Quantum Physics
Recursive QLSTM with Dynamic Variational Quantum Circuit Adaptation
Samuel Yen-Chi Chen, Yifeng Peng, Jiun-Cheng Jiang et al. (+8 more)
Recent advances in
quantum computing and machine learning have motivated the development of
quantum models for sequential data processing. In this paper, we propose a Recursive
Quantum Long Short-Term
Memory model, or Recursive QLSTM, which extends QLSTM through metacore-based recursive constructions. We numerically test the model under different input sequence lengths, metacore designs, and recursive rules, and identify the best-performing architecture among these variants. For this selected model, we further provide theoretical arguments explaining why its recursive structure improves temporal information propagation and enhances learning performance. Our results suggest that Recursive QLSTM offers a flexible and effective framework for
quantum recurrent learning over input time series of various lengths.
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All authors Samuel Yen-Chi Chen, Yifeng Peng, Jiun-Cheng Jiang, Chun-Hua Lin, Kuo-Chung Peng, Junghoon Justin Park, Huan-Hsin Tseng, Hsin-Yi Lin, Kuan-Cheng Chen, Chen-Yu Liu, Shinjae Yoo
Quantum Physics
Picosecond Schrödinger cat states for ultrafast optical quantum processing
Mamoru Endo, Kan Takase, Takefumi Nomura et al. (+12 more)
Non-Gaussian states are essential resources for universal, fault-tolerant optical
quantum computing, but their generation rate remains limited by low heralding probabilities and operation in nanosecond temporal modes. Here, we demonstrate multi-
photon generalized
photon subtraction in picosecond optical wave packets, establishing the state-generation capability required for high-rate operation by addressing the temporal-mode bottleneck that has constrained the achievable rate. Two interfering ultrashort squeezed
vacua are heralded by
photon-number-resolving detection with a high-speed transition-edge sensor and characterized by pulsed homodyne detection matched to 10-ps temporal modes at a 5-MHz pump repetition rate. We reconstruct Wigner functions without loss correction that exhibit up to four distinct negative regions for four-
photon heralding, together with an effective cat-state amplitude of $\alpha_{\mathrm{eff}} = 1.69$. This amplitude approaches the range of practical relevance for fault-tolerant cat-code architectures and for adaptive breeding toward logical-
qubit generation, while the picosecond temporal mode establishes a platform compatible with high-rate, scalable time-multiplexed photonic architectures.
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All authors Mamoru Endo, Kan Takase, Takefumi Nomura, Tatsuki Sonoyama, Kazuma Takahashi, Sachiko Takasu, Daiji Fukuda, Takahiro Kashiwazaki, Asuka Inoue, Takeshi Umeki, Peter van Loock, Petr Marek, Radim Filip, Warit Asavanant, Akira Furusawa
Quantum Physics
Suppressing Self-Discharging of Quantum Batteries by Cavity Interactions
Anass Jad, Abderrahim El Allati and Mohammad B. Arjmandi
We analyse a two-cavity architecture, in which a lossy cavity hosting $N$
qubits is coherently coupled to an auxiliary cavity, as a resource for the storage phase of an open
quantum battery at non-zero temperature. Within a local Lindblad treatment in the resonant configuration, we find that the inter-cavity coupling enhances the suppression of self-discharging across every initial preparation, battery size, and temperature we examine, with the protection degrading smoothly as the mean thermal occupation increases. For a single
qubit, the energy-basis coherence of a pure superposition leads to better long-time retention than fully excited state, highlighting the beneficial role of
quantum coherence in protecting stored energy against thermal degradation. For two-
qubit batteries, Bell-state preparations exhibit enhanced long-time ergotropy retention compared with the fully excited state, while the inclusion of
qubit-
qubit interactions produces only a weak dependence on the interaction type and strength within the parameter regime considered. Extending the analysis to multi-
qubit GHZ-charged batteries with all-to-all Heisenberg interactions, we find that the normalized retained ergotropy increases monotonically with the number of
qubits. This behavior is consistent with the collective enhancement of the
qubit-cavity coupling in the symmetric Dicke
manifold, indicating that larger
quantum batteries can benefit from improved protection against self-discharge. These findings establish cavity-assisted protection as a promising strategy for mitigating self-discharging and realizing of long-lived
quantum batteries in experimentally accessible platforms.
