We prove that the QAOA achieves this value, a long-standing
conjecture of Farhi, Goldstone, and Gutmann. Curiously we do this without finding the optimal parameters. Instead we show it is equivalent to find an optimal pair of Laurent
polynomials of degree at most 2p-1. This is made possible by recasting the QAOA on one
qubit in the
language of
quantum signal processing.
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Quantum Physics · Cross-listed from physics.optics
Nanometric voids as optical antennas for rewritable momentum-engineered photonics in silicon
Aleksei I. Noskov, Evan P. Garcia, Haiping Sun et al. (+2 more)
Optical
antennas are widely used to localize electromagnetic fields far below the diffraction limit, enabling enhanced light-matter interactions across nanophotonics. Yet the regime in which optical confinement approaches the electronic de Broglie wavelength in a solid - where the
photon momentum distribution broadens sufficiently to relax optical selection rules - remains largely unexplored. Here we show that nanometric voids embedded within crystalline silicon act as such optical
antennas, dramatically altering the optical response of an indirect
semiconductor without the introduction of any foreign material. Using an electrically induced melt-quench process, we generate nanometric voids throughout bulk silicon, confirmed by high-resolution electron microscopy, diffraction analysis,
Fourier-filtered
lattice reconstruction, elemental mapping, and supported by optical and vibrational spectroscopies. The void-containing silicon exhibits intense broadband photo- and electroluminescence spectrally indistinguishable from that produced by metallic or
semiconductor nanoconfiners of similar dimensions, establishing that dielectric discontinuity, not confiner composition, governs the observed momentum-assisted optical transitions. The luminescence can be repeatedly written, erased, and rewritten through alternating electrical conditioning and optical recrystallization. These findings establish nanometric voids as a previously unexplored platform for extreme optical confinement and demonstrate that photonic functionality can be embedded and reconfigured directly within bulk silicon.
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All authors Aleksei I. Noskov, Evan P. Garcia, Haiping Sun, Eric O. Potma, Dmitry A. Fishman
Quantum Physics
Quantum simulation of molecular excited-state manifolds and energies using the TEPID-ADAPT-VQE algorithm
Jason Saroni, Bharath Sambasivam, Ayush Asthana et al. (+1 more)
The simulation of molecular excited states is a key challenge in
quantum chemistry and a promising application for
quantum computing. In this work, we investigate the efficacy of the truncated
eigenvalue parametrized initial density adaptive variational
algorithm (TEPID-ADAPT-VQE) for computing low-lying excited states and potential energy surfaces. TEPID-ADAPT variationally diagonalizes a truncated low-temperature Gibbs state, enabling the simultaneous preparation of multiple excited states within a single optimization. We apply the method to H$_2$, LiH, and linear H$_4$ across bond lengths spanning weakly and strongly correlated regimes. The adaptive derivative-assembled problem-tailored (ADAPT) ansatz construction yields compact
circuits suitable for near-term hardware. We also implement a modified version of the MORE-ADAPT-VQE
algorithm for comparison with TEPID-ADAPT. We find that both
algorithms accurately reproduce excited-state spectra and potential energy curves within chemical accuracy for all the molecules and geometries studied. However, TEPID-ADAPT has the advantage of utilizing only a single, physically motivated hyperparameter (temperature) that controls the energy scale at which excited states are targeted, while MORE-ADAPT utilizes multiple hyperparameters whose optimal values depend sensitively on the target problem. These results demonstrate that combining adaptive ansatz construction with density-
matrix-based formulations provides an efficient framework for excited-state
quantum chemistry on near-term devices.
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All authors Jason Saroni, Bharath Sambasivam, Ayush Asthana, Edwin Barnes
Quantum Physics
Concentration of Measure Phenomena for Quantum States on a Higher Dimensional Equator
Kabgyun Jeong
We revisit Lévy's lemma, a widely used analytical tool in
quantum information theory. Concentration inequalities quantify the phenomenon in which Lipschitz observables concentrate around a median or mean, and serve as fundamental analytical tools across information theory, statistical physics, and learning theory. In particular, Lévy's lemma provides a crucial framework for describing functionals on pure
quantum states, with applications in
quantum entanglement and
quantum statistical query learning. In this work, we isolate the hyper-equatorial part of the standard spherical concentration argument. The resulting estimate is a Lévy-type bound for Lipschitz functions on a fixed hyperequator, with the natural dimension parameter $d-1$. We also formulate the accompanying
geometric localization in terms of neighborhoods of the boundary, hyperequator, and a codimension-two antipodal great subsphere. This viewpoint clarifies the structure of the usual proof and points to the measure-theoretic formulation needed for sharper constant-level statements.
