Quantum Physics
Achieving perfect completeness for one- and two-message quantum proof systems
Yupan Liu and Thomas Vidick
While
quantum interactive proof systems using at least three messages can achieve perfect completeness, as shown by Kitaev and Watrous (STOC 2000), whether perfect completeness is achievable for one- and two-message
quantum proof systems has remained open. For the one-message case, whether $\sf QMA$ can achieve perfect completeness was posed as an open problem in Watrous (FOCS 2000) and Aharonov and Naveh (2002); for the two-message case, the corresponding problems were (implicitly) posed in Jain, Upadhyay, and Watrous~(FOCS 2009) and Kobayashi, Le Gall, and Nishimura (SICOMP, 2019). In this work, we establish that ${\sf QIP}(2)$, ${\rm qq}\text{-}{\sf QAM}$, $\sf QAM$, and $\sf QMA$ can achieve perfect completeness. Here ${\rm qq}\text{-}{\sf QAM}$ denotes the class of promise problems admitting two-message
quantum-public-coin
quantum interactive proof systems in which the verifier's only message consists of half-EPR pairs. Our main technical contributions are the follows: 1. For $\sf QMA$ (and directly for $\sf QAM$), an exactly constructible block-encoded
matrix whose kernel certifies yes instances, constructed from the acceptance operator induced by the verification
circuit. This also yields an alternative proof of ${\sf QCMA}={\sf QCMA}_1$. 2. For ${\sf QIP}(2)$ (and implicitly ${\rm qq}\text{-}{\sf QAM}$), a new turn-halving transformation that preserves completeness and ensures that the resulting proof system retains at least two messages, provided that the terminal state before the final measurement is efficiently preparable.
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Quantum Physics
Quantum Codes for Generalized Amplitude-damping Noise
Sourav Dutta, Anubhab Rudra, Manav Seksaria et al. (+2 more)
Quantum error correcting (QEC) plays a crucial role in protecting
quantum information against decoherence and enabling scalable, reliable
quantum computing. One of the most realistic and ubiquitous sources of noise affecting
quantum hardware today is generalized amplitude-damping (GAD) noise. Conventional, deterministic QEC codes struggle to correct for GAD noise because of their inherent structure, leading to fidelity losses that scale linearly with the damping strength. In this work, we introduce the framework of probabilistic approximate
quantum error correction (PAQEC), that combines the flexibility of approximate QEC with the potential of post-selected recovery, enabling high-fidelity, resource-efficient error correction. We construct a five-
qubit permutation-invariant code that, under probabilistic recovery, achieves a fidelity loss quadratic in the damping strength, thus outperforming existing QEC codes. Formulating PAQEC as an optimization problem, we present a numerical technique based on Charnes-Cooper and semidefinite programming to identify the optimal recovery map for any PAQEC code. Our results establish PAQEC as a powerful tool for developing resource-efficient, high-fidelity
quantum codes tailored to realistic noise, with promising implications for near-term
quantum devices and future fault-tolerant architectures.
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All authors Sourav Dutta, Anubhab Rudra, Manav Seksaria, Anil Prabhakar, Prabha Mandayam
Quantum Physics
Relative Entropy Decay via BKM coercivity for Quantum Markov Semigroups
Li Gao and Jingyu Guo
We introduce the notion of the BKM coercivity constant and show that the positivity of this constant governs the exponential relative
entropy decay of the semigroup. Indeed, we prove that the modified log-Sobolev constant is equivalent to the BKM coercivity constant up to a constant depending on the asymptotic conditional expectation. We also discover that the BKM coercivity constants for
matrix amplifications are already attained with a
qubit auxiliary system, which also coincides with the GNS spectral gap of the generating Lindbladian. As a corollary, we establish a criterion that the complete modified log-Sobolev inequality holds if and only if the GNS gap of the semigroup is strictly positive. All the discussion above applies to
quantum Markov semigroups that admit a faithful asymptotic conditional expectation as long-time equilibration, with no detailed balance condition assumed. As an application, we show the finite-dimensional Chen-Kastoryano-Gilyén Gibbs samplers always satisfy the complete modified log-Sobolev inequality.
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Quantum Physics
Generation of entanglement and magic via continuous homodyne monitoring of a qubit pair
Debmalya Das, Giuseppe Magnifico and Maria Maffei
Continuous weak measurements of
quantum systems are of great relevance in
quantum foundations and applications. They can be achieved by probing the
quantum system of interest by repeatedly measuring an auxiliary system weakly coupled to it. Here we study two
qubits coupled in different points to a common one-dimensional electromagnetic field and simultaneously monitored in the right- and left-propagating output channels by homodyne detection. Using a collision-model description, we derive an analytical
Stochastic Master Equation (SME) governing the resulting diffusive
quantum trajectories, including the interference between the two measurement channels. For nonlinear functions of the
quantum state, such as
entropies, averages over
quantum trajectories generally differ from the corresponding quantities evaluated on the unconditional state. Through this mechanism, we show that continuous monitoring generates
entanglement, absent in the unconditional dynamics, and enhances
quantum magic in the
qubit pair during the decay. Both resources can be tuned through the optical phase accumulated between the
qubits and the phases of homodyne local oscillators. Our results establish continuous homodyne monitoring of multiple emitters as a tunable mechanism for generating
quantum resources.