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Quantum Physics
Polynomial-time exact diagonalization via sparse guided eigenwalks
Zachary E. Chin, Mario Motta, Javier Robledo Moreno et al. (+3 more)
Computing
quantum ground states is generically difficult, but additional structure can sometimes allow diagonalization to be recast as a more feasible problem. For example, when the desired ground state is sparse in a given basis, diagonalization can be facilitated via graph search. We make this reformulation precise by introducing the eigenwalk problem, which seeks the support of a sparse eigenvector of a Hermitian
matrix by exploring the graph induced by its nonzero entries. However, it is not obvious whether the relevant support vertices must always be efficiently reachable by a search on the graph. To resolve this question, we prove that for every sparse eigenvector, there exists a (possibly different) sparse eigenvector with the same
eigenvalue whose support is tightly localized in the graph, with diameter scaling only linearly in the sparsity and independently of the total number of vertices. As a consequence, if a $2^n$-dimensional, ${\rm poly}(n)$-sparse
Hamiltonian has an $\mathcal{O}(1)$-sparse extremal eigenvector and one support element is known, then an exact eigenvector with the same
eigenvalue can be computed classically in ${\rm poly}(n)$ time. The same conclusion follows when the $\mathcal{O}(1)$-sparse eigenvector is non-extremal, provided that it is sparser than every eigenvector with a different
eigenvalue. These results hold with no assumptions on the degeneracy, locality, spectral width, or spectral gap of the
Hamiltonian, and the underlying support-localization principle also extends to problems beyond exact diagonalization, such as sparse principal component analysis.
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All authors Zachary E. Chin, Mario Motta, Javier Robledo Moreno, Antonio Mezzacapo, Isaac L. Chuang, William Kirby
Quantum Physics
Biophysical EPR Using Superconducting Resonators
Austin R. Gamble Jarvi, Hamid R. Mohebbi, Ishit Raval et al. (+7 more)
We present innovations that enable the use of superconducting resonators for high sensitivity, high bandwidth pulsed electron paramagnetic resonance (EPR) measurements on biologically relevant samples with enhanced stability and throughput. A custom-built X-band pulsed EPR spectrometer with AWG and digital IF capability generated by an FPGA was used to control a novel patterned thin film planar superconducting microstrip resonator capable of generating Rabi fields sufficient to achieve 6 ns pi/2 Gaussian pulses using a 100 W solid-state HPA. The system allows automated sequential calibration, measurement, and analysis of five 3.5 uL samples contained in a sample cartridge. Performance was validated through measurements of double electron-electron resonance (DEER) distances in a variety of
spin-labeled
protein samples with biologically relevant concentrations, including measurements below 10 uM. The results enable broadening the scope of applications for both superconducting resonators and the use of EPR in biotechnology.
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All authors Austin R. Gamble Jarvi, Hamid R. Mohebbi, Ishit Raval, Omari Culzac, Jack Erickson, Sameh Hegazy, Don Carkner, Andrew Wiles, David G. Cory, Troy W. Borneman
Quantum Physics
Mølmer-Sørensen gates in trapped-ions chains in the presence of correlated noise
D. V. Donchenko, E. A. Anikin, O. Lakhmanskaya et al. (+1 more)
We analyze the impact of correlated
laser frequency noise on Mølmer-Sørensen gates in
qubit registers based on trapped-ion chains. Using perturbation theory, we calculate gate fidelities in the presence of noise with arbitrary power spectral density for different chain lengths and ion positions in the chain. With our approach, we account for simultaneous excitation of multiple
phonon modes during gate operation. We find out that the impact of medium-frequency
laser noise depends considerably on the positions of the ions in the chain. In contrast, low-frequency noise has similar effect for different chain lengths and ion positions.
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All authors D. V. Donchenko, E. A. Anikin, O. Lakhmanskaya, K. Lakhmanskiy
Quantum Physics
Augmenting Imaginary-Time Evolution with Local Geometric Information
Carlos L. Benavides-Riveros, Prachi Sharma, Pietropaolo Frisoni et al. (+1 more)
Imaginary-time
evolution (ITE) underpins a broad family of
algorithms for ground-state preparation in
quantum simulation and
quantum many-body physics. In these methods, convergence is governed by the energy variance of the instantaneous state, causing the flow to approach the ground state only asymptotically. We introduce an augmented imaginary-time
evolution (AITE) framework that replaces the standard gradient flow on the energy landscape with a geometrically informed descent along locally optimal directions, which are identified by exploiting the higher-order statistical structure of the instantaneous energy distribution. The resulting flow strictly outperforms standard ITE throughout the entire
evolution and exhibits two qualitatively distinct regimes: a superlinear convergence regime, followed by an extinction regime in which the energy error vanishes exactly at a finite value of the imaginary-time flow parameter, in contrast to the asymptotic exponential decay of ITE. Standard ITE is recovered in the zero-skewness limit of AITE, implying that the acceleration extends naturally across the broader ITE algorithmic family.