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Quantum Physics
A superconducting qutrit link beyond the qubit limit
Xiang Li, Zheng-Yang Mei, Yang He et al. (+7 more)
Superconducting microwave links have enabled deterministic state transfer and remote
entanglement between
qubits, but deterministic links have so far operated with an effectively two-dimensional transmitted
Hilbert space. Here we demonstrate a superconducting qutrit link between two independently packaged nodes connected by a microwave channel. Each node combines a transmon qutrit, a transmission resonator, and a tunable Purcell-filter interface, allowing the two remote microwave-
photon interfaces to be matched in both frequency and bandwidth. We implement two transition-selective
photon-mediated operations that transfer the $|e\rangle$ and $|f\rangle$ qutrit components in distinct temporal modes of the same channel. We tomographically characterize arbitrary qutrit-state transfer, obtaining a mean transferred-state fidelity of 83.68% and a qutrit process fidelity of 77.12%, exceeding both the classical qutrit-transfer
benchmark and the best possible average fidelity of an effective
qubit channel used to transmit an arbitrary qutrit. Using partial-transfer operations, we reconstruct a remote two-qutrit state with negativity 0.730, a
tomography-inferred dense-coding capacity of 2.273 bits, and a
tomography-inferred Collins-Gisin-Linden-Massar-Popescu (CGLMP) parameter $I_3=2.332$, all beyond the corresponding
qubit or local bounds. These results demonstrate a superconducting microwave link that uses the native three-level structure of transmons as a genuine high-dimensional communication resource.
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All authors Xiang Li, Zheng-Yang Mei, Yang He, Si-Lu Zhao, Yan-Jun Liu, Xiao-Hui Song, Kai Xu, Zhong-Cheng Xiang, Dong-Ning Zheng, Heng Fan
Quantum Physics
Stabilizer entropy is trustworthy for mixed states
Gianluca Esposito, Michele Viscardi and Alioscia Hamma
Quantifying non-stabilizerness in mixed states is provably intractable, as any strict monotone requires superexponential time. We propose a linear Stabilizer
Entropy that acts as a proper non-stabilizerness monotone with overwhelming probability when restricted to non-adaptive Clifford channels acting on flat mixed stabilizer states. Analytical and numerical results for Haar-random states, Clifford orbits, and random
matrix product states show that monotonicity violation probabilities decay as $\exp-\eta N$. We also prove the validity of Stabilizer
Entropy in specific many-body systems undergoing partial measurements, where the amount of resource never increases for each measurement outcome as well as when averaged over outcome probabilities. Given the hardness of strict alternatives, Stabilizer
Entropy emerges as a practical and theoretically justified resource measure.
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Quantum Physics
Steady-state Bell nonlocality in an autonomous quantum thermal machine
Bradley Longstaff
A
quantum thermal machine is presented that is able to autonomously generate steady-state Bell nonlocality. A Lindblad equation is derived for two
qubits that are incoherently coupled to a pair of thermal baths and the resulting Liouvillian is found to have a strong
symmetry. This out-of-
equilibrium system can generate Bell-nonlocal steady states across a range of parameters and at arbitrarily high temperatures for certain initial conditions. To analyse how the machine operates in a more realistic setting, experimental imperfections are then included via a
stochastic perturbation to the system
Hamiltonian. This breaks the strong
symmetry and results in collective and local dephasing noise. It is found that if the sole source of noise is local noise, then the steady state is Bell local. The co-presence of collective noise is then shown to be advantageous, with the ability to increase the degree of Bell-inequality violation and transform a steady state from Bell local to Bell nonlocal.