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Quantum Physics
Probing Residual Noise at a Decoherence Sweet Spot in a $^{28}$Si/SiGe Spin Qubit
Shinwoo Lee, Hanseo Sohn, Jaemin Park et al. (+7 more)
In $^{28}$Si/SiGe
spin qubits with micromagnets, the longitudinal stray field gradient transduces charge noise into
qubit frequency noise and limits coherence. We compare two neighboring
qubits in the same device with 800 ppm residual $^{29}$Si, one near a decoherence sweet spot where the gradient is locally minimized and the other at a position with a larger gradient. At the sweet spot, $T_2^*$ reaches 67 $\mu\mathrm{s}$ and the Carr-Purcell-Meiboom-Gill coherence time reaches 4.6 ms, whereas $T_2^*$ is 5.2 $\mu\mathrm{s}$ at the neighboring
qubit. Near 1 Hz, the frequency noise power spectral density at the sweet spot is nearly two orders of magnitude lower than at the neighboring
qubit. The magnitude and low-frequency decay of the residual spectrum are compatible with the prediction for $^{29}$Si nuclear
spin noise, and the weak interqubit correlation indicates that local noise becomes important for dephasing at the sweet spot. Despite a finite correlation with the charge sensor, sweet-spot operation strongly reduced the transduction of charge noise, bringing the residual frequency noise close to the level predicted for $^{29}$Si nuclear
spin fluctuations.
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All authors Shinwoo Lee, Hanseo Sohn, Jaemin Park, Hyeongyu Jang, Jonginn Yun, Jun Yoneda, Lucas E. A. Stehouwer, Davide Degli Esposti, Giordano Scappucci, Dohun Kim
Quantum Physics · Cross-listed from physics.atom-ph
Absolute frequency measurement of the $^{40}$Ca$^{+}$ clock transition using a GNSS link to the SI second
M. Guevara-Bertsch, M. K. Joshi, M. I Hussain et al. (+2 more)
We report the absolute frequency measurement of the $4s$ $ ^{2}S_{1/2}\leftrightarrow 3d$ $^{2}D_{5/2}$ $^{40}$Ca$^{+}$ clock transition with respect to the SI second. To perform this measurement, a link between our laboratory in Innsbruck and the clocks realizing the Coordinated Universal Time at the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig was installed and characterized using the Global Navigation Satellite System GNSS. The comparison between our clock and the ones at PTB was done using the Precise Point Positioning technique. After the evaluation of the systematic shifts, the measured transition frequency is 411 042 129 776 401.2$\pm$0.6 Hz with a fractional uncertainty of 1.5 $\times$ 10$^{-15}$. The stability of the clock measurements was also corroborated by comparing two different calcium ion clock experiments, which share the clock
laser source at our institute. Furthermore, after careful evaluation of the trap-drive induced ac
magnetic fields, we estimate ac Zeeman shifts on the $D_{5/2}$ sublevels and reevaluate the Landé g-factor of the $3d$ $^{2}D_{5/2}$ level to be g$_{5/2}= 1.200329(1)$.
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All authors M. Guevara-Bertsch, M. K. Joshi, M. I Hussain, R. Blatt, C. F. Roos
Quantum Physics
Limits on the atomic description of four-wave mixing
Irvin F. Ángeles-Aguillón, Nieves Arias-Téllez, Pablo Yanes-Thomas et al. (+2 more)
The limits of the well-established single-atom model for describing
photon-pair generation via four-wave mixing in a diamond configuration in atomic ensembles are experimentally tested. Using a cold-atom source, biphotons are generated and detected through polarization analyzers that resolve the emitted light into horizontal and vertical components in the laboratory frame. The pump
lasers driving the first and second excitation transitions are horizontally and vertically polarized, respectively. To predict single-
photon counts and coincidence rates in all polarization channels, the first- and second-order correlation functions are calculated directly using a comprehensive model that accounts for all Zeeman sublevels of the relevant hyperfine states. Furthermore, the population dynamics with and without the re-pump
laser of the magneto-optical trap are compared, revealing substantial differences in the populations of the Zeeman sublevels. This motivates the inclusion of two additional hyperfine levels and their corresponding Zeeman sublevels in the final model. The resulting density
matrix is used to calculate expectation values of the far-field electric-field operators and compare them with experimental measurements over a range of pump-
laser powers. Excellent agreement with the measured
photon counts is obtained over most of this range. For the coincidence measurements, good agreement is found when the polarization of the
photon generated by the first (second) decay is parallel to that of the first (second) pump beam. In contrast, the model consistently underestimates the experimental coincidence rates for the opposite polarization configuration. These findings indicate that effects beyond the internal level dynamics of individual atoms, most notably collective phenomena, are required to fully account for
photon coincidences generated by four-wave mixing in atomic ensembles.