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All authors Carlos L. Benavides-Riveros, Prachi Sharma, Pietropaolo Frisoni, Fedor Šimkovic IV
Quantum Physics · Cross-listed from physics.atom-ph
Analysis of the frequency shift in coherent population trapping resonance's dynamic continuous-wave spectroscopy at the phase-jump modulation and its comparison with the conventional approach
E. D. Chivilis and E. A. Tsygankov
We present the research of dynamic continuous-wave spectroscopy of the coherent population trapping resonance at the phase-jump modulation. {\Lambda} system of levels supplemented by a nonabsorbing state and bichromatic optical field, whose spectral components have different intensities, are considered. We demonstrate that the asymmetry leads to an additional nonlinear shift of the error-signal frequency under unisotropic relaxation of the ground-state density-
matrix elements. We also investigate the conventional approach where the frequency difference of the optical field components is harmonically modulated to obtain the error signal. Comparison demonstrates that in the high-frequency modulation regime the corresponding frequency shift is more linear than at the phase-jump modulation for nonshort integration times.
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Quantum Physics
When does dissipation help neural surrogates learn open quantum dynamics?
Alauddin Ahmed
Dissipation is usually viewed as an obstacle to predicting
quantum dynamics, yet it can also contract trajectories toward steady states and thereby suppress accumulated prediction errors, leaving it unclear whether dissipation ultimately helps or hinders the learnability of open
quantum dynamics. We investigate this question using Neural Ordinary Differential Equation (NODE) surrogates for open Heisenberg XYZ
spin chains. Closed-system learnability deteriorates rapidly with system size, culminating in a static-prediction collapse at four
qubits; dissipation reverses this trend, creating a broad high-fidelity regime at intermediate system sizes, while at four
qubits a fidelity-aware objective recovers learnable rollout structure that is absent under closed-system training. Comparison against static and steady-state baselines reveals that dissipation improves performance through two fundamentally different mechanisms: at weak-to-moderate dissipation the surrogate captures nontrivial transient dynamics and substantially outperforms trivial predictors, whereas at stronger damping high fidelity increasingly reflects trajectory simplification toward the steady state rather than improved learned dynamics. These results show that dissipation can enhance the learnability of open
quantum dynamics, but that fidelity alone is insufficient to distinguish genuine dynamical learning from steady-state trivialization: dissipative contraction and trajectory simplification are distinct effects that peak in different regimes and should be disentangled when evaluating learned
quantum-dynamical surrogates.
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Quantum Physics
Mode-selective nonlinear interference for high-brightness and high-purity fiber-coupled SPDC sources
Carlos Sevilla-Gutiérrez, Purujit Singh Chauhan, Varun Raj Kaipalath et al. (+1 more)
Single-mode-fiber-coupled spontaneous parametric down-conversion (SPDC) sources are a key resource for photonic
quantum technologies, but in single-
crystal geometries brightness, heralding efficiency, and spectral purity remain constrained by intrinsic trade-offs. Here, we show how nonlinear interference in a cascaded two-
crystal type-II SPDC source can be used to engineer the modal structure of SPDC emission, improving the brightness–heralding-efficiency trade-off by more than one order of magnitude beyond the single-
crystal limit. We further demonstrate two routes to near-unity spectral purity while retaining high brightness and/or heralding efficiency, even with standard periodically poled
crystals, and study the additional advantages of aperiodic poling with Gaussian phase matching. Using a spectrally resolved Laguerre–Gauss modal decomposition, we show that these improvements arise from mode-selective interference of spatial-spectral SPDC modes within the nonlinear interferometer. We experimentally validate the model through sum-frequency-generation measurements of the spatial-spectral state.
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All authors Carlos Sevilla-Gutiérrez, Purujit Singh Chauhan, Varun Raj Kaipalath, Fabian Steinlechner
Quantum Physics
Wavelet Matrix Product States for Quantum Fields
Molly Kaplan and Antoine Tilloy
We introduce a variational method to solve continuum
quantum models with discrete tensor network techniques. The method leverages wavelet
matrix product states (wMPS):
matrix product states built on top of sufficiently regular ($N\geq 6$) Daubechies scaling functions. These states live in the continuum field theory Fock space, have finite energy density, and can be optimized with standard
algorithms, without restriction to free theories. Further, exploiting the multi-resolution analysis built into wavelets, and its
quantum circuit description, we can iteratively refine wMPS to obtain accurate approximations at arbitrarily fine length-scales. We showcase the efficiency of the method on the Lieb-Liniger model, computing energy density and correlation functions.
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Quantum Physics · Cross-listed from cond-mat.quant-gas
Sayak Bhattacharjee, Mahendra K. Verma, Alexander V. Balatsky et al. (+1 more)
We discuss phenomenology associated with turbulent hydrodynamics in
quantum fluids from a condensed-matter perspective. We begin with weakly-interacting superfluids, often modeled by a mean-field theory governed by the Gross-Pitaevskii equation. Considering the effect of
quantum fluctuations beyond the mean-field approximation, we propose a study of many-body
quantum effects in turbulent hydrodynamics, especially near zero temperature. We motivate examples of
quantum many-body systems where such effects may be uncovered. These include
bosons confined in a periodic potential in low spatial dimensions (one and two), and the associated
quantum critical point of the superfluid-
insulator transition, realized in present-day ultracold-atom and
quantum computing platforms. We conclude by listing a set of (open) questions that may be answered using modern
quantum many-body techniques. This article is part of the theme issue 'Frontiers of
turbulence and statistical physics'.
