Quantum Physics
Design and Benchmarking of a Quantum Photonic Chip
Gabriele De Angelis, Nicolò Leone, Alessandro Luongo et al. (+7 more)
We present the design and benchmarking of RP000, a
quantum photonic processor capable of encoding a
quantum system in the degrees of freedom of single
photons, based on standard CMOS-compatible manufacturing processes, and working at room temperature. We
benchmark it against machine learning tasks, evaluating three
quantum-classical architectures of increasing complexity. Our experimental results and simulations show that RP000 achieves higher accuracy than classical networks of comparable size in multiple use cases. Compared to a superconducting
quantum processor, RP000 exhibits superior noise tolerance. These findings demonstrate that RP000 can provide a scalable route toward efficient
quantum applications.
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All authors Gabriele De Angelis, Nicolò Leone, Alessandro Luongo, Alberto Montanaro, Matteo Sanna, Roberto Siagri, Vito Sorianello, Luigi Tallone, Fabrizio Tamburini, Marco Venere
Quantum Physics
Typical Entanglement of Superpositions
Damien Quinn, Joshuah T. Heath and Graham Kells
We investigate universal
entanglement properties inherent to superpositions of randomized states. We find that an $m$-fold superposition of typical states may be classified into two distinct
entanglement classes via the 2nd Rényi
entropy density $s_2$. The maximally entangled regime is defined by $s_2 \sim \ln (2)$, for which superposition adds no additional
entanglement. The sub-maximally entangled regime, $s_2<\ln 2$, instead constrains the reduced density
matrices of independent components to be orthogonal in the thermodynamic limit, which fixes the
entanglement of the superposition to a logarithmic enhancement $\Delta S(m)=\ln (m)$. As a consequence, an exponentially large number of superpositions is required to transition from the sub-maximally entangled class to maximal
entanglement. We explicitly calculate $s_2$ and the logarithmic enhancement, and demonstrate orthogonality for two canonical examples of the sub-maximally entangled regime (superpositions of pure Gaussian states and of random
matrix-product states). We also examine the
entanglement of superpositions of random stabilizer states, and discuss their relaxation to the Haar limit.
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Quantum Physics
Provable learning separation for predicting time-evolution of quantum many-body systems
Rahul Bandyopadhyay, Riccardo Molteni, Jens Eisert et al. (+2 more)
Given that
quantum computers are naturally suited to simulate the behavior of
quantum many-body systems, an immediate question arises: can one formulate physically motivated
quantum machine learning (QML) tasks that exhibit learning separations? We address this problem by studying the learnability of
quantum many-body dynamics from the perspective of probably approximately correct (PAC)-learning. Concretely, we devise a supervised learning problem where the training set consists of specifications of randomized stabilizer probe states,
evolution times sampled uniformly from a polynomially large time interval $[0,T]$, coupled with expectation values of certain observables evaluated on the resulting time-evolved state under an unknown
Hamiltonian. For this learning task, we provide an efficient
quantum procedure whose training phase learns the underlying
Hamiltonian from short-time training samples, and whose deployment phase combines
Hamiltonian simulation with the classical shadows protocol to perform inference on a newly given data point. By contrast, the existence of $O(\mathsf{poly}(n))$-time instances ensures classical hardness: by
embedding a $\mathsf{BQP}$-complete computation into the polynomially long time-dynamics of a low-intersection variant of the Feynman-Kitaev clock
Hamiltonian construction, we show that, for a certain family of input distributions, no randomized classical
polynomial-time
algorithm can fulfill our learning condition, unless $\mathsf{BQP}\subseteq\mathsf{P/poly}$. Furthermore, we show that the classically hard instance maintains
quantum learnability. We also give an interpretation of our results in learning-assisted certified
quantum simulation. Taken together, our results demonstrate a rigorous learning separation for a natural ML task based on
Hamiltonian evolution, while building connections between
quantum learning theory,
quantum simulation, and QML.
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All authors Rahul Bandyopadhyay, Riccardo Molteni, Jens Eisert, Vedran Dunjko, Sofiene Jerbi
Quantum Physics
Unbiased Estimation of Conditional Covariance for Quantum Optomechanics
Katsuta Sakai and Nobuyuki Matsumoto
Continuous measurements can prepare macroscopic mechanical oscillators in conditional
quantum states, but their covariance is difficult to verify. The conventional retrodictive
estimator assumes a forward–backward covariance
symmetry and can be biased, because physical dynamics such as feedback damping reduces the observability of the state from future records. Here, we derive an exact linear-Gaussian
estimator from causal, retrodictive, and smoothed trajectories. For a milligram-scale mirror, it agrees with a Riccati prediction based on parameters fixed independently, while the conventional estimate exhibits a covariance-space bias of $d_M\simeq3.5$. Our method paves the way toward unbiased testing of macroscopic
entanglement within a calibrated linear-Gaussian model, applicable to both tabletop mirrors and kg-scale gravitational-wave test masses.
