Quantum Physics · Cross-listed from cond-mat.stat-mech
Decoherence-controlled collective criticality in a two-dimensional quantum Stag Hunt
Rajdeep Tah
Physical decoherence can preserve the microscopic strategic neutrality (i.e. zero effective field and equal potential of the two homogeneous strategic orientations) of a
quantum game while changing the thermodynamic regime of the corresponding interacting population. We demonstrate this for an Eisert–Wilkens–Lewenstein (EWL) Stag Hunt embedded as independent nearest neighbor encounters on a square
lattice. For the restricted strategies $\mathsf{Q}=i\mathbb{Z}$ and $\mathsf{D}=i\mathbb{Y}$, noisy two-player payoff
matrices are determined for phase damping, depolarization, and amplitude damping and mapped exactly to channel dependent Ising parameters $\mathfrak{J}(\Gamma,p)$ and $\mathfrak{H}(\Gamma,p)$. Phase damping and depolarization show the clearest contrast: they share the same microscopic neutrality branch $\mathfrak{H}=0$, while only depolarization suppresses the interaction as $(1-p)^2$. At $\beta=1$, this produces an exact depolarization-driven square-
lattice critical point at $p_{*}\approx 0.233460\ldots$, whereas phase damping remains in the ordered coexistence regime along the same neutrality branch. Monte Carlo finite size scaling is consistent with two-dimensional Ising criticality and distinguishes field driven coexistence below $p_{*}$ from a smooth crossover above it. Amplitude damping additionally reveals a strong dependence on channel placement: the post-strategy neutrality branch reaches $\Gamma=0$ at $p=1/3$ and then disappears. Resource negativity further shows that microscopic two-
qubit entanglement and collective interaction strength are distinct quantities. The resulting extended
lattice remains an ordinary classical Ising system.
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Quantum Physics
A quantum oracle separation between QMA(2) and QMA
John Bostanci, Sabee Grewal, Jonas Haferkamp et al. (+4 more)
We find a
quantum oracle relative to which $\mathsf{QMA} \neq \mathsf{QMA}(2)$. As a consequence, we resolve the no-disentanglers
conjecture of Watrous: for every $\epsilon+\delta<1$, any $(\epsilon,\delta)$-disentangler requires input size exponential in the number of output
qubits. Our proof combines the unitarily invariant
polynomial method of She and Yuen (ITCS '23) with a new construction based on the symmetric and antisymmetric subspace projectors, reducing the $\mathsf{QMA}$ lower bound to the approximate degree of $\mathrm{OR}$.
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All authors John Bostanci, Sabee Grewal, Jonas Haferkamp, Andrew Huang, Yeongwoo Hwang, Anand Natarajan, Chinmay Nirkhe
Quantum Physics
Exponential speedup of polarization stabilization for long distance DWDM quantum networks
Jinyi Du, En Teng Lim, Xingjian Zhang et al. (+4 more)
Fibre-based
quantum networks distributing polarization
entanglement require a stable and uninterrupted transmission basis for reliable operation. Bright classical reference light enables rapid polarization feedback but can introduce noise into
quantum channels. Entangled-
photon-based feedback avoids this noise, but typically interrupts the target
entanglement channel during calibration and becomes prohibitively slow over long distances due to the product loss of fibre links. Here, we overcome both limitations by combining wavelength-bracketed probing with switch-enabled path decomposition. Spectrally adjacent entangled-
photon sidebands track the polarization response of the central distribution channel without interrupting its transmission, while optical switches and local reference fibres independently determine the signal and idler network transformations. Transferring the resulting compensation settings to the central channel eliminates calibration-induced downtime and changes the acquisition-time scaling from the product of the link losses to the sum of losses. We demonstrate the method on a 133 km fibre testbed and achieve continuous closed-loop stabilization for more than 24 hours without classical reference light. The decomposition of multi-link
quantum feedback into single-link measurements provides a scalable stabilization strategy for wavelength-multiplexed
quantum networks.
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All authors Jinyi Du, En Teng Lim, Xingjian Zhang, Hongwei Gao, George F.R. Chen, Dawn T. H. Tan, Alexander Ling
Quantum Physics · Cross-listed from hep-ex
Effective Sub-Quantum Readout for Non-Monochromatic Axion Signals in High-$Q$ Haloscopes
Junu Jeong and Max Silva-Feaver
The search for wave-like dark matter using microwave cavity haloscopes is constrained by the Standard
Quantum Limit, which dictates that phase-preserving linear amplification results in a minimum of one
quantum of total system noise for a narrow-band signal. We demonstrate that this limit is effectively halved for a non-monochromatic
axion signal coupled to a high-$Q$ cavity. By operating a Josephson Parametric Amplifier such that the cavity resonance is centered exactly at the half-pump frequency, the
axion signal symmetrically populates both the signal and idler bands. Through quadrature analysis of the homodyne readout, we show that the incoherent sum of these mirrored spectral components doubles the measured signal power while the
vacuum noise remains constant. This operation yields an effective noise limit of 0.5 quanta per frequency bin, translating to an overall effective limit of $1/\sqrt{2}$ quanta after optimal matched filtering.