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All authors Irvin F. Ángeles-Aguillón, Nieves Arias-Téllez, Pablo Yanes-Thomas, Alejandro Kunold, Daniel Sahagún-Sánchez
Quantum Physics
Instantiating Microcrypt: Obstacles and opportunities via tailored state certification
Jose Carrasco, Jens Eisert, Soumik Ghosh et al. (+3 more)
Recent work has introduced the
Hamiltonian phase state (HPS) assumptions, which postulate that
Hamiltonian phase states can be used to instantiate pseudorandom and one-way state generators. Additionally, it has been conjectured that these assumptions can be true, even if one-way functions do not exist. This is exciting, because if true, then the HPS assumptions provide a route to the instantiation of Microcrypt. In this work we falsify this
conjecture, by proving that if the HPS assumptions are true, then one-way functions exist. While this removes the possibility of instantiating genuine Microcrypt cryptography with
Hamiltonian phase states, it shows that the HPS assumptions provide novel inherently
quantum assumptions for the construction of classical cryptography. Technically we achieve this via a method for the construction of one-way puzzles from one-way state generators and tailored "measure first, ask later" state certification protocols. This generalizes prior constructions of one-way puzzles from one-way state generators via classical shadows and allows us to relate properties of the one-way puzzle to properties of the state certification protocol used in the construction. Specifically, if the state certification protocol admits efficient classical post-processing then one obtains an efficiently verifiable one-way puzzle, and if the state certification protocol can be efficiently classically simulated in a certain sense, then one obtains a classical one-way puzzle, which implies one-way functions. The latter observation allows us to prove that the HPS assumptions imply one-way functions, by exploiting properties of state certification protocols for phase states. The former observation provides a new toolbox for the construction of efficiently verifiable one-way puzzles by exploiting tailored state certification protocols for pseudorandom and one-way state generators.
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All authors Jose Carrasco, Jens Eisert, Soumik Ghosh, Dominik Hangleiter, Nicky Kai Hong Li, Ryan Sweke
Quantum Physics
Higher-order quantum thermodynamics: equilibrium and causal structure
Simon Milz, Kyrylo Simonov, Zoltán Zimborás et al. (+3 more)
Quantum thermodynamics is traditionally formulated as a theory of
equilibrium states and state transformations. Recent advances in higher-order
quantum transformations, which describe physical scenarios beyond states and channels and provide a systematic framework for causal order, raise the question of how
equilibrium should be defined and preserved in this more general setting. Starting from the Gibbs state as the unique
equilibrium state, we identify
equilibrium preservation as its natural higher-order extension. We show that, while this principle can in general give rise to distinct classes of transformations, all such distinctions disappear when
equilibrium preservation is required completely, namely under arbitrary ancillary extensions. Remarkably, every transformation satisfying this condition is causally ordered, making causal order an emergent consequence of thermodynamic
equilibrium. We establish this result for arbitrary higher-order maps and use the resulting framework to introduce free-energy-like quantities for
quantum channels. Our findings reveal a fundamental connection between thermodynamic
equilibrium and causal structure, providing a foundation for a fully fledged theory of higher-order
quantum thermodynamics.
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All authors Simon Milz, Kyrylo Simonov, Zoltán Zimborás, Tamal Guha, Saptarshi Roy, Giulio Chiribella
Quantum Physics
Security framework for practical quantum key distribution with imperfect devices
Jerome Wiesemann, John Burniston, Devashish Tupkary et al. (+1 more)
Practical
quantum key distribution (QKD) systems inevitably exhibit imperfections in both the source and detector. At the same time, the behavior of these imperfect devices is never exactly known due to characterization uncertainty, parameter fluctuations, and potential influence by an adversary. In this work, we present a security proof for generic prepare-and-measure QKD protocols, including decoy-state BB84, with imperfect and imperfectly characterized sources and detectors using the marginal-constrained
entropy accumulation
theorem (MEAT). Our approach uses a sequence of proof technique independent source maps and squashing maps, yielding a very modular framework. We show that practical key rates can be achieved even when multiple imperfections are combined. More broadly, our work provides a unified foundation that avoids the need for dedicated protocol-specific arguments and can be readily extended to other protocols and device imperfections.
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All authors Jerome Wiesemann, John Burniston, Devashish Tupkary, Norbert Lütkenhaus
Quantum Physics
Low-Depth Initial-State Preparation for Ground-State Energy Estimation of Two-Dimensional Strongly Correlated Systems
Ryo Watanabe and Keisuke Fujii
Quantum algorithms for ground-state energy estimation require initial states with non-negligible fidelity to the ground state. Guided by classical simulations, we design shallow
circuits for the two-dimensional half-filled Hubbard model by first preparing an approximate Heisenberg ground state and then applying charge-fluctuation gates derived directly from the Schrieffer-Wolff (SW) generator. For the Heisenberg model, we demonstrate effective parameter transfer from a $4\times4$
lattice to
lattices up to $10\times10$ without further optimization. We obtain promising lower bounds on the ground-state fidelity using tensor-network simulations, variational Monte Carlo reference states, and estimates of the ground-state energy and singlet gap. Exact $4\times4$ calculations show that the SW gates substantially improve the Hubbard ground-state fidelity of the embedded Heisenberg state. The successful Heisenberg parameter transfer supports the use of these small-
lattice results to design shallow Hubbard
circuits on larger
lattices beyond the reach of classical simulations. For a $10\times10$
lattice, the estimated preparation cost is on the order of $10^5$ $T$ gates, including Clifford+$T$ synthesis, which is well within the megaquop regime. These results offer a route to low-cost initial-state preparation for ground-starongly correlated systems.