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Quantum Physics
Geometric obstructions to quadratic time scaling in multiparameter quantum estimation
Eoin O'Connor, Jiayu He, Matteo G. A. Paris et al. (+1 more)
Unitary encoding of a single parameter provides quadratic enhancement in precision, with the
quantum Fisher information scaling quadratically with the encoding time. However, when estimating multiple parameters simultaneously, this fundamental scaling is not guaranteed. Here, we establish a universal
geometric obstruction that dictates when multiparameter
quantum metrology fails to achieve simultaneous $t^{-2}$ scaling. By decomposing the
Hamiltonian derivatives into components that commute and do not commute with the system
Hamiltonian, we prove that linear dependence among the commuting components inevitably generates a slow parameter direction whose Fisher information remains bounded as O$(t^0)$, limiting the overall estimation precision. We demonstrate this mechanism in both discrete- and continuous-variable setups, including collective
spin magnetometry and a generalized
quantum harmonic oscillator, and contrast it with the Lipkin–Meshkov–Glick model where $t^{-2}$ decay is preserved. Remarkably, while the slow direction fundamentally limits the achievable precision, the measurement incompatibility between fast and slow directions decays as $1/t$, rendering the symmetric logarithmic derivative bound asymptotically saturable. Our framework provides a readily computable diagnostic, given by the Gram
matrix of the diagonal generators, for identifying such obstructions in arbitrary multiparameter estimation problems. We further show that the bottleneck can be circumvented by relegating slow directions to nuisance parameters or by employing adaptive
quantum control.
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All authors Eoin O'Connor, Jiayu He, Matteo G. A. Paris, Marco G. Genoni
Quantum Physics
Bosonic quantum error-correcting codes with finite stellar rank
Rui Wang, Adithi Udupa, Timo Hillmann et al. (+3 more)
Bosonic
quantum error correction (QEC) relies on non-Gaussian bosonic encodings whose preparation cost is a central practical constraint. In this work, we use
stellar rank as a resource measure to design and
benchmark bosonic codes under finite non-Gaussian resources. For fixed cat and Gottesman–Kitaev–Preskill (GKP) code families, we show that finite
stellar rank creates a trade-off among state approximability, energy, and logical protection under
photon loss and
photon-number dephasing, evaluated with optimal recovery. This trade-off implies that codewords with better ideal error-correction properties need not be optimal once finite-rank preparation constraints are imposed. Going beyond fixed-target codewords, we directly optimize bosonic encodings at fixed
stellar rank, revealing noise-adapted code structures and concrete resource thresholds. Grid-like encodings emerge under
photon loss, whereas approximately rotation-symmetric encodings arise under dephasing. In the optimized search,
stellar rank k=2 suffices to surpass break-even for all dephasing strengths considered, while under
photon loss the required rank increases with the loss rate. These results establish
stellar rank as an operationally meaningful resource measure for bosonic QEC under practical state-preparation constraints.
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All authors Rui Wang, Adithi Udupa, Timo Hillmann, Ulysse Chabaud, Alessandro Ferraro, Giulia Ferrini
Quantum Physics
Attosecond metrology of bright quantum light
P. Stammer, J. Rivera-Dean, E. Pisanty et al. (+1 more)
Attosecond metrology is the ability to measure ultrafast optical light-wave oscillations, yet its approach has been limited to classical fields. Hence, the influence of the fluctuations of a
quantum field on attosecond measurements has remained unexplored. Here, we close this gap by showing that the attosecond streaking measurement of bright
quantum light is sensitive to
quantum fluctuations of the optical field on the attosecond timescale. The distinct sub-cycle modulations allow to extract the properties of the squeezed field quadrature in regimes where conventional state
tomography approaches reach their limitation. With the full
quantum optical attosecond streaking scheme developed here, we provide a certification method that can measure
quantum squeezing below the shot noise limit, thereby overcoming the problem of tomographically measuring bright
quantum light. This opens the way towards
quantum optical metrology of field fluctuations with attosecond temporal resolution.
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All authors P. Stammer, J. Rivera-Dean, E. Pisanty, M. Lewenstein
Quantum Physics · Cross-listed from cond-mat.mes-hall
Radio frequency readout and control of Ge/SiGe hole spin qubits with a global accumulation gate
Tien-Ho Chang, Chi-Wei Lee, Jian-Chang Zeng et al. (+12 more)
Hole
spin qubits in undoped Ge/SiGe
quantum well structures have advanced rapidly in performance and scalability. However, stringent multi-layer patterning and overlay requirements of conventional overlapping-gate devices create a bottleneck for academic proof-of-concept experiments involving few-
qubit devices. Here we present fabrication and measurements of Ge/SiGe
spin qubit devices with a global accumulation gate and single-layer depletion fine gates, which substantially reduce fabrication complexity. With careful design of the gate-2DHG capacitance, we demonstrate RF-based single-shot
spin readout and coherent control of two single-
spin qubits. We also characterize the
spin coherence times and exchange tunability, which are similar to those reported in recent overlapping-gate Ge/SiGe
spin qubit devices. By simplifying fabrication without sacrificing performance, our approach offers a more accessible device design for
spin-based
quantum technology research.