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Quantum Physics
Squeezing-enhanced Pairwise Fusion of Photonic Qudits
Pradip Laha and Peter van Loock
Pairwise fusion gates are linear-optical measurements that herald Bell projections onto two-rail subspaces of two \(d\)-rail single-
photon qudits. Without ancillary input
photons, these passive measurements succeed with probability \(1-d^{-1}\), with all failures confined to the diagonal logical subspace. We show that identical single-mode squeezers applied to the \(2d\) interferometer outputs before
photon-number-resolving detection recover part of this structured failure sector.
Photon-number parity preserves the successful off-diagonal fusion signatures, while selected all-even patterns yield POVM elements proportional to definite pairwise Bell projectors. We derive the exact logical-space POVM and prove that a diagonal pattern is accepted if and only if its
photon-number-imbalance vector has exactly two nonzero components of equal magnitude. The resulting closed
elliptic-integral expression increases the ideal success probability, for instance, from \(75\%\) to \(79.62\%\) for \(d=4\), and from \(83.33\%\) to \(87.15\%\) for \(d=6\). With a representative finite detector saturation threshold, \(n_{\rm sat}=7\), the respective certified values remain \(78.84\%\) and \(86.71\%\). These results establish active Gaussian processing as a method for recycling structured measurement failures without ancillary input
photons, at the cost of \(2d\) squeezing operations and a larger
photon-number range at detection.
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Quantum Physics · Cross-listed from physics.med-ph
Pawel Moskal, Deepak Kumar, Sushil Sharma et al. (+37 more)
Annihilation
photons are
quantum-entangled in polarization, a phenomenon that has not been exploited in medical diagnostics so far. We present the first in vivo imaging of the degree of
quantum entanglement of
photons originating from positron-electron annihilation within a human subject. This study utilized the Jagiellonian Positron Emission
Tomography (J-PET) scanner, constructed from plastic scintillators. In plastics, annihilation
photons interact primarily via the Compton effect, which provides simultaneous information regarding the
photon interaction position and time, as well as the
photon polarization plane. The patient was injected with a DOTA-TATE radiopharmaceutical labeled with the $^{68}$Ga radionuclide. Using the J-PET scanner, we determined the image of the radiopharmaceutical uptake and, simultaneously, the image of the degree of
quantum entanglement. The latter was determined from the relative angle between the polarization planes of the annihilation
photons. The values of the degree of
quantum entanglement extracted for the liver and the spleen are smaller than those predicted for maximally entangled two-
photon states, yet larger than expected for separable
photons. This demonstration opens new perspectives for the application of
quantum entanglement in clinical diagnostics.
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All authors Pawel Moskal, Deepak Kumar, Sushil Sharma, Ermias Y. Beyene, Neha Chug, Catalina Curceanu, Eryk Czerwiński, Atharva Dalvi, Manish Das, Alicja Hubalewska-Dydejczyk, Sharareh Jalali, Krzysztof Kacprzak, Tevfik Kaplanoglu, Łukasz Kapłon, Kamila Kasperska, Aleksander Khreptak, Grzegorz Korcyl, Tomasz Kozik, Sumit Kumar Kundu, Anoop Kunimmal Venadan, Bartosz Leszczyński, Edward Lisowski, Filip Lisowski, Justyna Mędrala-Sowa, Simbarashe Moyo, Wiktor Mryka, Szymon Niedźwiecki, Marta Opalińska, Anand Pandey, Piyush Pandey, Alessio Porcelli, Bartłomiej Rachwał, Magdalena Skurzok, Anna Sowa-Staszczak, Tomasz Szumlak, Satyam Tiwari, Pooja Tanty, Keyvan Tayefi Ardebili, Kavya Valsan Eliyan, Ewa Ł. Stepień
Quantum Physics · Cross-listed from cond-mat.quant-gas
Breathing mode of quantum droplets in dipolar quantum gases: A sum-rule analysis
Xinran Zhang, Junli Liu, Huiyun Xiao et al. (+2 more)
We theoretically investigate the ground-state properties and breathing-mode collective excitations of three-dimensional dipolar Bose gases in anisotropic harmonic traps incorporating
quantum fluctuations. Combining a Gaussian variational ansatz with a non-perturbative sum-rule analysis, we derive explicit analytical expressions for both axial and radial breathing-mode frequencies, which are validated by numerical solutions of the time-dependent extended Gross-Pitaevskii equation. Our theoretical predictions show excellent agreement with existing experimental data for $^{166}$Er and $^{162}$Dy gases. By constructing comprehensive phase diagrams across the parameter space of the $s$-wave scattering length, atom number, and trap aspect ratio, we reveal both discontinuous first-order phase transitions and smooth crossovers between the dilute Bose-Einstein condensate and dense
