Quantum Physics
Energy-error tradeoff in encoding quantum error correction
Josey Stevens and Sebastian Deffner
While it has been widely recognized that genuine
quantum advantage for practical problems might only be achieved with fault-tolerant
quantum computers, it is still not entirely clear whether the required
quantum error correction will be physically feasible. In the present work, we carefully analyze the required energy resources to encode the logical
qubit states for repetition, perfect, and Steane codes. We find that there is a universal trade-off between the target precision and the required energetic resources. Importantly, we find that the energetic resources intimately depend on the specific physical realization of a
quantum error correction code, and that the required resources scale exponentially with the targeted precision of the encoding.
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Quantum Physics · Cross-listed from physics.optics
Seeing the forbidden: overcoming optical selection rules through nanophotonic integration
Alex H. Rubin, Vytautas Žalandauskas, Pranta Saha et al. (+7 more)
Optically addressable
spin defects in silicon carbide, including the neutral divacancy (VV$^0$) and the negative nitrogen-vacancy (NV$^-$), are among leading building blocks of solid-state
quantum technologies. Integrating these defects into photonic structures such as nanopillars improves
photon collection efficiency, but the consequences extend further. We show that the sub-wavelength geometry of nanopillars drastically modifies the local electromagnetic environment, providing optical access to defect transitions that are otherwise suppressed by selection rules in bulk material. Using low-temperature photoluminescence spectroscopy, we observe that emission from the PL3 divacancy, which is nearly absent in planar devices, becomes pronounced in nanopillars owing to a polarization transformation of the excitation field within the pillar. We further leverage the orientation-dependent collection of nanopillars to resolve the origin of previously ambiguous spectral lines. In particular, the NV4$'$ feature displays the signal enhancement expected for axially oriented NV$^-$ centres, consistent with assignment to a higher excited state of the $kh$ defect configuration. Our results establish nanophotonic integration as a
symmetry-sensitive probe that can both activate nominally dark transitions and identify the dipole character of poorly understood defect states.
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All authors Alex H. Rubin, Vytautas Žalandauskas, Pranta Saha, Rubek Poudel, Aurora Teien, Liam Hofmann, Nathan R. Gonzalez, Scott Dhuey, Marianne Etzelmüller Bathen, Marina Radulaski
Quantum Physics
Jan Østergaard, Shashi Raj Pandey, Christophe Biscio et al. (+2 more)
We study compression strategies for multipartite
entanglement distribution under uncertainty in the partitioning of the
quantum state. When the partition is not known at the time of state preparation, we show that a joint design of the resource state and a family of compression schemes can increase the
entanglement across partitions under a fixed transmission budget. We formulate this as a source coding problem and derive non-asymptotic upper and lower bounds on the achievable average
entanglement subject to an average coding rate. We furthermore design an efficient method for jointly optimizing states and lossless compression maps by exploiting the inherent
symmetry of weighted Dicke states. In the bipartite case, we propose practical constructions that closely approach the derived upper bound, and more generally we provide practical constructions for multipartite settings.
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All authors Jan Østergaard, Shashi Raj Pandey, Christophe Biscio, Torben Bach Pedersen, Petar Popovski
Quantum Physics · Cross-listed from cs.ET
Second-Order FALQON Parameter Transfer for the Max-Cut Problem on 3-Regular Graphs
Gabriel Fernandes Thomaz, Eduarda Rodrigues Monteiro, Jerusa Marchi et al. (+3 more)
The Feedback-based
Algorithm for
Quantum Optimization (FALQON) offers a deterministic alternative to variational
quantum algorithms by bypassing classical optimization loops. However, maintaining convergence on large problem instances often requires restricting the time step, necessitating
quantum circuit depths that exceed Noisy Intermediate-Scale
Quantum (NISQ) hardware capabilities. This paper investigates the parameter transferability of second-order FALQON applied to the Max-Cut problem on 3-regular graphs. Through numerical experiments evaluating
quantum circuits up to 16 layers on graphs up to 24 nodes, we demonstrate a highly advantageous scaling behavior: transferring feedback parameters optimized on small instances to larger target graphs yields significantly higher approximation ratios than natively optimizing the parameters directly on the larger graphs. This performance advantage arises because parameters trained on smaller instances can safely adopt aggressively larger time steps. By offloading the expensive parameter discovery phase to small-scale instances, this transfer strategy simultaneously reduces computational overhead and enhances the approximation ratio, thereby bringing FALQON closer to practical viability on near-term
quantum architectures.
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All authors Gabriel Fernandes Thomaz, Eduarda Rodrigues Monteiro, Jerusa Marchi, Marcelo Zen Pretto, Alisson dos Passos Fumaco, Evandro Chagas Ribeiro da Rosa
Quantum Physics · Cross-listed from hep-th
Nonlocal Nonstabilizerness from Holographic Schwinger Pair Production
Sebastian Grieninger
We analyze the emergence of nonlocal magic in Schwinger pair creation in strong non-Abelian (chromo)electric fields using holography. The produced
quark–antiquark pair is entangled into a color singlet, yet accelerates into causally disconnected Rindler wedges. Using the Casini–Huerta–Myers conformal mapping and the probe-brane framework, we compute the refined Rényi
entropy and its derivative, which captures the antiflatness of the
entanglement spectrum for a spherical bipartition. We find that for boundary spacetime dimension $d>2$, the
entanglement spectrum is non-flat, implying the dynamical generation of nonlocal magic in the pair creation process. Interestingly, the nonlocal magic in the
holographic dual can be obtained from the free energy of the probe action.