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All authors Tien-Ho Chang, Chi-Wei Lee, Jian-Chang Zeng, Chia-Hao Wei, Ching-Shiang Wang, Fu-Yuan Gu, Guan-Yu Yang, Ruei-Syuan Chiang, Ho-Chun Wu, Ming-Hao Lee, Ming-Wen Chu, Guang Li Luo, Ta-Chun Cho, Shawn S. H. Hsu, Tzu-Kan Hsiao
Quantum Physics · Cross-listed from math.CO
Enumeration of Laplacian integral and {-1,0,1}-diagonalizable graphs
Nathaniel Johnston, Sarah Plosker and Luis M. B. Varona
A graph with
Laplacian matrix $L$ is called
Laplacian integral if the
eigenvalues of $L$ are all integers, and it is called $\{-1,0,1\}$-diagonalizable if $L$ has a full set of eigenvectors with entries from $\{-1,0,1\}$. We herein develop a structure
theorem for both
Laplacian integral graphs and $\{-1,0,1\}$-diagonalizable graphs of
prime order, and combine it with some novel computational techniques to characterize all such graphs for orders larger than was previously possible. For example, we enumerate all
Laplacian integral and $\{-1,0,1\}$-diagonalizable graphs of order $13$ or less, all $\{-1,0,1\}$-diagonalizable graphs of
prime order $23$ or less, all regular integral graphs of order $15$ or less, and all regular $\{-1,0,1\}$-diagonalizable graphs of
prime order $53$ or less. As an immediate byproduct of our work, we show that the $S_{n,n}$
conjecture for
Laplacian integral graphs is true when $n = 12$, thus making $n = 16$ the smallest open case; additionally, we disprove two related
conjectures regarding
Laplacian spectra. We also establish an exponential lower bound on the number of connected $\{-1,0,1\}$-diagonalizable graphs of order $n$, thus beating the previously best-known (subexponential) lower bound. Finally, we show that every bipartite $\{-1,0,1\}$-diagonalizable graph is regular (a fact that fails to generalize to
Laplacian integral graphs).
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Quantum Physics
Quantum Probabilistic Local Differential Privacy: Structural Properties and Sample Complexity Bounds
Xian Shi
Differential
privacy provides a rigorous framework for quantifying
privacy leakage in data analysis, while its
quantum extensions have become increasingly relevant with the development of
quantum computing and
quantum machine learning. In this work, we introduce and study
quantum probabilistic local differential
privacy, a relaxation of
quantum local differential
privacy in which the
privacy constraint is allowed to fail on a spectral violation event with low probability. This quantity can be interpreted as the probability under the
quantum superoperation of a
quantum privacy-loss violation, and is closely related to the acceptance probability of the
quantum Neyman-Pearson test at a small threshold. We investigate the basic structural properties of this
privacy notion and clarify its relationship with existing forms of
quantum differential
privacy. We show the properties of
quantum probabilistic local differential
privacy under tensor-product composition and
unitary post-processing, while it is in general neither convex nor closed under post-processing by arbitrary
quantum channels. We further characterize when depolarizing noise satisfies
quantum probabilistic local differential
privacy under several representative scenarios. Finally, we connect
quantum probabilistic
privacy constraints with statistical inference by deriving a lower bound on probabilistically privatized contraction coefficients in terms of the hockey-stick divergence. As an application, we obtain sample complexity bounds of probabilistically privated asymmetric and symmetric
quantum hypothesis testing. These results provide a systematic foundation for studying probabilistic
privacy guarantees in
quantum information processing and their operational consequences for private
quantum statistical inference.
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Quantum Physics · Cross-listed from cond-mat.mes-hall
Composite-Fermion Study of Cavity-Modified Fractional Quantum Hall Excitation Gaps
Dalin Boriçi, Nicolas Regnault and Cristiano Ciuti
We investigate how cavity-mediated attractive electron-electron interactions modify the excitation gaps of fractional
quantum Hall states within the composite-
fermion framework. We compute both the neutral magnetoroton excitation spectrum and the charged excitation gap relevant to transport experiments for the Laughlin $\nu=1/3$ and $\nu=1/5$ states. We consider a
spin-polarized lowest-Landau-level model in which the interaction is mediated by a cavity mode with a spatially uniform
vacuum-field gradient and a finite interaction range controlled by a long-distance cutoff. Finite-size scaling reveals that the transport gap is consistently enhanced by the cavity-induced interaction, with the gap enhancement scaling quadratically with the electron number and with the fourth power of the
vacuum-field gradient. By contrast, the magnetoroton spectrum exhibits a richer dependence on the interaction range. The high-$k$ magnetoroton gap is enhanced for all interaction ranges considered, consistent with its close connection to the charged excitation gap, even with the long-range character of the interaction.