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All authors Sayak Bhattacharjee, Mahendra K. Verma, Alexander V. Balatsky, Srinivas Raghu
Quantum Physics
Jayden Webster, Florian Kanitschar, Emanuele Polino et al. (+5 more)
Photons encoded in discrete time bins can be routinely prepared in temporal superposition states, enabling high-dimensional
entanglement and enhanced
quantum communication rates. However, characterizing this high-dimensional
entanglement presents significant challenges, namely due to the involved measurement complexity or reliance on restrictive assumptions that compromise the generality of traditional approaches. Here, we develop and experimentally demonstrate a simple linear-optical scheme based on particle-exchange
symmetry that allows us to probe high-dimensional
entanglement in time-bin-encoded states. Combining Hong-Ou-Mandel interference with suitable transformations, our method not only certifies
entanglement but also lower-bounds its dimensionality using only two dichotomic
symmetry-based measurements. This bound is obtained through a new rigorous theoretical analysis and can be further improved by weak, physically motivated assumptions. The scheme remains effective at any timescale, even far below the temporal detector resolution used. Our work provides a powerful state-characterization tool and demonstrates that we can prove high-dimensional temporal
entanglement on timescales inaccessible to the setup.
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All authors Jayden Webster, Florian Kanitschar, Emanuele Polino, Simon J. U. White, Sven Rogge, Farzad Ghafari, Marcus Huber, Nora Tischler
Quantum Physics · Cross-listed from cond-mat.stat-mech
Universal Dynamical Response to Slow Driving in Chaotic Systems
Nachiket Karve, Nathan Rose, David Campbell et al. (+1 more)
We propose a unified perspective on classical and
quantum chaos based on the sensitivity of a system's stationary states to slow driving. We probe this sensitivity via the system's susceptibility to the average protocol speed, which we call the “speed-Fisher information," and relate it to irreversible
entropy production in the system. We show that chaotic dynamics manifests as a divergence of the speed-Fisher information with the protocol time, and that this response is controlled by the perturbation's low-frequency spectral weight. This approach to chaos applies to both classical and
quantum Hamiltonian systems, and naturally extends to non-
Hamiltonian classical flows. We illustrate this framework with simple classical and
quantum examples, along with a non-
Hamiltonian flow that qualitatively exhibits analogous low-frequency spectral behavior.
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All authors Nachiket Karve, Nathan Rose, David Campbell, Anatoli Polkovnikov
Quantum Physics
Efficient Graph State Purification with Factorized Graph-Preserving Operations across Local Clifford Orbits
Mingyuan Wang, Guus Avis, Kenneth Goodenough et al. (+1 more)
Graph states form a broad class of multipartite entangled states underlying measurement-based
quantum computation,
quantum networks, and stabilizer codes. However, systematic
entanglement distillation for arbitrary graph states remains challenging because the
circuit design space grows rapidly with the number of parties. We introduce a group of Clifford operations that we call "factorized graph-preserving". It enables us to efficiently enumerate and optimize graph-state purification
circuits at finite size for realistic noisy hardware. These operations map products of graph-basis states to products of graph-basis states, so their action can be represented as permutations of graph-basis labels. Moreover, this useful gate set admits a compact factorized description determined by simple graph-theoretic features. This structure also allows, after some initial cached precomputation, drastically lower computational complexity for simulating a gate. We further organize these operations over local-complementation (LC) orbits using minimum-edge representatives (MERs), which let us design purification
circuits that apply to all locally equivalent graph states (up to a basis change). Using this framework, we optimize noisy finite-size multipartite
distillation circuits for several graph-state families. Numerical results show that the resulting graph-preserving
circuits can outperform standard recurrence-based purification protocols under realistic gate and measurement noise. Our results establish LC-orbit structure and factorized graph-preserving operations as practical tools for scalable,
topology-aware and hardware-constrained graph-state
distillation protocol design. Our work can also be interpreted as a graph-based heuristic for finding transversal gates.