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Quantum Physics
Model-level synthetic-flux control of hyperchaos order and matched-resource sensing in dissipative optomechanics
Stella Rolande Mbokop Tchounda, Carolle Tchodimou, Philippe Djorwe et al. (+2 more)
Within a normalized six-dimensional model of dissipative optomechanics (one cavity + two mechanical resonators), a synthetic-flux phase $\Phi_{\rm syn}$ acts as a reproducible control coordinate that selects the order of a drive- and coupling-gated hyperchaos transition—up to four simultaneously positive Lyapunov exponents, beyond any reported single-mode
benchmark. A phase-consistent Floquet–Lyapunov protocol (cross-checked by monodromy multipliers, dissipative volume balance $\sum\lambda_i=\operatorname{Tr}(J)=-1.04$ and a 180-run three-seed audit) localizes the onset to a Neimark–Sacker bifurcation at $E^{*}=\num{1.060}$ ($\theta=0$). As a secondary, model-level
geometric clarification, the identical matched-resource force-sensing protocol returns a null gain on the chaotic attractor ($\mathcal{G}_{A/B}=\num{1.039}\pm\num{0.014}$), consistent with the matched-Fisher lemma: noise projected onto an unstable
manifold is stretched by the same factor $e^{\lambda t}$ as the deterministic signal. Truncated-Fock and truncated-Wigner checks support the mean-field description at selected points. All results remain strictly model-level: the strong-coupling sector lies $\num{2542}\times$ beyond anchored silicon optomechanical couplings. Closing that gap requires ultrasonic characterization of the mechanical degeneracy $\omega_2$, a measured inter-resonator hopping $J_m$, and the emergence of a genuine gigahertz platform.
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All authors Stella Rolande Mbokop Tchounda, Carolle Tchodimou, Philippe Djorwe, Sifeu Takougang Kingni, Serge Guy Nana Engo
Quantum Physics
Variational preparation of thermofield double states for SYK models via multi-angle QAOA: sequential angle pruning for circuit reduction
Haji Muhammad Husnain Ashfaq, Moongul Byun and Keun-Young Kim
Variational preparation of thermofield double (TFD) states can require deep
quantum circuits, particularly for interacting many-body systems. Reducing these
circuits while retaining high fidelity is therefore crucial for TFD-state preparation on noisy
quantum processors. We study this problem by applying the multi-angle
quantum approximate optimization
algorithm (ma-QAOA) to TFD-state preparation and introducing two top-down sequential angle-
pruning algorithms. Starting from the optimized initial ma-QAOA
circuit, both
algorithms sequentially remove Pauli-string evolutions with small optimized angles and reoptimize the remaining parameters after each removal. We apply these
algorithms to Gaussian and binary Sachdev–Ye–Kitaev (SYK) models in both dense and sparse cases. We find that ma-QAOA prepares the target TFD states with high fidelity and that sequential small-angle
pruning retains high fidelity while reducing the
circuit depth, particularly at low temperature. Moreover, using the post-reoptimization cost in sequential small-angle
pruning further improves the fidelity. For the binary sparse $N=10$ SYK model at $\beta=10$, $88.8\%$–$92.1\%$ of the nonlocal Pauli-string evolutions are removed while retaining an average fidelity of approximately $95\%$. Finally, we propose extensions of the sequential
pruning algorithms toward
quantum–classical hybrid implementation.
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Quantum Physics
Phonon-limited detection thresholds for genetically encoded fluorescent-protein spin-qubit relaxometry of neural radicals
Parul Raghuvanshi, Sagnik Ganguly, Sharika E et al. (+2 more)
The demonstration that enhanced yellow fluorescent
protein hosts an optically addressable
spin-1
qubit in its metastable triplet state raises the prospect of genetically encoded
quantum sensing at molecular length scales. We develop a detection-limit theory for using this fluorescent-
protein spin qubit (FPSQ) to sense paramagnetic neural signaling radicals by
spin relaxometry. We derive the transition-resolved Redfield relaxation
matrix of the zero-field-split triplet coupled to a diffusing radical bath, establish the regime in which it collapses to a single exponential, and validate it against Lindblad simulations and nitrogen-vacancy
benchmarks. Propagating the effects of
photon shot noise, photobleaching-grounded
photon budget, and finite measurement bandwidth, we find that the native room-temperature sensor falls short of physiological sensitivity by six to eight orders of magnitude with the bottleneck being the
phonon-limited intrinsic $\Tone$. Analyzing the underlying direct and two-
phonon Raman processes, we show that room-temperature relaxation is Raman-dominated by $\sim\!720\!:\!1$ and that, because the Raman coefficient scales as $v^{-10}$ with sound velocity, a $\sim\!2\times$ stiffening of the chromophore environment recovers $\Tone\sim\SI{100}{\micro\second}$, sufficient for micromolar sensing. Nanomolar sensing is obstructed by a direct-process ceiling of \SI{79}{\micro\second} that vibronic decoupling alone cannot breach. We obtain quantitative design rules, identify
photon yield as a co-equal bottleneck, and propose a frequency-resolved protocol for chemical specificity.