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Quantum Physics
Calculating strongly correlated ground states from the non-Markovian dissipative dynamics of Gaussian fermions
Giuliano Chiriacò
We introduce a mapping between the ground state of interacting fermionic
Hamiltonians and the non-
equilibrium steady state of a purely dissipative open
quantum system. Within the framework of third quantization, we map the
Fermi-Hubbard
Hamiltonian onto Lindblad jump operators acting on Majorana
fermions. Remarkably, both hopping and interaction terms map onto jump operators that preserve the Gaussianity of Majorana
fermions along individual
quantum trajectories. As a result, the dynamics can be unravelled and each trajectory can be simulated efficiently using only two-point correlation functions, with a computational cost that scales polynomially with system size. We further show that finite particle number requires negative dissipative rates, leading to an intrinsically non-Markovian dynamics. The corresponding trajectory unravelling involves both positive and negative
stochastic weights and exhibits a sign problem and large fluctuations, so that convergence requires an exponentially large number of trajectories. The overall computational cost remains exponential in system size despite the efficient Gaussian representation of individual trajectories, but is crucially dependent on the computational complexity of the non-Markovian unravelling, motivating further studies on the efficiency of such unravellings.
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Quantum Physics · Cross-listed from hep-th
Determination of thermodynamics from entanglement entropy in the finite-density O(N) model
Niko Jokela, Aatu Rajala and Tobias Rindlisbacher
We nonperturbatively compute Rényi
entropies for strip-shaped subregions in the three-dimensional O(4) model at finite density on the
lattice. By using a dual variable representation and a tailored worm
algorithm, we circumvent the sign problem when sampling the grand canonical ensemble. In the limit of large subregions, we also establish a direct, quantitative relationship between the derivative of
entanglement entropy with respect to the size of the entangling region and the thermal
entropy density for general
quantum field theories, providing a new way to study their thermodynamics. We corroborate this argument with our
lattice results by demonstrating that, in the appropriate limit, the derivative of
entanglement entropy satisfies the same Maxwell relation as the thermal
entropy density.
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Quantum Physics · Cross-listed from cond-mat.str-el
Bockstein Braiding Statistics Versus Three-Loop Braiding
Hanyu Xue
Braiding statistics of $p$- and $q$-dimensional topological excitations is conventionally defined in $p+q+2$ spatial dimensions. We find a novel statistical process $W_N(X,Y)=(Y^{-1}X^{-1})^N(YX)^N$ for two order-$N$ excitations in $p+q+1$ dimensions, detecting the Bockstein response $A\smile \beta(B)$. This new statistics and fermionic loop statistics exhaust all loop statistics in three dimensions whose fusion rules form an Abelian group $G$, classified by $H^5(B^2G,U(1))$. Surprisingly, conventional three-loop braiding goes beyond this classification, so it must have non-Abelian fusion rules. We suggest viewing three-loop braiding as particle-loop braiding together with exotic fusion rules between loops and point-like defects. We also try to clarify the relationship between statistics and
symmetry anomaly.
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Quantum Physics
Epitaxial single T centres in silicon-on-insulator
Christian H. Christiansen, Kasper H. Nielsen, Alisha Nanwani et al. (+26 more)
Spin-
photon interfaces based on silicon
quantum emitters offer a scalable platform for
quantum computing and networking. However, achieving coherent
photon emission remains a primary challenge due to stringent material quality requirements. To overcome this, we utilise high-purity molecular-beam epitaxy (MBE) to epitaxially incorporate single T centres in silicon-on-
insulator (SOI) wafers. We demonstrate single T-centre emission coupled to a nanophotonic waveguide and observe significant suppression of homogeneous broadening, yielding optical linewidths as narrow as 30 MHz using natural silicon for
crystal growth. These results establish epitaxial T centres as a robust foundation for coherent
spin-
photon interfaces in silicon
quantum photonics.
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All authors Christian H. Christiansen, Kasper H. Nielsen, Alisha Nanwani, Sebastiano Guaraldo, E. Laurits Piehorsch, Arnulf J. Snedker-Nielsen, Magnus L. Madsen, David R. Gongora, Emanuele Brusaschi, Rodrigo A. Thomas, Ian Farrer, D. Hieu Nguyen, Georgios Kountouris, Beñat M. d. A. Jokisch, Mohammad Khalifa, Claudia Piccinini, Mathias Ø. Augustesen, Hugo Laurell, Kokeb B. Benti, Maria S. Gonzalez, Amedeo Carbone, Elvedin Memisevic, Sangeeth Kallatt, Mark K. Svendsen, Marianne E. Bathen, Lasse Vines, Peter Granum, Peter Krogstrup, Stefano Paesani
Quantum Physics · Cross-listed from cond-mat.quant-gas
Chiral Graviton Modes in Non-Abelian lattice Fractional Quantum Hall states
Zeno Bacciconi, Min Long, Hernan Xavier et al. (+3 more)
Synthetic
quantum matter provides a highly tunable route to fractional
quantum Hall physics beyond the constraints of conventional electronic materials. However, previous theoretical studies have mostly focused on their ground state properties. It remains unclear to what extent such platforms could reveal key excitation properties of fractional
quantum Hall states. Here, we study charge-neutral collective excitations in a non-abelian
lattice fractional
quantum Hall state realized in the bosonic Harper-Hofstadter model at unity filling factior, realizing a Moore-Read ground state. Combining full exact diagonalization, band-projected exact diagonalization, and
matrix-product-state simulations, we demonstrate the existence of a long-lived
chiral graviton mode, probed by
chiral 3-body correlators, for the first time on
lattice non-Abelian states. The graviton signal is topological sector-independent and could be observed via
geometric quenches in small open droplets directly relevant to current cold-atom experiments, while other neutral modes, such as the magnetoroton and neutral
fermion, are less resolved at presently achievable volumes.