quantum droplet phases. We confirm that the enhanced incompressibility induced by
quantum fluctuations significantly elevates the breathing-mode frequencies in the droplet phase compared to conventional weakly interacting Bose gases. Furthermore, the system undergoes a phase transition and a crossover over the scattering length under the quasi-two-dimensional and quasi-one-dimensional confinements, characterized by discontinuous jumps and continuous crossovers in peak density and atomic cloud sizes, respectively. Our work offers a rigorous and highly accurate framework to characterize collective excitations in dipolar
quantum gases, providing quantitative insights for forthcoming ultracold atom experiments in lanthanide atoms and polar molecules.
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All authors Xinran Zhang, Junli Liu, Huiyun Xiao, Xiao-Long Chen, Yunbo Zhang
Quantum Physics
Generic Number-of-Copies Amplification for Pseudorandom States
Zvika Brakerski and Miri Zenilman
We show that any
quantum pseudorandom state that is secure against single-copy distinguishers, i.e. a $1$-PRS, can be amplified to $t$-copy security, i.e. to a $t$-PRS, without additional assumptions, for any
polynomial $t$ in the security parameter. Prior work (Ananth and Goldin, arXiv 2025) was only able to show this for a restricted class of $1$-PRS constructions, namely ones whose generators only use a small number of ancilla
qubits. Technically, we show that by carefully accounting for the randomness that is used in the construction, and using
quantum extractors, it is possible to eliminate an ancilla register of any length and obtain a meaningful $t$-PRS outcome.
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Quantum Physics · Cross-listed from cond-mat.dis-nn
Bongsu Kim, Oscar Lee, Sangjun Jeon et al. (+1 more)
Physical reservoir computing offers an energy-efficient alternative to conventional neural networks, where the material-specific intrinsic
memory capacity in a physical system plays an indispensable role. Substituting temporal
memory with spatial degrees of freedom, we demonstrate that the
memory capacity can be created extrinsically in systems with no intrinsic
memory by exploiting the computational space-time tradeoff. Our approach utilizes multidimensional input nodes to function as a spatial
memory axis, thereby replacing the dependency on intrinsic history-dependent dynamics in the reservoir. Our scheme is validated in a multi-terminal
quantum dot system, whose discrete energy levels provide strong nonlinearity and complexity crucial for reservoir computing, while its short relaxation time leaves no room for intrinsic
memory. Numerically, the
quantum dot reservoir with a tunable extrinsic
memory shows high performance on both chaotic future prediction and nonlinear transformation tasks. Furthermore, from the analysis of
quantum state trajectory acquired from task operations, the
geometric understanding of the extrinsic
memory capacity, nonlinearity, and complexity is provided and their correlations are systematically investigated.
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All authors Bongsu Kim, Oscar Lee, Sangjun Jeon, Kun Woo Kim
Quantum Physics · Cross-listed from cond-mat.mes-hall
Emergent energy scales in magnonic systems with relative motion
Daigo Oue
Relative motion between interacting systems can generate emergent energy scales that are absent in isolated systems. While uniform motion can be eliminated by a Galilean transformation, relative motion between interacting systems generally cannot. By coupling to an excitation's spatial structure, relative motion generates a Doppler frequency determined by its wavevector and the relative velocity, providing a mechanism for driving nonequilibrium phenomena. In this tutorial, we illustrate these ideas using magnonic systems as a concrete platform. We first discuss motion-induced
magnon transport between relatively moving ferromagnets, in which the Doppler frequency serves as an effective nonequilibrium bias in the perturbative regime. This mechanism produces
magnon currents even without conventional driving forces such as temperature gradients or chemical potential differences. We then introduce motion-induced parametric instabilities. When the emergent scale becomes sufficiently large to resonantly create
magnon pairs, the perturbative description breaks down, and the magnonic
vacuum becomes unstable. This instability occurs above a critical velocity threshold and leads to spontaneous
magnon-pair creation. Connections to related phenomena, including
quantum friction, Cherenkov emission, and Zel'dovich superradiance, are also highlighted. The concept of an emergent energy scale provides a unifying framework for understanding transport phenomena and instabilities in
quantum systems with relative motion.