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All authors Parul Raghuvanshi, Sagnik Ganguly, Sharika E, Mohana Priya T., Vishvendra S. Poonia
Quantum Physics
Conditional validity of quantum event classifiers under collider systematics and quantum estimation uncertainty
Roberto Fernández-Barrios, Iker Pastor-López, Asier González-Santocildes et al. (+1 more)
Claims about a deployed
quantum machine-learning classifier can fail when target data shift or when finite-shot
quantum evaluation randomizes the model itself. We develop an information-conditional, fail-closed auditing framework that returns supported, refuted or unresolved verdicts with anytime-valid per-claim error control under a declared sampling protocol. On a
Higgs-to-tau-tau
collider benchmark, stable classifier metrics do not guarantee valid signal-strength inference: at the studied finite-template statistics, the fixed-template profile can lose coverage, even in shift-free controls, when its templates are estimated independently and template-statistical uncertainty is not modeled explicitly. Across 30 frozen finite-shot
quantum-kernel deployments, every realized Gram
matrix is propagated through refitting, calibration and threshold selection; in the primary raw pipeline these perturbations leave ranking nearly unchanged yet move thresholded target metrics by about 0.02, flipping ideal-anchored claims, and the diagonal-loading sensitivity decomposes differently. Matched classical controls remove apparent
quantum-specific nominal-performance and sensing effects. We claim no
quantum advantage.
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All authors Roberto Fernández-Barrios, Iker Pastor-López, Asier González-Santocildes, Pablo García Bringas
Quantum Physics
Spin squeezing by geometric focusing in vacuum Rabi oscillations
Aoqin Liang, Qi Liu and Guoqing Wang
We show that
vacuum Rabi oscillations can directly generate
spin squeezing through
geometric focusing on the Bloch sphere. Starting from a coherent
spin state resonantly coupled to a cavity initially in the
vacuum state,
quantum fluctuations are focused by the
curvature of the Bloch sphere as the collective
spin approaches the atomic ground state, producing squeezing transverse to the direction of motion. The squeezing timescale is set by the collective Rabi frequency $t_s\sim 1/(g\sqrt{N})$. The optimal Wineland squeezing parameter scales as $\xi_{\rm opt}^2\propto N^{-1/3}$, which is an outcome of the competition between the
geometric focusing effects and the cavity-field
vacuum fluctuations. The squeezing remains robust against realistic dissipation. In the end, an application example of $^{171}$Yb is briefly discussed to show the feasibility of our protocol.
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Quantum Physics · Cross-listed from cond-mat.mes-hall
QArray+: A physics-informed GPU-accelerated simulator for quantum dot arrays
Pranav Vaidhyanathan, Barnaby van Straaten, Alice Petrillo et al. (+6 more)
Semiconductor quantum-dot arrays are a compelling platform for scalable
quantum technologies, yet their practical operation is hindered by the complexity of tuning large-scale devices. Existing automation tools rely on simplified physical models—such as constant-capacitance approximations and
equilibrium Hubbard models—which assume instantaneous relaxation to a steady state. These frameworks fail in experimentally critical regimes where measurement rates exceed tunneling dynamics, necessitating more sophisticated non-
equilibrium control strategies. To bridge this gap, we introduce QArray+, an extension of the QArray framework that incorporates gate-dependent tunnel coupling and a
quantum open-system description of dissipative processes. This approach enables the unified simulation of coherent interdot charge-state hybridization and the non-
equilibrium latching dynamics essential for training robust machine-learning models for automated device operation. Implemented in JAX with GPU acceleration, QArray+ scales across GPUs and multi-node systems. For example, a charge stability diagram for a 100X100 grid of gate voltages over 64 dots can be computed in $\sim0.17\,\mathrm{s}$ on multiple GPUs. Since interdot interactions are short-ranged and the corresponding tuning corrections are local, simulations at these scales capture the physics relevant to even larger devices. These capabilities support high-throughput
dataset generation for automated device tuning.
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All authors Pranav Vaidhyanathan, Barnaby van Straaten, Alice Petrillo, Rahul Marchand, Edwin De Nicolo, Menno Veldhorst, Brucek Khailany, Taylor L. Patti, Natalia Ares
Quantum Physics
Effective quasiparticle conserving Lindbladians in the thermodynamic limit
Lea Lenke and Kai Phillip Schmidt
Open
quantum many-body systems are commonly described by Lindblad master equations, yet the treatment of Lindbladian operators in the thermodynamic limit remains a major challenge. We develop a framework for constructing effective quasiparticle-conserving Lindbladian operators directly in the thermodynamic limit. Our approach extends continuous similarity transformations to non-Hermitian open
quantum systems and enables the systematic block diagonalization of Lindbladians with respect to the quasiparticle (qp) number. We formulate two complementary methods. The first, projective continuous similarity transformations (pcst++), generalizes perturbative continuous
unitary transformations to Lindblad operators so that a linked-cluster expansion allows us to obtain high-order series expansions of the infinite system. The second, deepCST, extends directly evaluated enhanced perturbative continuous
unitary transformations by combining the same qp-conserving generator with a perturbative truncation scheme that yields non-perturbative effective Lindbladian operators directly in thermodynamic limit. We apply pcst++ and deepCST to the dissipative transverse-field Ising chain with local dissipation. We focus on the low-Ising regime. We purify the Lindbladian by splitting each
spin into two sites. A
spin flip then corresponds to two qps. We derive and analyze the effective, qp-conserving Lindbladian in the sectors with zero to two qps. We show how the Ising interaction renormalizes the decay rates of elementary
spin-flip excitations and provides the microscopic mechanism for a competition between coherent interactions and dissipation. Our work establishes perturbative and non-perturbative continuous similarity transformations as a versatile tool for deriving effective qp pictures of open
quantum many-body systems in the thermodynamic limit.