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All authors Zeno Bacciconi, Min Long, Hernan Xavier, Hongyu Lu, Marcello Dalmonte, Zi Yang Meng
Quantum Physics
Learning to Reconstruct Wigner Functions in Phase Space
Xinyu Tang, Yi-hsin Lin, Yan Zhu et al. (+5 more)
Wigner function learning is a central tool for characterizing continuous variable
quantum systems. A fundamental challenge in this setting is to infer a continuous phase-space function from sparse pointwise measurement data, a task that becomes increasingly demanding as the effective dimension enlarges. Here, we develop a general machine learning framework to reconstruct Wigner functions directly as continuous functions from sparse phase-space data. For states with sparse Fock-space or coherent-state representations, such as binomial code states and cat states, we devise provably efficient regression models whose measurement complexity scales only logarithmically with the effective
Hilbert-space dimension. For more general states, such as the Gottesman-Kitaev-Preskill (GKP) states, we design a deep learning model that reconstructs the Wigner function from sparse measurements and generalizes to arbitrary phase-space resolution. We demonstrate the broad applicability of our framework on both simulated data and experimental data from a
circuit quantum electrodynamic (
circuit-QED) system. Interestingly, on experimental data, we find that our model reconstructs Wigner functions of GKP code states across multiple rounds of
quantum error correction and identifies the dominant error process using significantly fewer measurements than conventional estimation techniques.
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All authors Xinyu Tang, Yi-hsin Lin, Yan Zhu, Tailong Xiao, Yuxuan Du, Giulio Chiribella, Qiongyi He, Ya-dong Wu
Quantum Physics
Entanglement as a Structural Complexity Axis: A PAC-Bayesian View of Generalization in Quantum Policies and Value Functions
Jian Xu, Delu Zeng, John Paisley et al. (+1 more)
Parameterized
quantum circuits (PQCs) are increasingly used as policies and value functions in
quantum reinforcement learning, yet it remains unclear when and why
quantum policies generalize. We give a PAC-
Bayesian account in which generalization is governed not by the raw number of
circuit parameters, but by the effective dimension of the Fisher geometry induced by the
circuit. This quantity is inflated by
entanglement, making entangling connectivity an independent axis of complexity.In controlled experiments that fix the number of trainable rotations and vary only
entanglement, we find that
circuits with larger Fisher effective dimension exhibit larger train-test gaps, while parameter count is a weak predictor. The resulting bound acts primarily as a ranking certificate: it correctly orders
circuits with identical parameter count, which parameter-counting bounds cannot do. We validate this mechanism across supervised classification,
quantum contextual bandits, and value-function generalization, where entangled
circuits consistently generalize worse than non-entangled
circuits of equal parameter count, with gaps shrinking as sample size increases.Our strongest evidence comes from low-variance decision models, including single-observable classifiers, value heads, and one-step policies. In end-to-end multi-step policy learning,
entanglement effects remain statistically significant but high return variance leaves the full ordering only partially resolved. Partial-correlation analysis shows that Fisher effective dimension screens off entangling pattern, and controls for training accuracy, readout, and optimizer rule out major optimization confounders. The effect also persists on an IBM Heron
quantum processor under real noise. Overall, our results reframe
quantum policy design around an
entanglement–generalization trade-off rather than expressivity alone.
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All authors Jian Xu, Delu Zeng, John Paisley, Qibin Zhao
Quantum Physics · Cross-listed from cond-mat.mes-hall
Classical Reversible Computation by Quantum Coherence
Daniel Loss
Rising energy demand from data-center and AI applications has renewed interest in reversible computation, where logic need not dissipate heat at every step if information is uncomputed. Implementations have so far been classical: adiabatic CMOS recovers part of the switching energy but still moves thousands of $k_BT$ per logic node at room temperature. Here we propose classical reversible logic implemented by coherent
spin dynamics in a
spin quantum-dot array, with inputs and outputs in classical basis states and no algorithmic use of superposition. The same
spin stores, transports, and computes, with
unitary rotation replacing irreversible switching. The universal building block is an iToffoli gate driven by DC voltage pulses and exchange between hole
spins in Ge/SiGe
quantum dots. Simulations with realistic model parameters reproduce the Toffoli truth table and yield a testable error landscape. Because shuttling transports the bit without measurement, logic and data movement remain
unitary until readout. Millivolt pulses on femtofarad gates with superconducting lines dissipate only dielectric loss and have no thermodynamic floor. With the loss parameters assumed for a demonstrated device the gate energy is $\sim 10^{2}\,k_BT\ln 2$ at 4 K; at a design point within reach of existing devices it can fall below the Landauer scale $k_BT\ln 2$, five to seven orders of magnitude less than a CMOS Toffoli. The same
semiconductor hardware therefore serves both purposes, supporting
quantum algorithms when superposition is used and classical reversible logic otherwise.