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Quantum Physics
GHZ is All You Need: Quantum Sensing with VISTA
Oskar Novak, Christos N. Gagatsos and Narayanan Rengaswamy
Quantum metrology holds the potential to enhance
magnetic field sensing beyond current limits. However, in the presence of realistic noise, this advantage degrades to the Standard
Quantum Limit. While recent algorithmic and variational techniques attempt to recover this scaling, they are hindered by stringent control requirements on the probe state that are infeasible in the near term, or by barren plateaus and interpretability issues inherent to black-box variational
quantum circuits. Here, we introduce Variational Inference and Sensing with Twin Ansätze (VISTA), a closed-loop protocol that combines passive sensing, or where the probe state is left to evolve without any active control, with physics-informed variational optimization. In the VISTA framework, a probe state evolves under a Lindbladian master-equation, and is compared, via the Swap test, to a parameterized “
quantum twin", a shallow
quantum circuit designed to mimic the underlying pure-state or Lindbladian master-equation dynamics. By restricting the optimization space to the physical parameters of interest, VISTA circumvents barren plateaus. We demonstrate that by coupling the protocol with a classical optimizer and high shot counts, VISTA can temporarily achieve near-Heisenberg scaling for moderately noisy
qubits over a finite range of system sizes. Furthermore, we introduce a Quasi-Normalization technique that sharpens the loss gradients, enabling simultaneous extraction of both the coherent signal $\theta$ and the environmental noise rate $\gamma$ with low absolute error. Finally, we extend VISTA to the multi-parameter vector metrology regime, enabling simultaneous parameter extraction from a transverse-
magnetic-field
Hamiltonian. By eliminating the need for complex, open-loop control and processing, VISTA offers a highly practical, resource-efficient framework for near- to intermediate-term
quantum sensors.
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Quantum Physics
A Transferable Machine Learning Approach to Predict Optimized Orbitals for Electronic Structure Problems
Lucas van der Horst, Maniraman Periyasamy, Abhishek Y. Dubey et al. (+3 more)
Variational
quantum eigensolver ansätze hold considerable promise for ground-state energy calculations on near-term
quantum hardware, yet most promising ansatz designs currently strongly depend on how well the molecular orbital basis captures the electronic correlation of the system. Computing optimized orbital coefficients via classical routines is computationally expensive and must be performed independently for each molecular geometry – a bottleneck that limits scalability across chemical space. We present a graph neural network framework that predicts optimized orbital coefficients directly from molecular geometry and pair-wise bonding structure. Trained on hydrogenic systems of modest size ($H_4$ and $H_6$) across tens of thousands of geometries, our model transfers to larger, unseen systems ($H_8$, $H_{10}$ and $H_{12}$) without retraining – demonstrating strong out-of-distribution generalization with respect to system size. When evaluating on structured and random configurations, and comparing against energies obtained with full classical optimization, our model reaches mean absolute energy errors $\mathcal{O}(10^2)$ and $\mathcal{O}(10)$ milli-Hartrees, respectively. Beyond energy estimation, the predicted orbitals serve as high-quality warm-start initializations that substantially reduce optimizer iterations to ground-state energy convergence. These results establish graph neural networks as an effective and scalable strategy for accelerating orbital optimization in hybrid
quantum-classical workflows, directly reducing the classical pre-processing overhead that currently limits the practical deployment of variational
quantum eigensolver on near-term
quantum hardware.
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All authors Lucas van der Horst, Maniraman Periyasamy, Abhishek Y. Dubey, Davide Bincoletto, Jakob S. Kottmann, Daniel D. Scherer
Quantum Physics
Boson Sampling with a reconfigurable 128 modes 3D integrated photonic circuit
Simone Di Micco, Francesco Hoch, Alessandro Ciorra et al. (+10 more)
Integrated
quantum photonics has emerged as one of the leading platforms for scaling
quantum information processing, offering compact, stable, and low-loss hardware with precise phase and mode control. Advances in integrated photonics architectures and active programmability now enable complex, high-dimensional transformations essential for
quantum advantage tasks. We introduce an integrated, reconfigurable 3D photonic device with 128 modes for manipulation of single-
photon quantum states (Qolossus 3D). Leveraging a continuously coupled architecture and thermo-optic programmability, the platform implements reconfigurable
unitary transformations at unprecedented scale for integrated
quantum optics. Exploiting indistinguishable single
photons demultiplexed from a
quantum dot source, we perform
Boson Sampling across the large-dimensional chip and analyse the resulting output distributions for up to 4
photons. We then exploit it to demonstrate randomness generation via
Boson Sampling. Agreement with theoretical predictions validates both the device's reconfigurable operation and the generation of random numbers. Our results highlight the scalability, stability, and precise control of integrated photonics for
quantum information processing.