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Quantum Physics
Quantum amplitude estimation beyond power-of-two schedules
Farrokh Labib
Non-adaptive
quantum amplitude estimation (QAE) fixes its Grover depths in advance, so every
circuit can run in parallel, but it has so far needed more queries than the best adaptive methods. We show that most of this gap comes from two conventional choices: subspace-based post-processing and power-of-two depth ladders. We replace the first by the exact maximum-likelihood estimate, one
matrix multiplication per batch of estimates, and the second by a
geometric ladder with ratio $r \approx 1.45$. The result is a fully parallel, deterministic-schedule
estimator with total query complexity $2.8$-$3.1/\varepsilon$ at 95% confidence for target errors from $3.5\times 10^{-3}$ to $10^{-6}$. This matches the average-case complexity of chebAE, the best benchmarked adaptive method, within statistical uncertainty (with the lower point estimate at every scale tested), beats its maximum-observed complexity by $1.6\times$, and needs a maximum sequential depth of only $0.21/\varepsilon$ against chebAE's $2.9/\varepsilon$. Relative to csAE, the best non-adaptive
benchmark, the constants improve by 30-35% at 95% and $1.5$-$1.7\times$ at 99% confidence. The optimal ratio has a simple origin. Doubling is the fastest depth growth at which the data can still tell neighboring candidate values apart, so power-of-two ladders sit at the edge of confusion and must buy reliability with extra shots; a slightly denser ladder checks every scale redundantly. An error-probability analysis reproduces the measured failure rates and locates the optimum. The likelihood formulation extends directly to noise-aware estimation, and uniformly scaling the capped ladder covers the depth-limited regime, realizing the optimal trade-off $M N_{\mathrm{tot}} \approx (0.4$-$0.6)/\varepsilon^2$ within $\sim 1.1\times$ of the schedule's Cramér-Rao limit.
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Quantum Physics
Quantum Meta-Complexity Is All You Need: Characterizing One-Way Puzzles via Time-Bounded Kolmogorov Complexity
Morteza Saberikamarposhti
We initiate the time-bounded meta-complexity program for
quantum cryptography. Recent work characterizes one-way puzzles, the minimal search primitive of
quantum cryptography without one-way functions, by the average-case hardness of approximating the plain, uncomputable Kolmogorov complexity over quantumly samplable distributions; the classical program of Liu and Pass, by contrast, lives at
polynomial time bounds. We define a probabilistic time-bounded
quantum program complexity pKq^t for classical strings and prove two unconditional
theorems. First, a
quantum coding
theorem: any string output by a
quantum polynomial-time sampler with probability delta admits a description of the information-theoretically optimal length log(1/delta) plus logarithmic terms, decodable by a
quantum machine in time O(sqrt(1/delta)) times a
polynomial, via amplitude amplification over the coherently executed sampler. Second, an exact characterization at subexponential time: one-way puzzles exist if and only if the gap problem for pKq at time bound 2^(n/2) poly(n) is weakly
quantum-average-hard, refining the plain-complexity characterizations. We then isolate the
polynomial-time coding
theorem as the single load-bearing open
conjecture of the program, prove that it implies the full
polynomial-time characterization, analyze why the classical derandomization proof resists quantization, and formulate a relativized barrier
conjecture delimiting string-valued meta-complexity at one-way puzzles.
Conjectures are labeled as such throughout.
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Quantum Physics
Transversal Gates and Magic State Distillation in an Optimally Synthesized Spin-Qubit Shuttling Bus
Pau Escofet, Andrii Semenov, Niall Murphy et al. (+4 more)
Fault-tolerant
quantum computing requires not only reliable logical
qubit storage, but also the ability to perform high-fidelity logical operations between error-corrected
qubits at scale. While much of the existing literature focuses on optimizing syndrome extraction for a single logical
qubit, the co-design of physical architectures that support both robust error correction and efficient logical computation remains an open challenge. In this work, we propose a multi-
qubit spin-
qubit shuttling bus architecture that addresses both requirements simultaneously. The architecture optimizes the physical
qubit layout for syndrome extraction and supports transversal two-
qubit logical gates between an arbitrary number of logical
qubits, achieving all-to-all logical connectivity through coherent
spin shuttling. We further propose an ancilla-sharing scheme that encodes multiple logical
qubits within a single logical element, compressing the physical footprint of the processor and improving long-range gate fidelity. Extending the architecture from a one-dimensional bus to a two-dimensional grid of shuttling tracks reduces the inter-
qubit distance, yielding consistent improvements in logical error. Finally, we apply the
Quantum Reverse Mapping methodology at the logical level to optimize the layout of a 15-to-1 magic state
distillation circuit, demonstrating how the transversal capabilities of the proposed architecture can be leveraged for universal fault-tolerant computation. Taken together, these results establish a principled co-design framework that bridges the physical, error-correction, and logical computation layers of the
quantum stack, and demonstrate that
spin-
qubit shuttling architectures are a viable and flexible substrate for scalable fault-tolerant
quantum computation.