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Quantum Physics
Optimal entanglement-assisted source coding under a balanced-difference promise
Julius A. Zeiss
Entanglement can reduce the communication required for coding tasks, but establishing the minimum achievable cost is essential to understanding its limits. We address this question in a zero-error source-coding task where Alice receives a word and Bob knows an unordered pair of candidates containing it. Alice does not know the pair and must enable Bob to identify her word without error using shared
entanglement and one classical message. The candidates satisfy a balanced-difference promise: for words in $\mathbb{Z}_q^n$ with $n=q\ell$, each residue modulo $q$ occurs exactly $\ell$ times in their coordinatewise difference. For all integers $q\geq2$ and $\ell\geq1$, we prove that the task requires exactly $n$ messages when $(q-1)\ell$ is even and two messages when it is odd. These minima allow arbitrary finite-dimensional shared states independent of the inputs and arbitrary local measurements. In even parity, this establishes optimality of an existing
entanglement-assisted protocol. In odd parity, an explicit deterministic protocol achieves the optimum of one bit without
entanglement. Our proof combines
Fourier analysis with a combinatorial counting argument to determine the smallest
eigenvalue of the associated graphs. In even parity, this resolves the spectral assertion of Cao et al.'s
Conjecture 6.3 for balanced cyclic generalized Hadamard graphs. Together with an explicit odd-parity bipartition, this determines the
quantum chromatic number as $n$ in even parity and $2$ in odd parity, where the classical chromatic number is also $2$. All lemmas,
theorems, and corollaries are formalized and verified in Lean.
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Quantum Physics · Cross-listed from cond-mat.mes-hall
Effects of residual exchange coupling on simultaneously driven spin qubits
Heun Mo Yoo, Tanner M. Janda, Victor Yu et al. (+3 more)
Exchange coupling and microwave drives are widely used control mechanisms for
spin qubits. However, the influence of exchange coupling on microwave-driven
spin dynamics is not fully understood. We report simultaneous drive measurements of two
spin qubits as a function of exchange coupling and drive power. When the Rabi frequency exceeds exchange, we observe a beating pattern in the Rabi oscillations. In the opposite limit, two exchange-split patterns emerge in the Rabi chevrons. We find that these exchange-induced effects are suppressed when the difference in the Rabi frequencies exceeds the exchange coupling. A theoretical model is developed that reproduces the main features in our data.
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All authors Heun Mo Yoo, Tanner M. Janda, Victor Yu, Michael J. Gullans, Adam R. Mills, Jason R. Petta
Quantum Physics
Pauli spectrum and nonstabilizerness of random fermionic Gaussian states
Xhek Turkeshi, Piotr Sierant and Poetri Sonya Tarabunga
We characterize the magic of random fermionic Gaussian states through their Pauli spectrum and their stabilizer
entropies. For the Haar ensemble of Majorana Gaussian states we derive closed finite-$N$ expressions for the average stabilizer purities and reconstruct the Pauli spectrum exactly, as a mixture of products of beta-distributed variables resolved by Majorana weight. The stabilizer
entropies, and their filtered versions, freeze at Rényi index $q_c=2$: for $q>2$ the magic density is $1/(q-1)$, controlled by rare weight-two Majorana strings. Resolving these contributions requires exponentially many characteristic samples, making direct sampling estimates of the stabilizer
entropies inefficient in this frozen regime. For Haar-random Gaussian states at fixed filling, we express the averaged stabilizer purities at positive integer $q$ as coefficient integrals over $\mathrm{SU}(2q)$, with dimension independent of $N$, and evaluate the $q=2$ case exactly. We establish that the frozen density describes typical states in the Majorana and half-filled number-conserving ensembles and relate these ensembles to the corresponding $\mathrm{SYK}_2$ ground states. Finally, we show that typical fermionic Gaussian states retain certified magic after discarding any fixed fraction $\kappa<\kappa_\star^{\rm G}\simeq0.7613$ of their modes in the thermodynamic limit, remaining magical even beyond the $2/3$ threshold for Haar-random states in the full
Hilbert space.