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Quantum Physics
Coherence Estimation Beyond the Liouvillian Gap in a Finite Nonequilibrium System
Sonali Brahma, Trishna Kalita and Himangshu Prabal Goswami
We investigate the estimation of bath-induced coherence in a finite
quantum system interacting with thermal reservoirs. Enhancement of coherence estimation is transient and the estimation precision totally disappears at the steady state despite the system retaining finite coherence. By analyzing the full Liouvillian eigenspectrum, we demonstrate that the optimal sensing window emerges from the competition between identifiable contributory modes' temporal relaxation and statistical importance. Neither is the linear inverse scaling of Liouvillian gap with transient optimal time a signature of unimodal contribution to optimal sensing, nor is the existence of
multimodal dynamics a signature of nonlinear scaling. The inverse Liouvillian gap does not obey any general scaling with the optimal sensing time of coherence and we prove our numerical results analytically using a general Markovian framework. We further show that coupling the finite system to a
quantum cavity and maintaining a thermal bias, transforms the transient metrological optimization into a sustained steady-state resource.
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Quantum Physics
Using Tanner Spectral Reduction to Improve Multi-Layer Optical Lattice Routing for Hypergraph-Product and Bivariate Bicycle qLDPC Codes
Joshua M. Courtney
We characterize the Tanner graph spectrum of hypergraph-product (HGP) / lifted-product (LP) codes and bivariate-bicycle (BB) codes, informing
qubit routing for three-dimensional reconfigurable
qubit architectures. Syndrome-extraction routing depth on HGP/LP Tanner graphs reduces to a single SVD on the base parity-check
matrix, using a spectral ratio $\beta_\text{HGP} = (1 + \beta_\text{base})/2$ where $\beta_\text{base} = \sigma_2(H)/\sigma_1(H)$ for the base parity-check
matrix, and a diameter identity $D_T = 2 D_\text{base}$ where $D_\text{base}$ is the base Tanner graph diameter.
Fourier spectral reduction reveals that the BB Tanner graph spectrum equals the union, over the $l \times m$ grid of characters of $\mathbb{Z}_l \times \mathbb{Z}_m$, of the singular values of a single $2 \times 2$ symbol
matrix built from the two defining
polynomials. This reduces spectral analysis from an $O((lm)^3)$ diagonalization of the $4lm$-node Tanner graph to $lm$ independent $2 \times 2$ SVDs. These results compose into a multi-layer three-dimensional AOL routing protocol with one-time setup cost $T_\text{Valiant} = O(\log N)$ atom rearrangements amortizable over a
memory experiment of $R$ rounds. For a Tanner graph chromatic index $\chi'$ and $L_\text{layers}$ stacked AOL planes, the per-syndrome-cycle depth is $\lceil \chi'/L_\text{layers} \rceil$ AOL pattern activations with no atom motion, an $8\times$ step-count reduction at $L_\text{layers} \geq \chi' = 8$. Contingent on multi-layer AOL hardware, this yields an estimated $\sim50-300\times$ per-cycle wall-clock advantage over a single-layer AOD baseline (degrading to $\sim5-100\times$ under AOD-crosstalk overhead), reducing to equality in the single-layer limit. This paper therefore presents a route toward practical routing improvement for future
quantum hardware incorporating multi-layer reconfigurable
qubit architectures.
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Quantum Physics
On stochastic realism and CP bias in diffractive dissociations
A.Y. Klimenko
CP bias in
proton–antiproton diffractive dissociation can be viewed from several philosophical perspectives, notably
stochastic realism and
stochastic epistemicism. When combined with the presumption of temporal
symmetry,
stochastic realism suggests that the laws of physics allow the possibility of thermodynamic antisystems, although whether antisystemic characteristics can be physically realised in nature remains an open question. Here, this bias is interpreted as apparent, indicating significant non-
unitary contributions and a distinction from fundamental CP violations arising from
unitary Hamiltonian dynamics. Such apparent effects may arise from intrinsic stochasticity, from environmental interactions, or from interference between the two. This work investigates the relevant mechanisms and determines conditions for creating, transmitting, or screening a CP bias. In the environmental branch, an equilibrated radiation bath may transmit a CP bias from matter-dominated surroundings, although causality constraints may limit this possibility. In the intrinsic branch, the observed bias is consistent with subleading antisystemic effects required by CPT invariance. Further experiments are needed to distinguish between these mechanisms.