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All authors Simone Di Micco, Francesco Hoch, Alessandro Ciorra, Daniel Carvalho de Salles, Nicolò Spagnolo, Taira Giordani, Gonzalo Carvacho, Niki Di Giano, Marco Gardina, Andrea Crespi, Francesco Ceccarelli, Roberto Osellame, Fabio Sciarrino
Quantum Physics
Quantum state texture of dynamical criticality
Lucas C. Céleri, Krissia Zawadzki, Ivan Medina et al. (+1 more)
We investigate the role of
quantum state texture in dynamical
quantum phase transitions by establishing a direct connection between critical nonequilibrium dynamics and the recently introduced notion of rugosity, a measure of
quantum state texture. Considering a generic quench protocol, we analyze both standard formulations of dynamical
quantum phase transitions. For type-I transitions, characterized through the long-time behavior of a dynamical order parameter, we show that the time-averaged rugosity, evaluated in the eigenbasis of the pre-quench
Hamiltonian, exhibits an order-parameter-like behavior across the transition. In the Lipkin-Meshkov-Glick model, this behavior is traced back to the underlying semiclassical structure, where crossing the excited-state
quantum phase transition separatrix controls the redistribution of the
quantum state over the pre-quench energy basis. We further demonstrate that the response function associated with the time-averaged rugosity develops a pronounced critical peak whose maximum grows algebraically with the system size, providing quantitative finite-size evidence of its sensitivity to the dynamical critical point. For type-II transitions, characterized by nonanalyticities in the Loschmidt rate function, we establish an exact connection with rugosity. For a suitable choice of basis, the rate function is exactly given by the density of rugosity, establishing a model-independent equivalence. Moreover, we show that even in physically motivated bases, such as the pre-quench energy eigenbasis, rugosity provides clear signatures of dynamical criticality. Our results establish rugosity as a sensitive probe of dynamical
quantum phase transitions while highlighting its role as a basis-dependent measure of
quantum state texture, thereby providing an information-theoretic perspective on nonequilibrium
quantum critical phenomena.
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All authors Lucas C. Céleri, Krissia Zawadzki, Ivan Medina, Diogo O. Soares-Pinto
Quantum Physics
Establishing Mixed-State Phase Equivalence beyond Renormalization Fixed Points
Yuhan Liu
Understanding mixed-state
quantum phases is a central challenge in the era of
quantum simulation, where many existing studies focus on renormalization fixed points. In this work, we move beyond the renormalization fixed-point paradigm by constructing a
quantum phase transition connecting two distinct one-dimensional fixed points, both exhibiting finite conditional mutual information and one of which is intrinsically nontrivial. We analytically establish phase equivalence within each of the two phases by explicitly constructing low-depth, quasi-local channel
circuits that connect states within each phase. Crucially, our approach leverages the parent Lindbladian construction to generate the desired channel
circuits. We further demonstrate that this framework generalizes naturally to a broad class of intrinsically nontrivial mixed-state
quantum phases. Our method establishes a framework for rigorously analyzing phase equivalence of intrinsically non-trivial mixed states beyond the renormalization fixed points.
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Quantum Physics
Nonstabilizerness Mpemba Effects
Zhenyu Xiao, Hao-Kai Zhang and Shuo Liu
Quantum state preparation can be strikingly counterintuitive: the fastest route to a target state need not start from the apparently closest initial condition. We uncover such a
quantum Mpemba effect in the dynamical generation of
quantum magic (nonstabilizerness), quantified by the stabilizer Rényi
entropy, in $\mathrm{U(1)}$-symmetric random
circuits initialized from tilted product states. States with lower initial magic can generate magic faster than states with higher initial magic. The acceleration is not determined solely by the conserved-charge distribution. Two initial-state families with identical initial magic and identical charge distribution exhibit qualitatively different magic-growth dynamics, depending also on the spatial structure of the initial state within each charge sector. Analogous magic Mpemba effects in $\mathrm{SU(2)}$-symmetric
circuits and under nonintegrable
Hamiltonian dynamics further show that the phenomenon is tied neither to Abelian
symmetry nor to random-
circuit dynamics, establishing
quantum magic as a distinct arena for Mpemba physics.