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Quantum Physics
Disentangling QAOA: From Weakly Entangled Circuits to a Classical QUBO Solver
Boris I. Bantysh, Andrey Yu. Chernyavskiy, Denis A. Kulikov et al. (+2 more)
The role of
entanglement in
quantum optimization remains actively debated. To address this question, we focus on the fixed-parameter expanding-depth regime of the
quantum approximate optimization
algorithm (QAOA), where a compact two-parameter schedule is trained once on small instances and then applied as the problem size and
circuit depth increase. To probe this regime beyond full state-vector simulation, we perform approximate
matrix product state simulations for up to 50
qubits and 100 layers and quantify
entanglement by the bond dimension. We observe an entangle–disentangle profile, with the peak bond dimension decreasing with depth and eventually saturating. This observation motivates an extreme approximation: projecting the state onto the product-state
manifold (bond dimension one) after every two-
qubit interaction. Based on this approximation, we introduce BOND-1, a
quantum-inspired classical solver. Despite the drastic simplification, BOND-1 achieves cut ratios above 0.95 relative to the best known values on standard GSet MaxCut
benchmarks with up to 20000 variables, and in some cases it matches those values. It achieves these results without per-instance optimization and has linear
memory cost, while per-instance tuning can provide further improvement. These results show that, in this regime, a substantial fraction of the optimization power of QAOA survives even in the complete absence of
entanglement. Our conclusions, however, are specific to this setting and do not imply that
entanglement is unnecessary for
quantum optimization in general.
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All authors Boris I. Bantysh, Andrey Yu. Chernyavskiy, Denis A. Kulikov, Maksim A. Gavreev, Evgeniy O. Kiktenko
Quantum Physics
A Modular, Topology-Aware Software Stack for Entanglement-Based Distributed Quantum Computing
Luke Andreesen, Shobhit Gupta, Sean Sullivan et al. (+1 more)
Distributed
quantum computing (DQC) seeks to scale beyond the limits of monolithic processors by interconnecting multiple
quantum processing units (QPUs) through
entanglement-based links. Realizing this
vision requires the co-design of hardware and software across the domains of
quantum networking, compilation, and scheduling, yet existing tools remain fragmented between monolithic
circuit compilers and long-distance
quantum network simulators. We present an open-source,
topology-informed framework for the compilation and scheduling of distributed
quantum programs. Given an input
circuit and a description of the target network, the framework partitions and reconstructs the
circuit into a distributed program that respects the specified inter- and intra-QPU
topology, covers cross-QPU operations through gate and state teleportation, and schedules the result under either deterministic or
stochastic entanglement-generation models. By accepting and emitting standard OpenQASM, the framework interoperates with existing monolithic toolchains, and its standardized module interfaces allow partitioning and scheduling strategies to be interchanged and benchmarked. Using this framework, we show that the best-performing compilation strategy varies across the tested
circuits and network configurations, and that both network
topology and intra-QPU connectivity substantially affect the
entanglement cost of execution. These findings underscore the need for a co-design approach to distributed
quantum computing, which our framework is designed to support.
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All authors Luke Andreesen, Shobhit Gupta, Sean Sullivan, Manish Kumar Singh
Quantum Physics · Cross-listed from cs.ET
A Hybrid Quantum-Classical Coordination Architecture for Portfolio Optimization via Global Context Injection
Xiaoguang Yang, Menghan Dou and Guoping Guo
Near-term
quantum and
quantum-inspired solvers for portfolio optimization rely on local subproblem execution under severe size constraints, but this locality can omit cross-cluster covariance essential for global risk coordination. We propose Context-Aware Folding (CAF), a lightweight hybrid
quantum-classical coordination layer between one-shot static decomposition and full-
matrix optimization. CAF injects a compressed global risk state into each local subproblem via a state-dependent linear bias, decouples local candidate generation from global commitment, and retains a sequential acceptance rule with a monotonic non-divergence guarantee. On a 2016 Russell 3000 subset (N=484) with Simulated Annealing (SA), CAF improves the scalarized mean-variance objective by 6.59% over a static baseline (20/20 wins), and by 0.2579% on an additional 2018 panel (N=1397, 17/20 wins). We further report matched folded N=40 compatibility studies with the
Quantum Approximate Optimization
Algorithm (QAOA) and simulated
quantum annealing (SQA) as local solvers, together with a frozen hardware-in-the-loop run on Origin
Quantum's Wukong 180 (WK\_C180) for one warm-started QAOA subproblem at n=5. These results support CAF as a coordination architecture with strong classical large-scale evidence, cross-backend compatibility, and local executability on real
quantum hardware in the noisy intermediate-scale
quantum (NISQ) era.
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Quantum Physics
Detectability of Post-Quantum Correlations in Symmetry-Restricted Bell Scenarios
Marek Gazdzicki and Francesco Giacosa
Searches for post-
quantum correlations depend on two distinct ingredients: how abundant such correlations are in the underlying correlation space and how efficiently the chosen representation preserves their distinguishability from
quantum correlations. We quantify these effects by introducing the popularity rate and the projection acceptance, whose product defines the search performance. We apply these concepts in binary-outcome Bell scenarios with
symmetry-related sites, using the CHSH and pyramid representations. Particular
attention is paid to the distinction between exchange-symmetric synchronous correlations and correlations satisfying only exchange
symmetry. In the synchronous
quantum sector, the common-vector representation follows automatically from synchrony, whereas exchange
symmetry alone defines a substantially larger correlation space. We show that this distinction can qualitatively change the effectiveness of a projected search: the pyramid representation, which retains substantial discriminatory power in the synchronous sector, has no discriminatory power when only exchange
symmetry is imposed. Our results provide a simple framework for assessing and comparing searches for post-
quantum correlations.