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Quantum Physics
Intrinsic Indistinguishability of Identical Particles and How Particle Labels Affect It
Valery Shchesnovich
We investigate indistinguishability of identical
bosons and
fermions undergoing arbitrary particle-number-preserving evolutions of their visible degrees of freedom. For the projective indistinguishability measure, defined by the projection of the visible state onto the symmetric/anti-symmetric subspace, we derive an equivalent expression in terms of the dynamically invariant internal state. We further generalize the textbook symmetrization/anti-symmetrization framework for
bosons and
fermions to arbitrary partial distinguishability by deriving an explicit reconstruction formula for the multiparticle visible state in terms of the indistinguishability function encoding the dynamical invariants. We give complete characterization of the class-functions of indistinguishability by projective measures on generalized
symmetries. Finally, we reveal a strikingly counterintuitive effect: introducing additional particle label states can increase the multiparticle indistinguishability of identical particles. The effect originates from the cancellation of collective multiparticle phases.
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Quantum Physics · Cross-listed from cond-mat.stat-mech
Pseudo entropy and topological phases of matter
Pramod Kamal Kharel, Manghang Limbu, Nabaraj Khatri et al. (+2 more)
Entanglement entropy has proven to be a powerful probe of phenomena such as
quantum chaos and phase transitions. Pseudo
entropy is a recently proposed time-like generalization of an
entanglement measure, motivated by de Sitter holography. In this work, we find that pseudo
entropy can also serve as a novel probe for distinguishing topological phases of matter. For this, we consider the Su–Schrieffer–Heeger model as a representative example and investigate the averaged excess
entropy $\Delta S_{12}$, defined as the difference between pseudo
entropy and the average
entanglement entropy, across the topological-to-trivial and trivial-to-topological phase transitions. When the two states are in the same phase, we find that $ \Delta S_{12}$ is non-positive under periodic boundary conditions, while for open boundary conditions, it is non-positive only when the system is sufficiently large. Moreover, we analyze ground-state quench protocols for
topology-crossing quenches and find that the imaginary pseudo
entropy tracks the critical times predicted by the Fisher zeros.
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All authors Pramod Kamal Kharel, Manghang Limbu, Nabaraj Khatri, Ashish Khanal, Kiran Adhikari
Quantum Physics
Volume Law and Universality of Entanglement Entropy in Random Graph Fermi Systems
Saikat Sur
We study the ground-state
entanglement entropy of free
fermions on random graphs. We first establish a general criterion for the volume law on random graphs, based on the spectral characteristics of the
Hamiltonian. We then apply this criterion to the Erdős–Rényi random graph, where each of the possible edges is present independently with some probability. Using random
matrix theory and asymptotic freeness, we show that the ground-state
entanglement entropy obeys an exact volume law in Erdős–Rényi graphs in the thermodynamic limit, with a universal coefficient that is independent of the edge probability of the graph. This coefficient is confirmed numerically to take the value approximately $0.3863$ nats, strictly below the Page value. The volume law therefore reflects the absence of
geometric locality in the random graph.