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Quantum Physics
Quantum criticality beyond thermodynamic stability
Mariam Ughrelidze, Vincent P. Flynn, Emilio Cobanera et al. (+1 more)
For a many-body system in
equilibrium, described by a thermodynamically stable
Hamiltonian,
quantum criticality is associated with structural changes of the many-body ground state. However, there exist physically relevant models, notably, certain quadratic bosonic
Hamiltonians (QBHs), which fail to have a ground state. QBHs can be dynamically stable or unstable. We show the notion of criticality is meaningful for the entire class of QBHs that are dynamically stable or at the boundary of instability, regardless of thermodynamic stability, and that the key state for such QBHs is a naturally and unambiguously defined quasiparticle
vacuum (QPV). This state is Gaussian, and coincides with the ground state if the QBH is thermodynamically stable. We identify a relevant spectral gap, the Krein gap, associated to the minimal spectral separation between creation and annihilation operators, and show that the QPV is unique when the Krein gap is positive. We prove that, for dynamically stable QBHs with finite-range couplings, correlations are exponentially bounded unless the Krein gap closes, which is associated with one of two spectral degeneracies: an exceptional point or a Krein collision. Consequently, long-range QPV correlations can ensue. Thus, the Krein gap takes the role of the spectral gap for dynamically stable QBHs, and the boundary of dynamical stability and criticality (associated to exceptional points) or multicriticality (associated to Krein collisions) are the same. We also find that bosonic critical behavior beyond thermodynamic stability is witnessed by the scaling of the
entanglement entropy and other indicators of
equilibrium criticality from information geometry. Our framework opens the door to investigating all dynamically stable QBHs through the lens of critical phenomena, including thermodynamically unstable ones from photonics, cavity-QED, and magnonics.
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All authors Mariam Ughrelidze, Vincent P. Flynn, Emilio Cobanera, Lorenza Viola
Quantum Physics
Topological subsystem bivariate bicycle codes with four-qubit check operators
Zijian Liang and Yu-An Chen
High-rate bivariate bicycle (BB) codes are promising low-overhead
quantum memories, but their stabilizer checks typically have weight $6$ or higher, making syndrome extraction challenging. We introduce subsystem bivariate bicycle (SBB) codes, a translation-invariant CSS subsystem construction that realizes BB-code logical structure using local weight-$4$ gauge measurements. Their stabilizer syndromes are inferred by multiplying the corresponding gauge outcomes. We further show that nonlocal stabilizers in translation-invariant CSS subsystem codes can be detected using a determinantal-ideal criterion based on the gauge-operator commutation
matrix. When this criterion excludes nonlocal stabilizers, a finite-depth Clifford
circuit decouples gauge
qubits and identifies the protected subsystem with a corresponding BB stabilizer code. An SBB code is topological, meaning that it has no nontrivial local logical operators, if and only if the corresponding BB code is topological. A finite search yields low-overhead examples including $[[27,6,3]]$, $[[75,10,5]]$, and $[[108,12,6]]$; the latter encodes six times more logical
qubits than a subsystem surface code at the same block length and distance. These results show how gauge degrees of freedom can make high-rate BB logical structure compatible with local weight-$4$ syndrome extraction.
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Quantum Physics
Tunneling from an oscillating initial state in quantum mechanics
Oliver Janssen, Matthew Kleban and Cameron Norton
We study the decay of general initial states out of a metastable potential well in
quantum mechanics. We provide a closed-form expression for the probability current that tunnels through the barrier in terms of the resonant states into which the initial state can be decomposed. All ingredients in the equation are computed analytically to first subleading order in the semiclassical limit. Specializing to a coherently-oscillating initial state, we derive an approximation to the time-dependent decay rate and demonstrate its accuracy by comparing it to a numerical solution of the Schrödinger equation.
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Quantum Physics
Multiparameter function estimation for general Hamiltonians
Erfan Abbasgholinejad, Sean R. Muleady, Jacob Bringewatt et al. (+2 more)
Estimation of physical parameters encoded in a
Hamiltonian is a central task in
quantum sensing and learning. While the ultimate precision limit for estimating a single parameter coupled to a single generator is well established, the corresponding bound for estimating a function of multiple parameters-each coupled to distinct and possibly non-commuting generators-remains unknown in general. Here, we derive the ultimate
quantum limit and present an estimation protocol for any function of parameters in a general
Hamiltonian that attains this bound. We show that, although the task is fundamentally a multiparameter problem, our tight bound reduces to an optimized single-parameter
quantum Cramér-Rao bound, even for arbitrary generator sets. Our result unifies and extends previous works, providing a general framework for optimal function estimation in
quantum systems.
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All authors Erfan Abbasgholinejad, Sean R. Muleady, Jacob Bringewatt, Lorcan O. Conlon, Alexey V. Gorshkov
Quantum Physics
Emergent Quantum Dynamics as a Bayesian Inference Problem: A Critical Analysis
Thales B. S. F. Rodrigues, Lucas L. Brugger, Vinicius G. Valle et al. (+2 more)
Coarse-grained descriptions can be used to account for physical processes in which information is lost or not entirely accessible. In this paper, we start by proposing a connection between effective, coarse-grained descriptions of
quantum dynamics and the
quantum conditional states formalism. In doing so, we address necessary and sufficient conditions for the existence of emergent dynamics from a subjective
Bayesian point of view. Although our solution is (quasi-)optimal, the dynamics it determines are shown to be analytically limited – it solves the problem in a state-by-state case. Due to this limitation, we then implement semidefinite programming techniques to investigate the existence of effective dynamics in four paradigmatic scenarios. The existence of such an effective dynamics motivates the introduction of a new robustness measure that quantifies how much noise can be added to a microscopic dynamics without compromising its compatibility with a given coarse-grained description. Finally, we also show how one can analytically determine a valid emergent description in several examples.