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All authors Pau Escofet, Andrii Semenov, Niall Murphy, Elena Blokhina, Carmen G. Almudéver, Sergi Abadal, Eduard Alarcón
Quantum Physics
Heuristically optimizing, synthesizing, and prioritizing measurement settings for quantum state tomography
Sumukh S. Moudghalya, Anton Frisk Kockum and Akshay Gaikwad
A key task in many
quantum-computing applications, e.g.,
quantum simulation and
quantum state
tomography (QST), is to partition an arbitrary set of operators into mutually commuting subsets for efficient measurements. However, brute-force approaches to this task quickly become intractable as the number and dimensionality of operators grow. Here, we reformulate operator partitioning as a graph-coloring (GC) problem and develop an efficient computational framework to solve it, balancing accuracy and efficiency. Our framework enables leveraging a range of GC
algorithms, which we
benchmark for operator partitioning. Then, we demonstrate their utility in optimizing QST experiments, where determining non-overlapping data acquisition settings for QST is a major challenge, and prioritizing among these settings, i.e., selecting the experiments that provide the most information. We further show how to perform these experiments by synthesizing Clifford
circuits for joint measurement of commuting Pauli operators in multi-
qubit systems. We validate our framework across multi-
qubit (up to five
qubits), multi-qutrit (up to three qutrits), and hybrid
qubit-qutrit systems. Our results show that heuristic GC methods substantially reduce the number of required measurement settings for QST and enable priority-based scheduling that maximizes the information gain per experiment. The optimization converges within minutes on a student-grade laptop, providing speedups of several orders of magnitude over brute-force methods already for these relatively small
quantum systems. This demonstrates the potential of GC heuristics as a scalable and practical tool for characterization of noisy intermediate-scale
quantum devices. We have made the Python implementation of our GC framework to optimize and schedule QST experiments publicly available at https://github.com/ssm8015/QST_GT.git.
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Quantum Physics
The advantages of extended nonreciprocal quantum batteries
Meng-Long Song, Zan Cao, Hai-Tao Dong et al. (+4 more)
This study investigates the performance of extended nonreciprocal
quantum batteries (QBs), as well as its advantages in energy storage and energy transfer compared to reciprocal charging and the original nonreciprocal batteries. After analyzing the detuning between the charging system and the external pump, we discover that resonance is a key factor in maintaining high-energy batteries and high charging power; furthermore, the detuning of the charger or battery determines the stability of the charging process for different structures. Research on steady-state energy storage in batteries revealed that single-threaded or multi-threaded charging can achieve nearly infinite energy storage in weakly localized environments, thereby demonstrating the significant energy advantages of extended nonreciprocal
quantum batteries. Finally, by considering the energy distribution within the charging system, we observe that nonreciprocal charging offers energy transfer advantages unmatched by reciprocal charging; the former achieves a comprehensive balance between charging cost and energy storage capacity that the latter cannot match. As a novel and superior charging protocol, our findings are expected to provide a potent reference for the promotion and practical implementation of nonreciprocal charging.
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All authors Meng-Long Song, Zan Cao, Hai-Tao Dong, Si-Yu Zhang, Xue-Ke Song, Liu Ye, Dong Wang
Quantum Physics · Cross-listed from physics.optics
Exceptional Topological Signatures of Non-Hermitian Photonic Hopf-Link Braids
Samit Kumar Gupta
Non-Hermitian physics endows the non-Abelian systems with exceptional
topology characterized by non-commutative braid patterns. Interplay of distinct competing sources of non-Hermiticity may induce novel topological effects. Here, we provide a generalized Hatano-Nelson model with higher-order nonreciprocal hoppings, non-Abelian gauge fields, and staggered gain-loss processes showing the exceptional topological structure of the Hopf-link braids that undergoes a EP-mediated topological phase transition. We demonstrate that mixing multiple nonreciprocal channels drives the system into highly intricate, nested complex energy Hopf-link braids and expands the topological landscape up to higher-order braiding sectors. Furthermore, utilizing biorthogonal eigenvector tracking, we map the structural
evolution of the exceptional phase boundaries via the maximum Petermann factor in the phase angle parameter planes. We show that the gain-loss non-Hermiticity drives a topological crossover where the extended exceptional contours constrict into isolated regimes. The demonstration of EP-mediated topological phase transition of Hopf-link braids and the associated rich exceptional phase portraits may offer new physical insights with promising applications in robust, fault-tolerant communication channels and
quantum computing platforms.