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All authors Mingyuan Wang, Guus Avis, Kenneth Goodenough, Stefan Krastanov
Quantum Physics
Infinite-Level Hierarchy of Solvable Quantum Circuits
Michael A. Rampp, Suhail A. Rather and Pieter W. Claeys
Dual-
unitary circuits have emerged as a paradigm of exactly solvable yet non-integrable
quantum dynamics. Recently, a generalization of dual unitarity attempting to extend the phenomenology of exactly solvable
circuits has been introduced through a hierarchy of conditions, with dual unitarity as the first level. However, beyond the second level the proposed generalized dual-
unitary hierarchy ceases to be solvable in the whole spacetime. We present an infinite hierarchy of solvability conditions remedying this problem. These new conditions can be combined with the generalized dual-
unitary hierarchy to obtain
circuits for which correlation functions and
entanglement dynamics can be analyzed exactly in the whole spacetime. We show that this novel hierarchy possesses non-trivial solutions at every level. Our results demonstrate that dual unitarity can be systematically extended while preserving solvability, opening up investigations of exactly solvable non-integrable systems with more general properties.
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Quantum Physics
Fabian Ballar Trigueros, Zheng-Hang Sun, Xhek Turkeshi et al. (+2 more)
Matchgate
circuits realize free-
fermion dynamics: they are efficiently classically simulable, yet cannot on their own generate the generic randomness required for universal computation or
unitary design formation. We study a controlled route beyond this integrable limit by doping matchgate
circuits with non-Gaussian gates-physically, the injection of fermionic interactions into an otherwise free system. Using the matchgate commutant framework, we obtain analytic control over
unitary $2$-design formation. For globally scrambled dynamics, the design problem maps exactly onto a classical birth-death Markov chain with an Ornstein-Uhlenbeck continuum limit, recasting the emergence of
quantum randomness in terms of spectral gaps and mixing times and yielding rigorous bounds on the number of non-Gaussian gates needed for approximate $2$-designs. These bounds hold for a broad class of parity-preserving non-Gaussian gates, independently of microscopic details, with numerics indicating that the same mechanism governs higher-order designs. Used as local building blocks in a glued-
circuit architecture, they yield approximate parity-preserving $2$-designs in polylogarithmic depth with a sparse non-Gaussian gate count, with implications for Page-like
entanglement growth and fermionic classical-shadow protocols. Finally, locality reshapes this picture: in local brickwork dynamics, design formation is
diffusion-limited and far slower. Our results establish doped matchgate
circuits as a controlled, analytically tractable route from free
fermions to interaction-generated
quantum designs.
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All authors Fabian Ballar Trigueros, Zheng-Hang Sun, Xhek Turkeshi, Piotr Sierant, Poetri Sonya Tarabunga
Quantum Physics · Cross-listed from cond-mat.str-el
Fermi surface change and $d$-wave superconductivity in the square lattice Kondo-Heisenberg model
Alexander Nikolaenko, Riccardo Rende, Luciano Loris Viteritti et al. (+2 more)
We study the two-dimensional Kondo-Heisenberg model on a square
lattice, with the conduction electrons away from half-filling, using neural network
quantum states. Mapping the ground-state phase diagram as a function of the Kondo and Heisenberg couplings, we identify (i) at weak Kondo coupling,
antiferromagnetic Néel order with a
Fermi surface whose enclosed area counts only the conduction electrons and is insensitive to the Néel order, and (ii) at strong coupling, a heavy
Fermi liquid with a
Fermi surface whose enclosed area counts both the conduction electrons and the
spins. In the crossover between these regimes, we find $d_{x^2-y^2}$ superconductivity, evidenced by off-diagonal long-range order in the pair-pair correlations and a pairing-amplitude dome that coexists with the underlying
magnetic phase. Our results establish
Fermi volume change and unconventional superconductivity as intrinsic features of the two-dimensional Kondo-Heisenberg model.
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All authors Alexander Nikolaenko, Riccardo Rende, Luciano Loris Viteritti, Subir Sachdev, Ya-Hui Zhang
Quantum Physics
A no-go theorem for privacy in distributed sensing using Gaussian states
Jason L. Pereira and Damian Markham
In the discrete variable setting, entangled resource states allow a set of parties to learn a global function of a set of spatially separated systems, whilst keeping the local parameters of those systems completely private. In the continuous variable setting, distributed sensing has been carried out using Gaussian resource states, but without the same guarantees about
privacy. Here, we show that perfect
privacy is impossible to achieve for any distributed sensing protocol that uses Gaussian states as a resource. We also introduce a measure of relative
privacy, bounding the degree to which any Gaussian distributed sensing protocol can keep local parameters hidden.