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Quantum Physics
OpenQARP: a modular framework for quantum application research
Stefano Scali, Vicente P. Soloviev, Antonio Márquez Romero et al. (+7 more)
We introduce OpenQARP, the Open
Quantum Application Research Package: an open-source Python framework for
quantum application research, built on a compiled C++ core. In OpenQARP, an application is assembled from an
algorithm library that orchestrates three interchangeable layers: blocks describe
circuits, primitives describe what to extract from them, and engines describe how they run. The release spans near-term through fault-tolerant methods, targeting problems from electronic structure to combinatorial optimization, with device-aware compilation,
circuit cutting, and resource estimation over a single set of numerical conventions that the code must follow. Every numerical feature is checked against an independent oracle, and every published timing carries a correctness check. Against OpenFermion, Qiskit/Aer, PennyLane/Lightning, Qulacs, qsim, and pytket, OpenQARP is one to two orders of magnitude faster on operator
algebra and among the fastest on simulation. It matches mature compilers on two-
qubit gate count, and expresses a complete
algorithm in the lines a framework needs rather than the plumbing a primitive stack demands.
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All authors Stefano Scali, Vicente P. Soloviev, Antonio Márquez Romero, Brian Coyle, Giuseppe Buonaiuto, Annie Paine, Jonathan H. Fetherolf, Marcos Diez García, Michal Krompiec, Josh Kirsopp
Quantum Physics
Distribution of light-matter quantum correlations with a temporally multiplexed solid-state quantum memory array
Aya Mneimneh, Susana Plascencia, Manuel Gundin et al. (+4 more)
Multiplexed
quantum memories increase the
entanglement distribution rate in long-distance
quantum repeater architectures by harnessing storage in several degrees of freedom. Here, we report on the distribution of light-matter
quantum correlations using an array of time-multiplexed solid-state
quantum memories. We store telecom-heralded single
photons sequentially in up to ten
memory cells using the full atomic frequency comb protocol with on-demand read-out in a Pr$^{3+}$:Y$_2$SiO$_5$
crystal. Leveraging both spatial and temporal multiplexing, we demonstrate
quantum correlations between the telecom
photon and up to 60 spatio-temporal modes of the
quantum memory array. We then transmit the heralding telecom
photon over 39.1 km of deployed optical fiber in the Metropolitan Area of Barcelona. In a realistic scenario where the generation rate is limited by the two-way communication time, we show that up to 15 % of the $393 \mu s$ round-trip communication time is filled with communication trials, leading to a 60-fold enhancement in the rate of detected telecom
photons correlated with the
quantum memory array, compared to a single-mode
memory. With increased storage times and efficiencies, our multiplexed
quantum memory array will constitute the backbone of a long-distance
quantum network, establishing remote
entanglement at high rates.
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All authors Aya Mneimneh, Susana Plascencia, Manuel Gundin, Jonathan Hänni, Samuele Grandi, Markus Teller, Hugues de Riedmatten
Quantum Physics · Cross-listed from cond-mat.mes-hall
Opto-Electrical Detection of Donor Bound Excitons in Silicon-on-Insulator Substrate
A. Kanniainen, A. S. Kumar, A. Sammak et al. (+2 more)
Spin of a donor bound electron in silicon has been shown to be a very coherent
qubit system but lacks a coherent optical interface. There does, however, exist a donor bound
exciton transition that can be excited optically but decays dominantly via an Auger recombination producing an electrical signal. This provides an opto-electrical pathway for
spin readout. Scaling this readout to single-
spin level will require interfacing the
spins with silicon photonics for efficient guiding of
photons, which in turn will require moving to silicon-on-
insulator (SOI) substrates. Here we demonstrate first ensemble opto-electronic measurements of donor bound
excitons in SOI material, including isotopically purified 28-Si device layers. The experiments show pronounced shifts in the resonance wavelengths and broadenings of the transition linewidths compared to the bulk experiments.
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All authors A. Kanniainen, A. S. Kumar, A. Sammak, G. Scappucci, J. T. Muhonen
Quantum Physics
Efficient quantum state tomography with two complementary projective measurements
Xiang Li, Yong Wang, Lijun Liu et al. (+3 more)
Quantum state
tomography (QST) is of fundamental importance to characterize
quantum systems in
quantum information processing, but its practical implementation is severely hindered by the exponential scaling of measurement and computational costs. In this paper, we present a novel QST protocol that utilizes Kirkwood-Dirac (KD) quasiprobability to reconstruct
quantum states. First, it enables state reconstruction with only two complementary rank-one projective measurements, thus significantly reducing the measurement cost. Then, a complex logistic regression
estimator is proposed to process collected KD data, together with a projected gradient
algorithm to mitigate numerical instability and to accelerate convergence. The product-operator structure of KD quasiprobability is further exploited to reduce the computational cost. Finally, extensive experiments are implemented to confirm the validity of our protocol. Notably, the full reconstruction of randomly generated 15-
qubit mixed-state instances can be accomplished within 20 minutes under the GPU implementation. These results suggest a promising route toward scalable QST and benchmarking large-scale
quantum systems.