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All authors Thales B. S. F. Rodrigues, Lucas L. Brugger, Vinicius G. Valle, Bruno F. Rizzuti, Cristhiano Duarte
Quantum Physics
FTPrimitiveBench: A Benchmark Suite For Logical Computation Under Hardware-Motivated and Biased Noise Models
Shuwen Kan, Adrian Harkness, Zefan Du et al. (+5 more)
Fault-tolerant
quantum computing requires understanding how error-correcting codes perform on diverse physical hardware. This is typically assessed via noisy stabilizer simulation of logical
circuits at HPC scale, combined with a noise model that yields a logical error rate for the relevant code distances and depths. The uniform depolarizing model is the standard baseline, but its homogeneous assumptions fail to capture the heterogeneity, asymmetries, and correlations of real devices, where Pauli, measurement, and spatio-temporal errors are not weakly coupled. Yet these same structured features create opportunities for joint code-hardware co-design, motivating noise models that more faithfully reflect target hardware while remaining tractable to simulate. We introduce FTPrimitiveBench, a systematic benchmarking approach for studying how logical primitives interact with hardware-motivated noise. It supports both custom specifications and representative structured noise families: Pauli bias, measurement bias, and spatial or spatio-temporal non-uniformity – together with generators for core surface-code Clifford primitives: logical
memory,
lattice surgery, transversal logical Hadamard, and the logical phase gate via
lattice surgery. We find that structured noise affects these primitives in qualitatively distinct ways, with outcomes shaped by the interplay between noise model, primitive, and decoder choice. These results extend
memory benchmarks to active logical computation, where the interaction between noise structure and primitive implementation matters. By standardizing the link between noise-model specification and primitive construction, FTPrimitiveBench enables reproducible comparative studies of QEC protocols and decoders, supporting hardware-aware co-design of fault-tolerant architectures. Code: https://github.com/ShuwenKan/FTPrimitiveBench.
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All authors Shuwen Kan, Adrian Harkness, Zefan Du, Rod Rofougaran, Sean Garner, Chenxu Liu, Ying Mao, Samuel Stein
Quantum Physics
Sequential vs. Synchronized Entanglement Swapping: QKD Secret-Key Rate under Optimized Link-Layer Control
Priyam Srivastava, Akshat R. Sabavat, Siddharth Jain et al. (+6 more)
Quantum repeater chains build long-distance
entanglement for
quantum key distribution (QKD) by creating pairs on short links and joining them through
entanglement swapping. Because stored pairs lose fidelity while they wait, swap timing can affect the secret-key rate. We study how network-layer swap scheduling affects six-state QKD on a simulated four-hop chain under an idealized Werner-state noise model. We compare a sequential (swap-and-wait) discipline, which builds and stores partial chains, with a synchronized (wait-and-swap) discipline, which waits until all links are ready before swapping. Both use the same reinforcement-learned link controllers, isolating the effect of the swapping discipline. When stored pairs cannot retain sufficient fidelity for one network time step, our sequential implementation produces no key, while synchronized retains a small key rate from simultaneous link deliveries. Once sequential becomes viable, it outperforms synchronized by up to 19.1% in the homogeneous chains studied, with the advantage decreasing as
memory lifetime increases. We predict the synchronized short-
memory pair-delivery rate from link-delivery statistics and derive a closed-form condition for when sequential can preserve progress across time steps. We also examine chains with one slower link, where partial chains can wait and decay at the bottleneck. In this setting, synchronized slightly outperforms sequential at longer
memory lifetimes. Moving the slow link has little effect on key rate away from the transition region but strongly affects sequential link-buffer occupancy. Choosing a swapping discipline therefore depends not only on whether
memories can preserve partial progress, but also on how the link configuration determines where that progress must wait and decay.
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All authors Priyam Srivastava, Akshat R. Sabavat, Siddharth Jain, Alan Scheller-Wolf, Sridhar Tayur, David Tipper, Prashant Krishnamurthy, Amy Babay, Kaushik P. Seshadreesan
Quantum Physics
Ergotropy Protection via Cavity Detuning in Collective Open Quantum Batteries
Tariq Zeyad Jawad
This study investigates the performance and ergotropy protection of open collective
quantum batteries subject to superradiant decay. By employing a passive spectral detuning strategy within an intermediate cavity, an optimal detuning value ($\Delta^*$) is analytically derived and numerically verified to spectrally isolate the system and protect
quantum coherence, achieving up to 1088% ergotropy improvement for single
qubits and superextensive collective advantage for $N \ge 3$. Our analysis resolves a "non-Markovian paradox," revealing that maximizing ergotropy does not strictly require non-Markovian
memory; rather, suppressing environmental
memory via detuning optimally preserves coherence, which serves as the fundamental resource. Survival maps across different environments demonstrate that thermal noise dissipates coherence more severely than telegraph noise. Finally, we establish that collective amplification of the effective coupling ($g_{\rm eff} = g\sqrt{N})$ inevitably drives large
qubit arrays into the ultra-strong coupling regime, providing a quantitative ceiling $N_{\rm max}$ on the validity of the Tavis-Cummings description and the current ergotropy protection protocol.