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Quantum Physics · Cross-listed from hep-th
Controlled Chaos in 4D SCFTs
Florent Baume, Atakan Çavuşoğlu, Vivek Chakrabhavi et al. (+1 more)
Chaotic dynamics play an important role in a number of physical systems. One of the qualitative hallmarks of this behavior is the appearance of a sufficiently "complex" spectrum of energy levels. This also makes it challenging to directly verify the onset of chaos in interacting
quantum field theories. We present a class of 4D superconformal field theories (SCFTs) given by orbifolds of 4D $\mathcal{N} = 4$ Super Yang–Mills theory in which operator mixing in a controlled subsector is described by an effective
spin chain in one spatial dimension with nearest neighbor interactions tuned by the marginal couplings of the SCFT. Tuning the marginal couplings results in a chaotic spectrum, while generically the
spin chain exhibits Anderson localization. We diagnose the onset of chaos by analyzing the statistical distribution of
eigenvalues of the dilatation operator, in particular properties such as
eigenvalue level repulsion, spectral
rigidity, and the spectral form factor. We also show that other diagnostics such as Krylov complexity sometimes do not faithfully capture this information. This structure defines a chaotic billiard in the target space of the stringy realization. We also comment on the large $N$
holographic dual description, where the controlled single
spin chain approximation must be supplemented by multi-trace dynamics, i.e., the splitting and joining of multiple
spin chains.
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All authors Florent Baume, Atakan Çavuşoğlu, Vivek Chakrabhavi, Jonathan J. Heckman
Quantum Physics
Sebastian Murk, Ian Tan, Fabian Müller et al. (+1 more)
Quantum tomography and
quantum retrodiction are traditionally viewed as separate inference tasks:
tomography reconstructs
quantum states from measurement data, whereas retrodiction infers past
quantum states from observed outcomes. We show that the two are manifestations of the same underlying principle. We prove that the Petz recovery map associated with a measurement channel is precisely the gradient update of the log-likelihood used in maximum-likelihood
tomography. Consequently, repeated applications of the Petz map monotonically increase the likelihood. Extending beyond measurement channels, we derive a noncommutative generalization of the Petz map from the gradient of a generalized likelihood for arbitrary
quantum channels. The resulting iterative procedure maximizes the likelihood and provides a general framework for
quantum tomography, establishing a direct bridge between retrodiction, recovery maps, and statistical inference.
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All authors Sebastian Murk, Ian Tan, Fabian Müller, Dominik Šafránek
Quantum Physics
Challenges in Barren Plateau Mitigation with Dynamic Parameterized Quantum Circuits
Sumeet Shirgure, Efekan Kökcü and Siyuan Niu
Variational
quantum algorithms (VQAs) are a promising paradigm for
quantum advantage, yet their trainability is severely hampered by barren plateaus (BPs). Several recent works have proposed dynamic parameterized
quantum circuits (DPQCs), which interleave
unitary layers with parameterized CPTP maps, such as engineered dissipation, feedforward gadgets, and periodic resets, as a possible strategy for mitigating BPs. We unify this class of
circuits into a formalization for DPQCs.We identify constraints on the nature and the structure of DPQCs if they are to prevent a significant number of parameters from becoming untrainable. Using purification and Pauli-path analysis, we further identify a mechanism by which the cost function can remain anti-concentrated even when many parameters remain untrainable. Our analysis reveals ways to design DPQCs that do not have an exponentially concentrated cost function, and our results suggest that BP mitigation via DPQCs is at least as hard as designing BP-free
unitaries.
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Quantum Physics · Cross-listed from cond-mat.str-el
Emergent Andreev Reflection from a Lattice Duality Defect
Atsushi Ueda, Tokiro Numasawa, Boris De Vos et al. (+1 more)
Andreev reflection converts an incoming
fermion into an outgoing hole and is usually tied to a superconducting interface. We show that an analogous charge-conjugating boundary condition emerges from a purely
lattice duality defect. Starting from a Majorana representation of the transverse-field Ising chain, we construct a folded
lattice model in which a boundary Majorana
impurity implements a one-site translation of a staggered Majorana chain. In the continuum, this translation becomes a
chiral fermion-parity defect: it flips the sign of the only left-moving Majorana mode while leaving the right-moving mode unchanged. When the two Majorana modes are recombined into a complex
fermion in the folded geometry, this sign flip becomes the Andreev-like boundary condition. Our
lattice formulation gives a microscopic interpretation of the Emery–Kivelson boundary of the two-channel Kondo problem and of Maldacena–Ludwig monopole scattering, while identifying the boundary as the interface between a Kitaev-chain SPT phase and a gapless chain. The same Majorana translation defect also provides a
lattice realization of an axial $U(1)_A$-symmetric charge-flip boundary.