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Quantum Physics
Long-lived telecom-heralded single-photon storage in an absorptive spin-rephased quantum memory
Alberto E. Rodríguez-Moldes, Félicien Appas, Jonathan Hänni et al. (+3 more)
Long-lived storage of single
photons under the form of atomic excitations is at the foundation of long-distance
entanglement distribution in
quantum networks. To mitigate decoherence effects induced by the environment, rephasing of the hyperfine coherences using microwave pulses have been implemented in a variety of single-emitter and ensemble-based solid-state systems. However, the demonstration of storage of single
photons in an absorptive
quantum memory including such
spin rephasing mechanism remains elusive. In this work, we show non-classical storage of telecom-heralded single
photons in a Pr$^{3+}$:Y$_2$SiO$_5$ rare-earth ion doped
crystal quantum memory using the atomic frequency comb (AFC)
spin-wave protocol combined with a XY4
spin rephasing sequence. Long-lived AFC
photon echoes are first observed in the classical regime for storage times of up to approximately 3 ms. We then demonstrate non-classical correlations between heralding
photons and stored signal
photons generated by a cavity-enhanced parametric
photon-pair source for storage times of up to 180 $\mu$s and with measured cross-correlation values as high as 4.6(4). Together with the capacity of Pr$^{3+}$:Y$_2$SiO$_5$ QMs to support highly efficient and multiplexed storage, this result represents a significant step towards scalable long-distance
quantum repeater links.
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All authors Alberto E. Rodríguez-Moldes, Félicien Appas, Jonathan Hänni, Jelena V. Rakonjac, Samuele Grandi, Hugues de Riedmatten
Quantum Physics
Thouless pumping and generation of squeezed Fock-state superpositions in a Fock-state lattice
Ruo Kun Cai, Ling Lin, Chun Wang Wu et al. (+2 more)
In this paper, Thouless pumping in a one-dimensional semi-infinite Fock-state
lattice is investigated. A distinctive feature of such
lattices is the intrinsic $\sqrt{n}$-dependent coupling arising from the bosonic mode, which leads to spatially nonuniform hopping amplitudes. In the dimer limit, the topological invariants and the quantized transport dynamics in the Fock-state basis are numerically evaluated and analyzed. By introducing an additional inter-
cell coupling and applying a squeezing transformation, the framework is then extended to Thouless pumping in the squeezed Fock-state basis, where a topologically protected scheme for preparing superpositions of squeezed Fock states is proposed. This study establishes Thouless pumping in Fock-state
lattices as a useful tool for
quantum state engineering, shifting the focus from observing topological transport to harnessing it for the preparation of non-classical states of the bosonic mode.
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All authors Ruo Kun Cai, Ling Lin, Chun Wang Wu, Zhi Jiao Deng, Ping Xing Chen
Quantum Physics
Combinatorial optimization of connected UAV communication bridges for emergency response
Matteo Vandelli and Daniele Dragoni
We present a combinatorial optimization problem for the strategic deployment of
UAVs equipped with 5G
antennas to assist rescue operations in regions hit by natural disasters. Our goal is to optimize the placement of
UAVs to provide coverage in flying ad-hoc networks among given candidate sites. Our formulation aims to maximize signal coverage and minimize interference while ensuring network connectivity. To mitigate interference effects, we incorporate the use of multiple frequencies. We formulate this problem as an integer quadratic program (IQP). We present numerical solutions obtained via the CPLEX solver and conduct a preliminary analysis of the problem's scalability in realistic network configurations. Our findings reveal a significant exponential increase in Time-to-Solution (TTS) as the number of sites grows, which poses a critical challenge in urgent, time-sensitive scenarios. To address this issue, approximate suboptimal solutions can be produced by enforcing a time limit on the solver. Although these solutions are not optimal, they preserve connectivity in most cases, providing a practical trade-off between solution quality and computational times that remain within feasible limits for real-time
UAV redeployment. Recognizing the limitations of classical solvers in these contexts, we explore
quantum computing as a promising alternative. Specifically, we reformulate the problem as a quadratic unconstrained binary optimization (QUBO) problem, suitable for most
quantum algorithms. Through high-performance computing emulation, we show that the
quantum adiabatic
algorithm (QAA) can accurately solve small-scale instances, paving the way for future application of
quantum computing to large-scale, time-critical optimization problems in disaster response.
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Quantum Physics
Optimal Fusion Strategies for Quantum Computation
Kenneth Goodenough, Andrew Landahl, Joon Lee et al. (+2 more)
Logical fusions are important for a number of tasks in
quantum information, such as
quantum error correction and
quantum repeaters. In the photonic setting one must contend with the fact that physical fusions can fail, i.e. individual
qubits are measured in a user-controlled product state basis with some probability, which can lead to a failure on the logical level. The choice of failure basis of each
qubit is known as a fusion strategy, and finding good fusion strategies is important for optimizing performance of fusion-based
quantum computation. Here we provide a complete characterization when $k=1$
qubits are encoded, and in particular characterize those codes and fusion strategies such that all but one physical fusion can fail, i.e.~perfect fusion strategies. In doing so, we recover previously known perfect fusion strategies, and find perfect fusion strategies for
quantum parity-check codes, answering an open question. We furthermore show that perfect fusion strategies are generic: random $[[n, 1, d]]$ graph codes admit a perfect fusion strategy with probability exponentially close to $1$. Additionally we motivate the study of a new graph parameter, namely the maximum degree of a graph at a given vertex taken over all LC-equivalent graphs, by giving a new operationally meaningful interpretation of it.