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Quantum Physics
Private training in quantum machine learning
Tigran Sedrakyan, Frédéric Grosshans and Elham Kashefi
With the emergence of machine learning (ML) models trained on large
datasets containing potentially sensitive data, a major question in AI safety is how to make learning private with respect to the training data. Similar to classical machine learning,
quantum machine learning (QML) models are not devoid of
privacy vulnerabilities. Differential
privacy (DP) is a standard tool for training ML models on sensitive data, but its impact in QML remains poorly understood. In this work we study private training in hybrid variational QML models using a classical private DP-SGD optimizer applied to pipelines with classical inputs and outputs. We analyze the interplay between gradient clipping and calibrated noise addition in DP-SGD, and its impact on optimization and accuracy for noisy and noiseless
quantum models. We first explain why
quantum noise does not provide a satisfactory replacement for the calibrated noise in DP-SGD for ensuring
privacy. We then show how the deterministic bounds on gradient norms for a wide class of
quantum models translate into explicit control of the detrimental clipping bias introduced by DP-SGD. Finally, we formulate a numerical comparison protocol under fixed clipping threshold and
privacy budget and evaluate it on synthetic and image-classification tasks for equivalent
quantum and classical models. Our results suggest that
quantum models can retain higher accuracy in private-training regimes where the formal
privacy guarantee is ensured by a classical DP-SGD mechanism.
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Quantum Physics · Cross-listed from cond-mat.mes-hall
Learning Inhomogeneous Heisenberg Hamiltonians in Nanographene Spin Chains
Greta Lupi, Saketh Ravuri, Chenxiao Zhao et al. (+9 more)
Inferring microscopic
Hamiltonians from experimental data is a central challenge in
quantum materials and
quantum simulation. In low-dimensional
spin systems, exchange interactions are often assumed to be spatially uniform, despite structural and environmental inhomogeneities that can locally modify the coupling. Here, we leverage a local, length-independent machine learning methodology to reconstruct spatially modulated exchange interactions directly from inelastic scanning tunneling spectroscopy maps. We demonstrate this approach with nanographene
spin chains, identifying both near-uniform and inhomogeneous regimes across the synthesized magnets. The reconstructed models quantitatively reproduce the experimental spectra and recover the correct scaling of the excitation gap with system size. Our results establish a general strategy to bridge local spectroscopic measurements with effective many-body
Hamiltonians.
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All authors Greta Lupi, Saketh Ravuri, Chenxiao Zhao, Weidan Zhang, Cesare Roncaglia, Renxiang Liu, Xinliang Feng, Daniele Passerone, Pascal Ruffieux, Roman Fasel, Jose L. Lado, Gonçalo Catarina
Quantum Physics
Quantum algorithm for the nonlinear Schrödinger equation via the Lax-pair scattering
Chenjia Zhu, Zhaoyuan Meng, Yousheng Zhang et al. (+1 more)
The nonlinear Schrödinger equation (NLSE) governs a broad class of wave phenomena, including deep-water waves,
quantum turbulence, and
solitons. The multiscale spatiotemporal coupling inherent in these systems imposes severe computational bottlenecks on classical high-fidelity numerical simulations. While
quantum computing offers the potential for exponential speedup, its
unitary dynamics pose a fundamental challenge to solve the NLSE. We propose a
quantum framework based on the Lax-pair scattering for solving the 1D NLSE. Specifically, the physical field is first mapped into the spectral space via a
quantum direct scattering
circuit. Following a decoupled linear time
evolution, the physical solution is reconstructed through an inverse scattering transform utilizing the
quantum singular value transformation. Since the temporal
evolution is performed analytically in the scattering domain, the framework bypasses iterative time stepping, rendering it highly advantageous for long-time simulations. To demonstrate the accuracy and noise resilience of this approach, we simulate a Gaussian wave packet under
quantum noise, two-
soliton collisions, breather dynamics, and modulational instability on a
quantum emulator.