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All authors Xiang Li, Yong Wang, Lijun Liu, Yiguang Hong, Qing Gao, Shuming Cheng
Quantum Physics
A Nonrecursive Lindblad Quantization of Dissipative Polynomial Dynamics
Tingfei Li
We present a direct and nonrecursive construction that maps an arbitrary planar
polynomial dissipative flow $\dot{\alpha}=h(\alpha,\alpha^*)$ to an open
quantum system in Gorini–Kossakowski–Sudarshan–Lindblad form. Given the homogeneous components of the classical drift, the corresponding
Hamiltonian and collapse-operator blocks are obtained algebraically and independently at each degree. No recursive cancellation of lower-order terms is required: in the large-amplitude limit $|\alpha|\sim S\to\infty$, ordering-generated lower-degree terms are parametrically suppressed, while mean-field closure yields the prescribed leading $O(S)$ drift for semiclassically localized states. This provides a simple and systematic route from dissipative classical dynamics to explicit open-
quantum-system realizations, without claiming a unique microscopic quantization of the classical flow. We demonstrate the construction for stable fixed points, a Hopf bifurcation, and a bistable-ring flow. The corresponding Liouvillian spectra recover the classical relaxation exponents, the radial Floquet exponent and neutral phase direction of a limit cycle, and the separation between local relaxation and inter-ring switching in the bistable case. These examples show that the construction can be used not only to analyze a given
quantum model, but also to design open
quantum systems with prescribed classical dynamical structures in the semiclassical limit.
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Quantum Physics
Reducing Decoding Latency in Quantum Error Correction by Early Starting Clustering
Tommaso Peduzzi, Lukas Bödeker, Markus Müller et al. (+1 more)
In
quantum error correction, fast low-latency decoding is essential for fault-tolerant
quantum computation, as delays in processing syndrome data can lead to the backlog problem. Existing decoders, including parallelizable approaches such as Union-Find, begin decoding only after all stabilizer measurement outcomes from an error-correction cycle have been received, inherently introducing a delay before decoding begins. We introduce Cluster-As-You-Go (CAYG), a modification of the Union-Find decoder that processes syndrome information during stabilizer measurements by clustering and correcting errors as they appear. This approach reduces the size of the remaining decoding problem at the end of the
quantum error correction cycle and, consequently, the time required to complete the decoding process. While this early start of clustering incurs a modest reduction in decoding accuracy, it preserves the decoder's scalability. Surface-code simulations show that the resulting reduction in post-measurement idling can outweigh the accuracy loss, yielding an improved speed-accuracy trade-off. These results demonstrate that real-time, early-starting decoding during QEC cycles is both feasible and can be advantageous for
quantum error correction. Demonstrated here for the surface code, CAYG is broadly applicable to other
quantum error-correcting codes, which allow for clustering-based decoding approaches, and is extensible to dedicated real-time decoding hardware.
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All authors Tommaso Peduzzi, Lukas Bödeker, Markus Müller, Luis Colmenarez
Quantum Physics
Parametric two-qubit gates via Landau-Zener interference
Simon Geisert, Albert Hertel, Soeren Ihssen et al. (+23 more)
We propose and demonstrate gates between two superconducting
qubits based on
quantum interference of consecutive Landau-Zener (LZ) transitions. This gate mechanism bridges between baseband and parametric two-
qubit control, enabling in situ tuning of the control frequency across a continuous interval up to hundreds of MHz. Another advantage compared to dispersive couplers is that the speed of the LZ gate is on the order of the full coupling strength. We experimentally demonstrate the gate on two platforms, a modular chiplet architecture of coupled generalized flux
qubits, and on a monolithic transmon architecture. The combination of tunability and gate speed establishes the LZ gate as a unique tool for multiplexing control pulses and interconnecting superconducting chiplet architectures.
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All authors Simon Geisert, Albert Hertel, Soeren Ihssen, Zhongyi Jiang, Paul Kugler, Nicolas Zapata, Nicolas Gosling, Ameya Nambisan, Yuan Gao, Asier Galicia, Jéferson R. Guimarães, Yorgo Haddad, Marc Neis, Harsh Bhardwaj, Dmitriy A. Volkov, Juan Cereijo, Marcello Guardascione, Yebin Liu, Markus Jerger, Pavel Bushev, Frank Wilhelm-Mauch, Wolfgang Wernsdorfer, Shai Machnes, Mohammad Ansari, Rami Barends, Ioan M. Pop
Quantum Physics
Information Erasure and Quantum Imprint in Quantum Measurement and First-Order SPAM Error Separation
Taiga Suzuki, Yuki Ito and Masayuki Ohzeki
We introduce information erasure and
quantum imprint as two properties that classify
quantum instruments. Information erasure is the property that an appropriate postselection can render the distribution of earlier measurement outcomes independent of the initial
quantum state while retaining all outcome branches.