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All authors Kenneth Goodenough, Andrew Landahl, Joon Lee, Antonio Russo, Kevin Thompson
Quantum Physics
Experimental Evaluation of Passive Polarization Compensation Techniques for Fiber-Distributed Polarization-Entangled Photons
Gayatri Thik, Amit Loyal, Srinivasan K et al. (+1 more)
Entanglement distribution through optical fibers is essential for
quantum communication networks; however, fiber transmission can alter
photon polarization and modify the observed correlations of polarization-entangled states, necessitating polarization compensation to recover the desired entangled state in the measurement basis. Here, we experimentally evaluate two passive polarization compensation techniques: a free-space Quarter-Half-Quarter (QHQ) waveplate configuration and an in-fiber three-paddle Fiber Polarization Controller (FPC). The polarization transformation along each downconverted-
photon path is independently compensated through a systematic path-by-path optimization procedure. Using both techniques, we recover high-quality polarization correlations and
entanglement, with visibilities exceeding $93\%$ in three mutually unbiased bases and fidelities above $94.5\%$. The results demonstrate comparable restoration using free-space waveplate-based and fiber-based control, establishing a systematic framework for laboratory and short-reach
quantum communication links.
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All authors Gayatri Thik, Amit Loyal, Srinivasan K, Raghavan G
Quantum Physics · Cross-listed from cond-mat.stat-mech
Branching stochastic mechanics: Relative localization and collective poles from Bohm/Fisher feedback
Benoit Bischoff and Eric Dumonteil
Branching
stochastic mechanics (BSM) provides a reciprocal branching representation of the Schrödinger–Nagasawa pair. Its centered forward–backward kernel $C_{\rm FB}=\mathbb E_\omega[\psi_F\psi_B]$ resolves an organized connected sector, with weight $\rho_{\rm BSM}=-C_{\rm FB}(x,x)$ on the anticorrelated branch. Here we investigate how this sector forms, localizes, and propagates under Bohm/Fisher feedback. We retain the branching covariance on the prescribed background as the bare noise kernel and truncate the nonlinear interaction to Bohm/Fisher drift vertices. A Martin–Siggia–Rose–Janssen–de Dominicis formulation and a causal two-loop two-particle-irreducible (2PI) closure determine response and correlation functions self-consistently. While the free connected theory exhibits secular growth and ultraviolet accumulation, the dressed theory develops a finite relative screening length. A reduced numerical
evolution shows bounded formation of the localized sector, and a self-similar Fisher construction defines a saturated information velocity $c_\star$. A Born–Oppenheimer separation connects this internal organization to collective propagation. The instantaneous adiabatic kernel has two pole families at fixed internal momentum: a gapless difference branch and a gapped sum branch. Projecting both inverse response kernels onto the same localized internal profile defines their collective coefficients. If both propagation speeds match $c_\star$ and the projected gap matches $mc_\star^2/\hbar$, the gapped branch takes the infrared Klein–Gordon form. These matching conditions define a candidate relativistic fixed point whose dynamical realization remains to be tested.
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Quantum Physics
Efficient Simulation of Nonreciprocal Many-body Physics via Quantum Feedback
Kseniia Vodenkova, Andrew Pocklington, Fan Yang et al. (+3 more)
We present a scheme to efficiently simulate nonreciprocal many-body
spin models using a non-Markovian open system. Our scheme utilizes a single
quantum emitter coupled to a waveguide mode that is fed back to the same emitter after a time delay. With this coherent delayed feedback, an effective nonreciprocal interaction can be engineered, wherein the emitter at earlier times affects itself at later times, creating a scalable
spin chain. We show that all the steady-state quantities of such
spin models can be accessed through measurement of the emitter and the output field. Furthermore, using classical feedback that resets the emitter, quench dynamics from arbitrary product states can be simulated. We demonstrate striking features of nonreciprocal models, such as the Liouvillian skin effect, anomalous relaxation dynamics, and quasi-long-range order of output
photons. Finally, we show that the proposal is amenable to experimental realization and robust to imperfections. Our work establishes a feasible way to scale up a nonreciprocal many-body system, and provides a new route towards generation of exotic states of light.
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All authors Kseniia Vodenkova, Andrew Pocklington, Fan Yang, Andrew Lingenfelter, Aashish A. Clerk, Hannes Pichler
Quantum Physics
Single-Shot Fidelity Reveals Hard and Soft Limits: A Universal Yardstick for Photon-Number-Resolving Detectors
Tetsuya Tsuruta, Akio Yoshizawa and Daiji Fukuda
Photon-number-resolving (PNR) detectors are essential for photonic
quantum computing, where a single measurement outcome must reliably herald a specific
quantum state. However, detector fidelity is conventionally evaluated using ensemble-averaged statistics obtained from many measurements, which can remain high even when individual
photon-number assignments are frequently misidentified. Here we introduce a universal single-shot fidelity that directly quantifies the probability of correctly identifying a
photon number in a single measurement. The framework combines an efficiency-based POVM with a resolution-driven confusion
matrix derived from the detector response, allowing
photon loss and
photon-number misidentification to be treated separately and then recombined into a single operational metric. This distinction reveals two fundamentally different limitations. Detection-efficiency loss represents an unrecoverable hardware constraint, whereas resolution-driven misidentification can be reduced by introducing a rejection region, trading generation rate for confidence. Because the metric is defined independently of detector architecture, it enables direct comparison between energy-resolving detectors such as transition-edge sensors and multiplexed click-based detectors on the same footing. Applying the framework to calibration data from three distinct detector architectures, we demonstrate quantitative comparison across
photon-number regimes relevant to both discrete-variable and continuous-variable photonic
quantum computing. The resulting
benchmark provides a common operational metric for evaluating
photon-number-resolving detectors and connecting detector performance to photonic
quantum-computing requirements.