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All authors Atsushi Ueda, Tokiro Numasawa, Boris De Vos, Masataka Watanabe
Quantum Physics
Robust Structure Learning of $k$-local Lindbladians
Tim Möbus, Thiago Bergamaschi, Daniel Stilck França et al. (+1 more)
We present an efficient protocol for learning an unknown $k$-local Lindblad generator on $n$
qubits using only product-state preparations, short-time
evolution, and single-
qubit Pauli measurements, without prior knowledge of the interaction structure. For fixed $k$ and a supplied weighted interaction-strength bound $\bar\alpha$, the protocol estimates all
Hamiltonian and dissipative Pauli–GKSL coefficients with total coefficient error at each site at most $\varepsilon$ with probability at least $1-\delta$ using $\widetilde{\mathcal O}_k(\bar\alpha^2n^{2k-2}\varepsilon^{-2}\log(n/\delta))$ samples and polylogarithmically many distinct
evolution times. This guarantee requires no degree, sparsity, or tail promise. A convex optimization converts these estimates into a valid $k$-local Lindblad generator with diamond-norm error at most $\varepsilon$ using $\widetilde{\mathcal O}_k(\bar\alpha^2n^{2k}\varepsilon^{-2}\log(n/\delta))$ samples and
polynomial-time classical postprocessing. With a supplied local effective-sparsity bound $r$ and a sufficiently small sitewise subthreshold tail $\mathcal O_k(\varepsilon)$, coefficient recovery requires only $\widetilde{\mathcal O}_k(\bar\alpha^2r^2\varepsilon^{-2}\log(n/\delta))$ samples, without a supplied threshold or coefficient locations and without a coefficient-gap assumption. In particular, exactly sparse bounded-degree models have logarithmic sample dependence on $n$. We also provide guard-band support recovery and complementary supplied-candidate guarantees, extend the guarantees to model misspecification, and prove complementary sample-complexity lower bounds.
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All authors Tim Möbus, Thiago Bergamaschi, Daniel Stilck França, Cambyse Rouzé
Quantum Physics
Genuine certification of incompatible quantum instruments through sequential communication tasks
Arindam Mitra, Satyaki Manna and Debashis Saha
Quantum instruments constitute the general description of
quantum dynamics, encompassing both
quantum measurements and
quantum channels as special cases. Consequently, the incompatibility of
quantum instruments represents a fundamental manifestation of nonclassicality in
quantum theory. Here, we establish the operational significance of this notion by demonstrating communication tasks with classical inputs and outputs that enable the semi-device-independent certification of incompatible
quantum instruments. We introduce a class of three-party communication tasks involving a sender, a relayer, and a receiver, and derive the tight upper bounds of the figure of merits of these tasks achievable by all compatible instruments implemented by the relayer. Furthermore, we show that these bounds coincide with the optimal performance attainable in a classical communication subject to the same dimensional constraints. Violation of this bound certifies the incompatibility of the pair of
quantum instruments implemented by the relayer. This identifies certification of incompatible instruments as a manifestation of
quantum advantage in communication. This certification protocol is genuine as it is able to certify the incompatibility of a pair of instruments where the set of induced measurements and the set of induced channels are compatible and, therefore it does not depend on the incompability of measurements and channels induced by the instruments. Finally, we identify the simplest instance of our communication scenario that enable the certification of incompatible
quantum instruments.
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Quantum Physics · Cross-listed from physics.ins-det
Multi-scale reconstruction of single-ion damage tracks in diamond via nitrogen-vacancy centers
Daniel G. Ang, Jiashen Tang, Maximilian Shen et al. (+9 more)
Understanding particle-induced damage tracks in solid-state materials underpins emerging applications in rare-event detection and
quantum defect engineering. Resolving these tracks requires multi-scale readout, from event localization at the millimeter scale to track-morphology reconstruction at the nanoscale. Nitrogen-vacancy (NV) centers in diamond provide such a platform, combining optical localization with
quantum sensing of track morphology. Here, we implant sub-MeV carbon ions into nitrogen-rich diamond and detect individual
recoil events via spatially localized NV formation. We develop a simulation framework that explains the observed NV yield and predicts that directional information is retained in the NV distribution after annealing. Machine learning further recovers much of the information lost to defect
diffusion and limited NV yield, improving head-tail classification to a level comparable to pre-annealed vacancy tracks. Measurements of NV
spin coherence indicate compatibility with nanoscale track reconstruction via NV strain mapping and
magnetic gradient-based techniques. These results identify promising pathways toward NV-diamond directional detectors for rare events, while the track-modeling framework has broader implications for paleodetection and
quantum material synthesis.