Quantum imprint is the complementary property that no admissible postselection can eliminate this state dependence. We show that this classification has a nontrivial structure and that the natural intuition that measurements providing more information about the initial
quantum state should be less likely to exhibit information erasure does not hold in general. We further show that, under a sufficiently reliable postselection, information erasure enables first-order separation of state-preparation and measurement (SPAM) errors. Specifically, the first-order contribution of state-preparation error vanishes from the posterior distribution, whereas visible first-order contributions of measurement error remain. This result recasts SPAM error separation from the problem of simultaneously characterizing state preparation and measurement into the problem of realizing a reliable postselection.
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Quantum Physics
Bell-inequality violation in light transmitted through disordered emitter ensembles
Ruolin Guan, Vineesha Srivastava, Kasper J. Kusmierek et al. (+2 more)
We show that light transmitted through a disordered ensemble of weakly coupled two-level emitters can violate a Bell inequality in a continuous-wave Franson-type measurement. The Bell signal is determined by two steady-state output-field correlations, the equal-time intensity correlation $g^{(2)}(0)$ and the phase-sensitive two-
photon coherence $R$. We compute these quantities for bidirectional propagation through disordered emitter ensembles using a fourth-order cumulant expansion of the many-body master equation. The resulting Bell inequality violation appears in two distinct regimes, an antibunched regime where equal-time coincidences are suppressed and a bunched regime where
photon pairs remain phase coherent. Extrapolating the numerics to a representative weak single-emitter coupling $\beta=0.01$ predicts violation in the antibunched regime for atom numbers $N\simeq75$-$220$, and in the bunched regime for $N\gtrsim250$.
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All authors Ruolin Guan, Vineesha Srivastava, Kasper J. Kusmierek, Ivan Vybornyi, Klemens Hammerer
Quantum Physics
Engineering dissipation and control pulses for high-fidelity fault-tolerance quantum computing
Shao-Wei Xu, Zhe-Yuan Zhang, Yi-Tong Shi et al. (+2 more)
Cat-state
qubits, a prominent class of bosonic encodings, offer a promising pathway toward hardware-efficient fault-tolerant
quantum computing. In this manuscript, we propose an optimally robust control protocol for the cat-state
qubits which are stabilized by engineering two-
photon dissipation. By deriving an effective two-level description in the cat-state subspace and applying shortcut-to-adiabaticity via inverse engineering, we design a robust protocol to achieve fast and high-fidelity state transfer in the cat-state
qubit. We analyze the sensitivity to systematic control errors and identify an optimal robustness condition that strongly suppresses errors induced by imperfections in the driving fields. Furthermore, we show that dissipative confinement efficiently suppresses leakage out of the cat-state subspace caused by the pure dephasing, highlighting an intrinsic advantage of dissipative-cat
qubits. This work establishes a robust and leakage-suppressing framework for high-fidelity bosonic
qubit control, offering a promising route toward scalable fault-tolerant
quantum computing.
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All authors Shao-Wei Xu, Zhe-Yuan Zhang, Yi-Tong Shi, Ye-Hong Chen, Yan Xia
Quantum Physics
Electromagnetic Selection Rules and Coherent Manipulation of Quantum Skyrmion via Surface Acoustic Wave Phonons
Geng Li, Yu-Yuan Chen and Yu-xi Liu
Skyrmions are competitive candidates for information-storage units and have great prospect in
quantum information processing. We here study coherent control of a
quantum skyrmion via surface acoustic wave (SAW)
phonons in a piezoelectric SAW cavity. Exact diagonalization of a finite Dzyaloshinskii-Moriya cluster reveals the eigenstates of
quantum skyrmion with robust scalar chirality. The underlying
lattice-
spin symmetry imposes polarization-dependent selection rules for the electromagnetic transitions between eigenstates states. Considering the small mode volume of the SAW
phonon easy to reach strong coupling, we derive the interaction
Hamiltonian between the
quantum skyrmion as a
qubit and a single-mode quantized SAW via the electric field induced by piezoelectric effect, and find that the coupling strength at the single-
phonon level increases linearly with skyrmion radius. This enables enhanced
spin-acoustic coupling for larger skyrmionic textures and for low
magnetic dissipation. Our results establish few-
spin quantum skyrmions as compact building blocks for hybrid
quantum information processing and suggest a route toward more densely integrated on-chip
quantum devices.
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Quantum Physics
Noise robust self testing from genuine local operation shared randomness multipartite nonlocality Tests
Som Kanjilal
Violation of the $N$-partite inequality introduced in Phys. Rev. Lett. 129, 150401 (2022) for genuine multipartite nonlocality under local operations and shared randomness (LOSR) rules out all causal-network models obtained by locally composing arbitrary resources involving at most $N-1$ parties, even when supplemented by shared randomness among all $N$ parties. Here, we demonstrate a device-independent self-test protocol for this. We further establish an analytic noise-robust self-test. For a Bell-score deficit $\epsilon$, we derive explicit $O(\sqrt{\epsilon})$ vector-norm bounds for the state and the corresponding measurement actions.
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