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All authors Chenjia Zhu, Zhaoyuan Meng, Yousheng Zhang, Yue Yang
Quantum Physics
QmDFT for Polycyclic Aromatics: Balancing Embedding Ground-State Fidelity and Experimental Gap Estimation
N. Manglani, Tejjan Arora, Samrit Maity et al. (+3 more)
Quantum Embedding density functional theory (QmDFT)
embedding offers a highly scalable approach to improve treatment for large highly correlated pi conjugated systems. However, estimating advanced electronic structure properties in polycyclic aromatic hydrocarbons (PAHs) needs advanced exchange correlation functionals that frequently trigger convergence instabilities during the
embedding cycle. In this work, we introduce an adaptive damping and direct inversion in the iterative subspace (DIIS) accelerated protocol that stabilizes the
embedding procedure, enabling robust integration of hybrid functionals like B3LYP and CAM B3LYP. Using 10 selected PAHs (linear and fused) molecules as a
benchmark. We demonstrate a clear functional-dependent ground-state energetics and frontier-orbital gap estimation. While LDA based approaches yield near-quantitative agreement with FCI in DFT reference energies and is further supported by thermochemical isomerization
benchmarks, while B3LYP provide significantly improved agreement with experimental E0-0 transition values. This mapping allows us to bypass explicit excited-state calculations for E0 0 values, thereby significantly reducing computational overhead. Among the hybrid functionals screened, CAM B3LYP offers a balanced overall performance. Our results establish a stable QmDFT framework and provide useful guidance for functional selection for
quantum embedding studies of PAHs and related low-dimensional pi-conjugated materials.
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All authors N. Manglani, Tejjan Arora, Samrit Maity, Ranjit Thapa, Shashank Sharma, Sanjay Wandhekar
Quantum Physics
Imaginary pseudo entropy encodes temporal orientation
Tatsuhiro Misumi
Pseudo
entropy between
quantum states at different times is generally complex, yet its imaginary part has lacked a bounded operational meaning. We show that a calibrated replica interferometer converts the pseudo-Rényi phase into a directly measurable record of transition orientation. Together with replica visibility, it exactly determines the trace distance between forward and backward ancilla outputs and hence the Helstrom-optimal single-shot success probability. At short times, the symmetrized covariance of the modular and physical
Hamiltonians sets the initial distinguishability response. Under any common
quantum channel, the corresponding orientation information can only decrease, with equality characterized by Petz recovery. Imaginary pseudo
entropy therefore records a reversible distinction between temporal orientations, while coarse graining can make the loss of that record irreversible.
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Quantum Physics
Rodeo Filtering for Direct Steady-State Estimation in Open Quantum Systems
Hyeonjun Yeo, Jongin Jeong, Soyoung Shin et al. (+1 more)
Computing non-
equilibrium steady states of open
quantum systems is a challenging task on conventional computers, motivating
quantum algorithms for direct steady-state estimation. A natural route is to regard the steady state as the zero mode of the Liouvillian and to isolate this sector spectrally. We formulate this task as a known-zero-sector projection problem and implement the corresponding filter using the Rodeo
algorithm, which performs
stochastic spectral filtering through repeated controlled evolutions and measurement-conditioned filtering steps. In the steady-state setting, the filter can be centered directly at the known zero
eigenvalue, avoiding the spectral search required in generic eigenstate preparation. Compared with a phase-estimation-based implementation of the same projection, the Rodeo approach enables restart on failure and reduces the target-error dependence of the filtering cost and controlled-
evolution depth from power-law to logarithmic. This advantage becomes more pronounced as the spectral separation of the Hermitian Liouvillian
embedding increases, allowing Rodeo filtering to outperform phase-estimation filtering already at modest controlled-
evolution depths. Our results identify Rodeo filtering as a resource-efficient primitive for estimating steady-state observables in open
quantum systems.
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All authors Hyeonjun Yeo, Jongin Jeong, Soyoung Shin, Ha Eum Kim
Quantum Physics · Cross-listed from nucl-th
Embedded Random Matrix Ensembles to Statistical Shell Model: Operation of $q$-normal forms
V.K.B. Kota, N.D. Chavda and Manan Vyas
Embedded random
matrix ensembles operating in
nuclear shell model spaces, with nucleons occupying a finite set of single particle orbits and interacting via a two-body interaction, form the basis for statistical shell model. With sufficiently strong interaction, the level densities in shell model spaces take close to a Gaussian form and transition strength distributions close to a bivariate Gaussian form. In practice, partitioning via spherical configurations ($\tilde{m}$) and angular momentum $J$ (also isospin where appropriate) are essential. The resulting statistical spectroscopy or statistical shell model was applied successfully in the past in some studies of
nuclear level densities, orbit occupancies, $\beta$-decay
matrix elements and so on. Going beyond these, recently it is recognized that embedded ensembles, in a better approximation, generate in-fact $q$-normal form ($q=1$ gives Gaussian and $q=0$ Wigner's semi-circle) for density of
eigenvalues, bivariate $q$-normal form for transition strengths and conditional $q$-normal form for strength functions. These then allow us to develop statistical shell model with $q$-normal forms. These new developments in embedded ensembles and statistical shell model are briefly reviewed in this paper. Also described, using some examples, is the role of the $q$ parameter in generating statistical properties of general
quantum many-particle systems.