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All authors Daniel G. Ang, Jiashen Tang, Maximilian Shen, Michael Titze, Gavishta Liyanage, Jordan Chapman, Chinmay Bharathulwar, Andrew T. Gilpin, Tanguy Terlier, Edward S. Bielejec, Vsevolod Ivanov, Ronald L. Walsworth
Quantum Physics
Log-concavity and tunneling: adiabatic quantum optimization for convex functions (with a spike)
Arthur Braida, Elie Bermot and Simon Apers
Quantum tunneling is expected to provide a computational speedup in
quantum computing, a phenomenon that Adiabatic
Quantum Optimization (AQO) aims to leverage. While some academic proofs of concept have been studied, such as the "Hamming weight with a spike" (HWS) problem, the algorithmic gains of this effect remain underexplored. In this work we extend the analysis underlying HWS to more general potentials. In the first half of the work, we establish (discrete) log-concavity of the ground state as a key structural property in this context. We devise a framework for establishing log-concavity of the ground state for a large family of discrete, 1-dimensional Schrödinger operators. The family includes convex potentials, but also certain potentials with local minima. In the convex case, this provides a discrete version of a continuous result by Brascamp and Lieb ('76). We demonstrate the utility of our result by establishing new spectral gap bounds, going beyond related results by Jarret and Jordan ('14) for convex potentials. In the second half of the work, we use our results on log-concavity to extend the perturbative analysis of HWS by Reichardt ('04) to the larger family of potentials with log-concave ground state. As a concrete instantiation, we use our result to extend the HWS analysis from a linear potential (which is exactly solvable) to a quadratic potential (which is no longer solvable). Our result strongly suggests the broader applicability of tunneling to convex potentials with spikes
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Quantum Physics
Gopikrishnan Muraleedharan, Minh Thuy Truc Pham, Vir Pathak et al. (+10 more)
We provide a pre-obfuscation
circuit-level implementation of an efficient one shot signature scheme, which has known applications to delegated signatures, secure
token transfer, and publicly verifiable randomness. The
algorithm consists of two stages: a key generation stage where a classical public key/
quantum secret key pair is produced, and a signing stage where the
quantum secret key is processed with a message string to produce a classical signature. There is no algorithmic error in the construction and the signed message can be efficiently checked by a classical verifier. Our scheme works by preparing a superposition over elements of a random affine coset determined by the output of a puncturable pseudorandom function, together with a
circuit that tests coset membership. The logical
qubit number scales like $\Theta( \kappa\log(r) + n + l)$ and the gate complexity scales like $\Theta(n^3 + nl)$, where $r$ is the public key size, $n+l$ is the signature size, $l$ is the message size, and $\kappa = \Omega(n)$ is the cryptographic security parameter. We provide explicit
qubit and gate counts before obfuscation. A secure instantiation additionally requires modified key generation and a security proof for the resulting obfuscated construction.
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All authors Gopikrishnan Muraleedharan, Minh Thuy Truc Pham, Vir Pathak, Thomas Gardner, Chuanqi Zhang, Samuel Crew, Tom Hartweg, Adrian Aasen, Guido Masella, Vase Moeini, Asier Piñeiro Orioli, Hugo Perrin, Gavin K. Brennen
Quantum Physics
Random dimension reduction and learning symmetric properties of quantum states
Angus Lowe and Xinyu Tan
We describe a procedure that simultaneously reduces the dimensions of many, potentially distinct
quantum states while preserving properties invariant under the tensor power action of an isometry. This serves as a black-box method to replace the dimension with the maximum rank in the sample complexity of learning symmetric properties, even those depending on multiple input states. Combined with full state
tomography, random dimension reduction yields improved upper bounds for estimating distances, fidelities, and relative
entropies between pairs of states. Expressing the action of this procedure through the Choi–Jamiołkowski isomorphism reveals an intimate connection with the recently introduced random purification channel by Tang, Wright, and Zhandry. With this new perspective, we obtain a simple description of the rank-dependent version of the random purification channel and give an end-to-end analysis of sample-optimal
tomography without requiring a reference to the Schur transform or Schur
polynomials. Finally, we prove that there does not exist a random purification channel that simultaneously purifies copies of multiple, potentially different input states. Hence, the procedure is related to, but distinct from, random purification.
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Quantum Physics
Genuine Global Kochen-Specker Contextuality as Classical Coordination Cost
Ming Yang
We formulate classical simulation of
quantum correlations as coordination across a spacetime separator. All past-dependent information capable of influencing the future must cross the separator either through communicated messages or through a persistent classical state. We prove a general lower bound on this classical coordination cost in terms of the nonnegative rank of the separator correlation
matrix, with approximate and amortized extensions governed by approximate nonnegative rank and Wyner common information. For sequential processes, we introduce a causal positive-realization rank that enforces consistent response and state-update maps, and we explain how bounded local computation and support-covering models arise as restricted cases. We apply the framework to genuine global Kochen–Specker contextuality, in which local subsystems are noncontextual and the tested multipartite blocks are generalized-Bell-local, while the complete empirical model has no global noncontextual explanation. At the symmetric point of a polarization–path Hardy construction, exact classical simulation requires two coordination bits, whereas the corresponding
quantum separator requires one
qubit. Under exact repetition, the classical lower bound is the base-two logarithm of three bits per copy; with asymptotically vanishing error, it becomes one and a half classical bits per copy, compared with one
qubit per copy. We further construct a finite stabilizer–Peres–Mermin family for which the exact classical coordination cost grows quadratically with system size, while the
quantum boundary cost grows only linearly. The separation applies to arbitrary finite-state causal online simulators. Establishing a constant-error quadratic separation and an intrinsic construction without flag conditioning remain open.
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