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Quantum Physics
Entanglement transitions in translation-invariant tensor networks
Yi-Cheng Wang, Samuel J. Garratt and Ehud Altman
We study the complexity of approximately contracting translation-invariant tensor networks. The computational cost of row-by-row tensor network contraction, which defines a discrete time
evolution governed by a fixed transfer
matrix, is associated with the
entanglement of the state of a row. By analyzing a family of tensor networks whose transfer
matrices interpolate between chaotic Floquet and strongly non-
unitary limits, we uncover a transition between volume- and area-law
entanglement in states evolved under the transfer
matrix. We show that deep in the volume-law phase the spectrum of the transfer
matrix in the complex plane consists of a dense ring with a sharp outer edge, reminiscent of behavior identified for non-
unitary random
matrices. At late times an evolving row state therefore has significant contributions from many eigenvectors with nearly degenerate
eigenvalue magnitudes. In the area-law phase, there is instead a distinct leading
eigenvalue. Our results establish connections between contraction complexity, spectral properties of the transfer
matrix, and purification under non-
unitary dynamics.
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Quantum Physics
Fast, accurate, high-resolution simulation of large-scale Fermi-Hubbard models on a digital quantum processor
Gavin S. Hartnett, Khadijeh Sona Najafi, Aleksei Khindanov et al. (+5 more)
We report experimental digital
quantum simulation of the one-dimensional
Fermi-Hubbard model on a superconducting
quantum processor at a scale beyond the reach of exact statevector simulation and challenging for state-of-the-art tensor-network methods. We encode this problem using up to 120
qubits through an efficient mapping that reduces
circuit complexity, and we improve accuracy through error suppression to simulate dynamical
evolution using up to 90 Trotter steps. From a vacancy defect introduced in the middle of an $L=31$-site (62-
qubit) Néel initial state, we directly observe
spin-charge separation to $t=9$ in natural units and quantitatively extract velocities $v_c$, $v_s$, which match classical simulations across model parameters, and analytics in appropriate regimes. We then extend experiments to $L=60$
fermions (120
qubits) and
evolution times to $t=6$ using 30 Trotter steps.
Quantum-processor outputs agree quantitatively with approximate classical simulations performed using a time-dependent variational principle (TDVP) solver and we observe that increasing the TDVP bond dimension through $\chi = 4096$ expands the range of
evolution times within which agreement has RMSE $\sim 1\%$ before the approaches diverge. Owing to the large scale of the simulation and the use of overhead-free error-suppression techniques, for simulated
evolution times at the limit of
quantum/classical agreement ($t\gtrsim 5$ in natural hopping units), the wall-clock runtime of the
quantum processor is nearly three orders of magnitude faster than TDVP simulations with $\chi = 4096$ and implemented using the ITensor and TeNPy packages. These results establish contemporary digital
quantum processors as a versatile, quantitatively accurate, and competitive platform for the study of fermionic many-body dynamics in regimes where leading classical methods can become prohibitively expensive.
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All authors Gavin S. Hartnett, Khadijeh Sona Najafi, Aleksei Khindanov, Haoran Liao, Michael Schutzman, Michael R. Hush, Michael J. Biercuk, Yuval Baum
Quantum Physics
Quantum work beyond classical (commuting) limits
Sumit Rout, Aravinth Balaji Ravichandran, Paweł Horodecki et al. (+1 more)
Free energy fixes the maximum work of a thermodynamic process once the state and
Hamiltonian are specified. A work-extraction task asks a different question: how much average work can a single device realize across several preparations and
Hamiltonian settings? A classical work device is one whose
Hamiltonian settings are mutually commuting. We place every branch at its best free-energy-limited work envelope and derive the corresponding classical limit on the task average. For pure preparations, the source is specified only by pairwise maximal-energy constraints: for each pair, the intrinsic maximal average energy under one common normalized
Hamiltonian is bounded as part of the task data, while the work device is otherwise microscopically unrestricted. The
benchmark is optimized over arbitrary-dimensional classical implementations. Incompatible
Hamiltonian settings exceed this limit, even though every branch remains bounded by its own free-energy maximum. The advantage therefore does not arise in any single process, but in the average work of the task: incompatible
Hamiltonians realize a value that no classical work device can attain.
Hamiltonian incompatibility is thus a thermodynamic resource for work extraction.
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All authors Sumit Rout, Aravinth Balaji Ravichandran, Paweł Horodecki, Anubhav Chaturvedi
Quantum Physics · Cross-listed from hep-th
Entangling gates for the SU(N) anyons
Sergey Mironov and Andrey Morozov
The model of a topological
quantum computer is a promising one due to its natural resistance to noise and other errors. Operations in such a computer are implemented by braiding the trajectories of anyons. While it is easy to understand how to build one-
qubit operations, two-
qubit operations are more difficult. In arXiv:2412.20931 we suggested an approach to build such operations for a topological
quantum computer based on SU(2) Chern-Simons theory with arbitrary level using cabling of
knots. In this paper we discuss how this approach should be generalized to the SU(N) case, what the differences are, and which new problems arise.