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Quantum Physics · Cross-listed from hep-ph
Quantum decoherence: a study applied to quarkonium-like bound states in strongly interacting matter
Gabriele Coci, Salvatore Plumari and Giuseppe Falci
We study the
quantum decoherence of a bound state interacting with a reservoir of strongly interacting matter within the framework of open
quantum systems. The bound state is modeled as a
quantum harmonic oscillator whose parameters are tuned to reproduce the root-mean-square radius of $J/\Psi$ particle. The surrounding medium, representing the many degrees of freedom of strongly interacting matter, acts as an environment that induces dissipation and decoherence through system-reservoir coupling. By analyzing the time
evolution of the reduced density
matrix, we quantify the loss of
quantum coherence and its dependence on medium properties. Subsequently, we extend the model by introducing a time dependence in the system-thermal bath coupling, thereby simulating a temperature
evolution similar to that occurring during the expansion of a fireball in the central region of heavy-ion collisions. We find that a temperature
evolution has a relevant impact on the way the system loses coherence through the coupling with the expanding medium. Finally, we estimate the impact of the time-dependent temperature on the decoherence process, also analyzing a scenario that includes viscous effects without finding a significant change with respect to ideal hydrodynamical
evolution.
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Quantum Physics · Cross-listed from cs.NI
Packet Routing for the Quantum Internet
Robert Malaney
We present a new design for
quantum packet routing within the emerging
Quantum Internet, highlighting how a little-used feature of Internet Protocol Version 6 (IPv6), namely Extension Headers, can lead to a significant amount of quantumness within the IP layer. Taking a minimalist approach to alterations of established standards, we outline the changes required in order for
quantum teleportation,
quantum routing, and superpositions of these processes to be enabled. Relative to other proposals for routing within the
Quantum Internet, the architecture we propose enables a wider range of outcomes allowed by
quantum mechanics. We do not claim any optimally in our design, but rather a pathway to invoke new
quantum routing outcomes via small additions to the current IPv6.
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Quantum Physics
Quantization of the classical Mpemba effect
Jannis Melles, Hartmut Löwen, Benno Liebchen et al. (+4 more)
The Mpemba effect refers to the counterintuitive phenomenon that an initially hot system can freeze faster than an initially warm one. Recent years have brought major advances in both classical and
quantum realizations, with the asymmetric bistable potential emerging as the paradigmatic classical
benchmark because its mechanism is especially transparent and controllable. Yet for precisely this
benchmark problem, the impact of quantization remains unexplored. Here we show that quantization shifts Mpemba behavior to qualitatively new regimes, moving it to ultra-cold temperatures that are orders of magnitude lower than those relevant for classical thermal barrier crossing. In addition, quantization produces inverse and double inverse Mpemba effects that are absent in the corresponding classical dynamics. Our results establish quantization as a robust route to
quantum-enabled Mpemba effects inaccessible in classical regimes.
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All authors Jannis Melles, Hartmut Löwen, Benno Liebchen, Michael te Vrugt, Giovanna Morigi, Artur Widera, Alexander P. Antonov
Quantum Physics
SQGen: Structured Quantum Image Generation with Latent-Modulated Quantized Tensor Trains
Guang Lin and Qibin Zhao
Generating images directly from
quantum systems is an attractive but unresolved goal on NISQ hardware. Existing
quantum generators face several coupled obstacles: barren plateaus that block trainability, expensive
quantum circuit preparation, and hardware noise that erodes
quantum information with depth. A further difficulty is producing image-scale output without a classical decoder, whose use would otherwise break the end-to-end
quantum advantage. We propose SQGen, a full
quantum generator built on a quantized tensor train (QTT) with a latent modulation architecture. Specifically, SQGen promotes the QTT bond index of the target pixel distribution to ancilla bond
qubits, so that each
circuit site operates locally on a bond register plus the two physical
qubits that carry the row- and column-bit of one image scale. We further introduce latent modulation: each re-uploading rotation is factorized at the angle level into a trainable main path plus an additive latent term, reducing to the trainable main path when the latent term is disabled. During training, we create a differentiable model in the classical system under gate-compatibility constraints, with a
torus prior as the latent distribution. After training, every operator maps one-to-one to a native
quantum gate, yielding a compact, deployable
quantum circuit with no classical decoder in the inference path. Together, these design choices address the obstacles raised above. Extensive experiments on image
datasets and synthetic data demonstrate that SQGen trains stably, generates images end-to-end from a shallow
circuit with no classical decoder, and shows promising feasibility on real
quantum hardware.