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Quantum Physics
Gaussian quantum steering of coupled three-mode squeezed vacuum in an expanding universe
Guang-Wei Mi, Xiaofen Huang and Tinggui Zhang
The coupled three-mode squeezed
vacuum is a representative multimode squeezed Gaussian state featuring unique steerability. This work investigates Gaussian
quantum steering distributions of the coupled three-mode squeezed
vacuum under an expanding universe. Due to causal separation between the interior and exterior spacetime regions,
quantum information behind the event horizon is inaccessible to Alice, Bob and Charlie. We separately analyze steering behaviors for physically accessible and inaccessible modes. Our analysis shows that greater total mean
photon number and momentum, combined with reduced expansion volume and expansion rate, enhance
quantum steering strength. Notably, Gaussian
quantum steering for physically inaccessible modes undergoes the "sudden death" phenomenon when a critical threshold parameter $\phi$ is exceeded. These results deliver novel insights into
quantum correlations in curved spacetime.
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Quantum Physics
Quantum scrambling of algebras of observables: the $\mathbb{Z}_2$-symmetric case
Paolo Zanardi and J. Karson Lewis
We consider
unitary quantum scrambling for an entire class of observable
algebras $\mathcal A$ whose commutant ${\mathcal A}'=\mathbb{C}\mathbb{Z}_2$ is generated by a Hermitian involution $S$, i.e., a parity operator. We adopt two different, although related, scrambling measures: (a) the algebraic out-of-time-order correlator (A-OTOC) introduced in [Andreadakis, Anand, and Zanardi, Phys. Rev. A 107, 042217 (2023)], and (b) a Slater-determinant-type quantity, dubbed the Plücker fidelity, obtained by
embedding operator
algebras into a higher-dimensional projective operator space. Both measures admit a simple
geometric interpretation in terms of distances between
algebras and their dynamical images. Moreover, they can be expressed in terms of the norm of the $\mathbb{Z}_2$-
symmetry-breaking component of the
unitary, which is in turn controlled by the time autocorrelation function of the $\mathbb{Z}_2$ generator $S$. We derive exact expressions for the A-OTOC and the Plücker fidelity in the general case, as well as their values for a random
unitary. For
Hamiltonian-generated channels, we compute their long-time averages, as well as the typical values of these long-time averages for random generators $S$. Finally, we numerically explore the results of our formalism for interacting
spin chains in different phases and for different physical parity operators.
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Quantum Physics
Shadow tomography for classical tensor network simulations
Jiace Sun and Garnet Kin-Lic Chan
Shadow
tomography has appeared as a powerful tool for estimating observables on
quantum computers from a small number of samples. We show that shadow-
tomography-inspired ideas can offer similarly improved sample scaling for estimating observables on tensor network states on classical computers after proper adaptation. We develop strategies for both
spin (bosonic) and fermionic systems, tailored to the contraction requirements of tensor networks, and generate scaling improvements of factors of $O(N)$ to $O(N^{3})$ (where $N$ is system size), depending on the specific task and system type. For the important and difficult task of evaluating the expectation value of long-range interacting
Hamiltonians, we achieve the optimal $O(1)$ overall scaling (up to logarithmic factors) for an arbitrarily fixed relative Monte Carlo error in both
spin and fermionic systems. Additionally, we show that shadow
estimators offer more stable gradients of observables in variational optimization tasks than standard Monte Carlo
estimators. We demonstrate practical advantage by simulating systems with long-range interactions, including the 2D long-range Heisenberg model and an ab-initio
quantum chemistry
Hamiltonian.
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