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Quantum Physics
Nonlinear Compton scattering in a frequency-modulated field
Antonino Di Piazza and Kenan Qu
When an electron is accelerated, it emits radiation. In the relativistic
quantum realm the elementary radiation process is the emission of a single
photon, a process known as nonlinear Compton scattering in the case of an electron moving in the presence of a strong electromagnetic wave. This process is typically described within the Furry picture, where the electromagnetic wave is described as a classical background field and the electron-positron field is quantized in the presence of that background field. Equivalently one can quantize the electron-positron field in the
vacuum but then the
photon emission process is described as a transition from an initial state to a final state both featuring, apart from the electron (the initial state) and the electron and the
photon (the final state), the same coherent state of
photons appropriately related to the electromagnetic wave. Here, we consider a more general situation where the initial and the final state feature the same squeezed coherent state. Then, we specialize to the case where the coherent state corresponds to a plane-wave field and the mostly populated modes of the coherent state are also squeezed. We show that, when
quantum fluctuations induced by the squeezing in the coherent field are negligible, a condition well satisfied at available squeezing levels, the squeezing effects effectively reduce to a frequency modulation of the plane-wave field corresponding to the coherent state. By means of numerical examples we show that at already available squeezing levels the emission spectrum of nonlinear Compton scattering and the total
photon yield can be altered significantly. Analytical explanations of the main numerical results are also provided.
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Quantum Physics
Quantum Dispersive Waves and Multimode Squeezing in Pure-Kerr Parametrically Driven Cavity Solitons
Rafael Romero Mendez, Sashank Kaushik Sridhar, Samyak Gothi et al. (+4 more)
Parametrically driven cavity
solitons (PDCS), unlike single-pumped cavity
solitons, are localized optical pulses arising from parametric processes. These cavity
solitons, recently discovered in pure-Kerr media, offer great promise for nonlinear dynamics studies and metrology. Here, we present the first multimode
quantum description of pure-Kerr PDCS. In the below threshold regime, we verify single- and two-mode squeezing, while above threshold we uncover novel "
quantum" dispersive waves - the
quantum analog of
soliton Cherenkov radiation. Besides revealing these unexplored
quantum properties, we show that PDCS generates up to 20 dB of squeezing, only limited by overcoupling and intrinsic losses for experimentally routine parameters. We therefore provide a pathway to observe strong multimode
quantum noise reduction in these systems.
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All authors Rafael Romero Mendez, Sashank Kaushik Sridhar, Samyak Gothi, Pradyoth Shandilya, Yichen Shen, Curtis Menyuk, Avik Dutt
Quantum Physics · Cross-listed from physics.chem-ph
Selecting optimal unrestricted Hartree-Fock trial wavefunctions for phaseless auxiliary-field quantum Monte Carlo: Accuracy and limitations in modeling three iron-sulfur clusters
Don Danilov, Brad Ganoe, Leon Otis et al. (+3 more)
Phaseless auxiliary-field
quantum Monte Carlo (ph-AFQMC) has emerged as a promising electronic structure method for correlated electronic systems. However, the quality of its predictions depends critically on the choice of trial wavefunction, and it is not obvious how to make an optimal choice especially for strongly correlated states of large systems. Mean-field wavefunctions are compelling trial wavefunction candidates as they map directly to chemical concepts and can be obtained with $O(N^4)$ cost. Yet in the strongly correlated regime one faces a
symmetry dilemma and the existence of multiple nearly-degenerate solutions. In this work we investigate active space models of [2Fe-2S]$^{2+}$, mixed-valent [4Fe-4S]$^{2+}$, and [4Fe-4S]$^{4+}$ and explore the sensitivity of ph-AFQMC to the choice of unrestricted Hartree-Fock trial wavefunction. We find that chemical properties and physical
symmetries, rather than the variational energy, ought to guide the choice of mean-field trial for ph-AFQMC (or reference state for coupled cluster models), and show that surprisingly accurate ground-state energies for these systems can be obtained. However, in all cases we find a rapidly vanishing overlap between the
stochastic wavefunction and the UHF trial, indicating that the trials are suboptimal importance functions. By analogy to a similar situation in the stretched helium dimer cation, we show how this sampling bias pushes ph-AFQMC towards artificially negative energies, which evidently can be compensated for by the phaseless bias in certain cases.