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Quantum Physics · Cross-listed from hep-th
Genuine Multi-Entropy in the Toric Code
Sriram Akella, Norihiro Iizuka and Akihiro Miyata
We study genuine multi-
entropy as a diagnostic of multipartite
entanglement in the toric code, which provides a controlled setting for probing multipartite structures in topologically ordered states. Our main question is whether genuine multi-
entropy captures information that is not reducible to conventional lower-party entropic data, such as topological
entanglement entropy. We first analyze toric-code ground states that admit a stabilizer-state description, where the relevant quantities can be evaluated exactly. We
conjecture that, for stabilizer states and $\mathtt{q}\ge4$, the $\mathtt{q}$-partite genuine multi-
entropy at replica index $n<\mathtt{q}$ collapses to a linear combination of multi-
entropies involving at most $\mathtt{q}-2$ parties. We establish this pattern explicitly for $\mathtt{q}=4$ in the toric code stabilizer sector: for $n=2,3$, the genuine multi-
entropy is proportional to the tripartite information $I_3$ and, for the Kitaev-Preskill partition, contains no independent genuine four-partite information beyond that captured by the topological
entanglement entropy. At $n=4$, however, this reduction breaks down: the genuine multi-
entropy is no longer proportional to $I_3$, but remains a topological invariant of the toric-code stabilizer ground states under Kitaev-Preskill-preserving deformations that do not change whether the numbers of vertex- and plaquette-type junctions coincide. For generic non-stabilizer superpositions within the ground-state
manifold and for coherent superpositions of local excitations, the low-$n$ reduction also fails. These results show that genuine multi-
entropy probes multipartite
entanglement structure beyond the tripartite information, and hence beyond the topological
entanglement entropy in the Kitaev-Preskill partition, whereas for stabilizer states at low replica index it reduces to lower-partite entropic data.
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Quantum Physics · Cross-listed from physics.optics
Single-photon polarization tomography with an integrated metal-superconductor nanowire array
Pierre Brosseau, Jiawei Wang, Giorgio Adamo et al. (+2 more)
Light polarization is a primary degree of freedom for encoding
quantum information. The scaling up of photonic
quantum networks and computer architecture depends crucially on its precise characterization. This is typically achieved by placing external waveplates, polarizers, moving mounts, and recently metasurfaces, on top of the detectors. All these solutions complicate integration and scaling. Here we break convention with traditional architecture and present a monolithic, self-aligned metal-
superconductor nanowire single
photon detector (M-SNSPD) possessing intrinsic full polarization selectivity. Gold nanowires, co-fabricated atop NbTiN superconducting nanowires within the same lithographic footprint, act as polarization-selective plasmonic metamaterials inducing resonant absorption in the NbTiN. U-shaped wires provide linear polarization selectivity, while S-shaped meanders distinguish circular polarization, while retaining the high-count rates and low dark count rates of conventional SNSPDs. By arranging them into a four-pixel array we realize simultaneous projection onto four polarizations and demonstrate continuous polarization state
tomography with an ensemble average fidelity exceeding 98%. Our approach opens new avenues towards scalable detector arrays with integrated plasmonic functionalities, for single
photon polarimetry, imaging and spectroscopy.
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All authors Pierre Brosseau, Jiawei Wang, Giorgio Adamo, Anton N. Vetlugin, Cesare Soci
Quantum Physics
Hybrid quantum floating-point method for sharp arithmetic
Gabriele Agliardi and Enrico Prati
There are several possible ways to encode random variables in a
quantum state. The basis encoding of bit strings has paramount importance because it allows to load the values of a random variable through the superposition of corresponding basis states, and to then exploit
quantum parallelism in processing
algorithms. The basis encoding offers a natural way to represent an unsigned integer random variable, and extends to signed integers, as well as to fixed-point and floating-point variables. Each
quantum representation of fractional numbers, however, involves a trade-off between accuracy and depth of manipulation
circuits. Here, an efficient hybrid
quantum-classical representation of
quantum floating points is introduced. It combines a
quantum register containing the values, with a classical register storing global information about the variable, namely the range and approximation tolerances. The sum and product operations are defined, in such a way as to ensure they are performed without overflow. By taking advantage of the stored classical information, the precision degradation that occurs due to rounding after repeated data manipulations, can be significantly reduced compared to known strategies. Ad hoc examples show up to around $90\%$ reduction in approximation, compared to previous techniques, after repeated additions. The method finds application in many
algorithms of practical relevance and constitutes a significant advance in the design of arithmetic
circuits with low depth and high accuracy.
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Quantum Physics
Simplified quantum key distribution implementation secure in the presence of state preparation flaws
Ainhoa Agulleiro, Fadri Grünenfelder, Raphaël Houlmann et al. (+3 more)
We present an implementation of a three-state BB84 protocol with time-bin encoding, one decoy state and a simplified measurement scheme that uses passive basis choice. Our system simplifies the state characterization with respect to previous iterations. We also adapt the loss-tolerant method to our protocol, thus dealing with the measured state preparation flaws. We compare the obtained phase error rate and secret key rate when including the state imperfections and when assuming perfect states. Our results highlight the importance of characterization and implementation security.
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All authors Ainhoa Agulleiro, Fadri Grünenfelder, Raphaël Houlmann, Ana Blázquez, Hugo Zbinden, Davide Rusca