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All authors Don Danilov, Brad Ganoe, Leon Otis, Zhi Gong, Zixiang Lu, James Shee
Quantum Physics
Phase-Reference Control of Steady-State Entanglement in Open Quantum Systems
Areeda Ayoub, Alfonso Castillo-Gonzalez and Eric R Bittner
We show that steady-state
entanglement in open
quantum systems is controlled by the phase reference of a phase-sensitive reservoir. Using a covariance-
matrix approach for Gaussian-preserving dynamics, we demonstrate that purely local, phase-sensitive dissipation can generate
entanglement when combined with coherent coupling. The steady state exhibits a finite entangled region with an optimal squeezing strength that maximizes both the magnitude and thermal robustness of
entanglement. We find that coherent coupling does not enhance
entanglement monotonically, but instead regulates the conversion of local squeezing into nonlocal correlations. Importantly, the coupling dependence is controlled by the phase reference of the squeezed reservoir: phase-locked (rotating-frame) and laboratory-frame implementations yield qualitatively distinct steady states and
entanglement structure. These results establish phase-sensitive reservoir engineering as a controllable route to steady-state
entanglement in continuous-variable systems. Steady-state
entanglement in phase-sensitive open systems depends explicitly on the reservoir phase reference and is not invariant under changes of that reference.}
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Quantum Physics
Quantum metrology of mixed states via purification
Sisi Zhou
Multiparameter
quantum metrology is ubiquitous in physics, yet it remains fundamentally challenging. As reflected by the uncertainty principle, distinct unknown parameters generally cannot be estimated simultaneously at their individual ultimate precision limits. The
quantum Cramér–Rao bound (QCRB) and the Holevo Cramér–Rao bound (HCRB) represent the ultimate precision bounds in multiparameter
quantum metrology. Here we introduce new formulations of these bounds based on
quantum state purification. We show that, for any mixed state, their values are directly related to those of its purification, provided that nuisance parameters are introduced on the environmental system. Leveraging this connection, we develop a remarkably simple method for asymptotically attaining either the HCRB or twice the QCRB for arbitrary mixed states, using random purification channels and individual measurements.
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Quantum Physics
Regev's reduction as a candidate quantum algorithm for the discrete logarithm problem in finite abelian groups
M. Isabel Franco Garrido and André Chailloux
In this article, we investigate whether combining Regev's reduction with the Cheng-Wan reduction can yield an alternative
quantum algorithm for the discrete logarithm problem (DLOG) in finite abelian groups. Cheng and Wan reduce DLOG over finite fields to bounded-distance decoding of low-rate Reed-Solomon codes at a large decoding radius. Regev's reduction, in turn, exploits a decoder for the high-rate dual code with a lower noise level to obtain a
quantum algorithm for decoding the original low-rate code at a larger radius. The Cheng-Wan instances therefore provide a natural setting in which to explore the power of Regev's framework. We first extend the Cheng-Wan reduction to the discrete logarithm problem in finite abelian groups, assuming access to
embeddings of their cyclic factors. We also establish NP-hardness of Reed-Solomon bounded-distance decoding at asymptotically vanishing rate, reaching the rate regime relevant to Cheng-Wan, while the radius at which we establish hardness remains beyond the Cheng-Wan radius. We then instantiate Regev's reduction at the Cheng-Wan parameters and, under a
conjecture on the performance of the induced BDD
algorithm on the structured Cheng-Wan instances, characterize the noise threshold for the
quantum decoding problem(QDP) sufficient to solve DLOG. We evaluate known decoders in this regime and show that a
quantum decoder based on unambiguous state discrimination(USD) enables Regev's reduction to reach a strictly stronger decoding regime than the standard classical Reed-Solomon decoders we consider. Nevertheless, the QDP noise threshold required by the Cheng-Wan reduction is asymptotically 4/3 times that achieved by USD. Going beyond the USD framework, we show that every Cheng-Wan instance can be solved through Regev's reduction using a suitable Pretty Good Measurement and its inverse, although no efficient implementation of it is known.
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Quantum Physics
Fisher-Informational Time: A Causal-Geometric Framework for Emergent Clock Time Physical Distinguishability
J. Sumaya-Martinez
We develop a Fisher-informational reformulation of physical time in which clock time is not regarded as a fundamental ontological substance, but as an emergent calibration of causally ordered distinguishability among physical states. The operational starting point is that clocks do not measure time itself; rather, they instantiate reproducible physical processes whose distinguishable states are correlated with other events. We introduce a causal-informational parameter, denoted by Lambda_F, defined as an accumulated Fisher-
geometric distance along a causally admissible trajectory in state space. In classical statistical systems, this parameter is generated by the Fisher information metric; in
quantum systems, the corresponding construction is associated with
quantum Fisher information, the Bures metric, and the Fubini-Study geometry of projective
Hilbert space. The manuscript distinguishes model-dependent Fisher information from
quantum Fisher information, clarifies the reparameterization of Schrodinger dynamics, and gives explicit examples involving a
qubit clock, an exponential decay process, and a Fisher characterization of clock quality. The proposal is positioned relative to relational time, the Page-Wootters mechanism, thermal time,
quantum speed-limit relations, information geometry, and the problem of time in
quantum gravity. We do not claim that relational or emergent time is new. The specific contribution is the use of Fisher distinguishability as an operational precursor from which ordinary clock time can be reconstructed. In this sense, the central statement of the paper is: time is not measured by clocks; clock time is reconstructed from the Fisher distinguishability accumulated along causally ordered physical changes.
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