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Quantum Physics
Engineering of Non-Hermitian Trajectories and Phase Structure in an Open Bose-Hubbard Model via Rate Operator Transformations
Jaakko Luomala, Kimmo Luoma, Iiro Vilja et al. (+1 more)
Non-Hermitian
evolution can be realized through post-selection on
stochastic pure-state trajectories arising in continuously monitored open
quantum systems. The rate operator formalism provides a versatile and systematic framework for unraveling a master equation into
stochastic pure-state evolutions, offering enhanced control over the resulting non-Hermitian dynamics. In the present work, we explore the applicability of the rate operator formalism as a tool for engineering non-Hermitian dynamics. Specifically, we apply this approach to the Bose-Hubbard model subject to environmental dephasing, examining its consequences for controlled state manipulation. Our analysis is framed within the broader contexts of
quantum state engineering and measurement-induced phase transitions. We demonstrate that the rate operator formalism enables the construction of effective non-Hermitian
Hamiltonians exhibiting a unique steady state-even in regimes where the standard Monte Carlo wavefunction method fails to produce one. Furthermore, we show that this framework facilitates transitions between distinct steady-state phases, governed by tunable parameters such as the interaction strength and a non-Hermiticity control parameter introduced via the rate operator formalism.
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All authors Jaakko Luomala, Kimmo Luoma, Iiro Vilja, Jyrki Piilo
Quantum Physics · Cross-listed from math-ph
Precise spectral asymptotics, exponential localization, and spectral gap estimates for the three-boson lattice Schrödinger operator
Janikul Abdullaev, Abdumalik Eshniyozov, Abdikhurayra Toshturdiev et al. (+1 more)
We study three identical
bosons with contact attraction of strength mu on the square
lattice at total quasimomentum K = 0 and K = pi. First, at K = 0, for all sufficiently large mu there are exactly two
eigenvalues below the essential spectrum. The trimer branch follows from Kato perturbation theory at the simple isolated
eigenvalue 3 of the pair-contact operator, uniformly in K; the atom-dimer branch follows from a Feshbach-Schur reduction whose limiting equation coincides with the second Birman-Schwinger (Faddeev)
eigenvalue of the finite-rank principal part. The spectral gap between the two branches grows linearly in mu. Second, at K = pi the trimer branch is unchanged up to O(mu^-2), the two-particle threshold equals -mu + 4 exactly, and the gap to it is 2mu - 2 plus lower order corrections. The second bound state disappears because the exchange operator changes sign, S_pi = -S_0: the s-channel
eigenvalue that exceeds 1 at K = 0 becomes negative, while the only positive atom-dimer channel at K = pi is the odd p-channel, bounded by 2 - 4/pi < 1. Consequently the operator has exactly one
eigenvalue below the threshold minus 36/(mu - 10). Third, the ground state is exponentially localized in the relative coordinates. The decay exponent is positive unconditionally; under a non-cancellation hypothesis it is bounded above by log(3mu) plus lower order terms, while finite-volume computations indicate the slowest tail is the atom-dimer dissociation channel with decay rate one half log(2mu). A dictionary between the principal-part method, Kato theory, and the atom-dimer threshold
matrix is given, together with the three-dimensional constants for comparison.
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All authors Janikul Abdullaev, Abdumalik Eshniyozov, Abdikhurayra Toshturdiev, Mikhail Dolgopolov
Quantum Physics
Accurate theoretical methods for photoionization of H$_2$ molecules
Hakon Volkmann, Jannis Schürmann and Alejandro Saenz
Single-
photon ionization cross sections of molecular hydrogen in the electric dipole limit are numerically computed by employing three independent methods. Both time-dependent and -independent approaches within the clamped-
nuclei approximation at
equilibrium internuclear distance are used, featuring the explicit time-propagation of the time-dependent Schrödinger equation and newly developed multi-channel configuration-interaction free-boundary as well as complex-scaling methods using an explicitly correlated geminal basis set. The found results show convincing mutual agreement despite their entirely different fundamental formulations. They further highlight the challenges in bringing together different theoretical predictions from literature with the experimental data at high
photon energies. Overall, the novel CI-based approach demonstrates fast and controllable convergence while being able to provide full channel-resolved information, indicating the need for more accurate experimental data.
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Quantum Physics
Circuit-Level Loss Performance of RHG and Foliated Floquet Color Codes in a Compound Photon–Atom Quantum Architecture
Dana Ben Porath, Juval Bechar, Daniel Azses et al. (+1 more)
A central question for fault-tolerant
quantum computing is which
quantum error-correcting codes are best suited to a given hardware architecture. Here we compare the Raussendorf–Harrington–Goyal (RHG) code, the Foliated Floquet Color Code (FFCC), and the reduced FFCC in a compound
photon–atom architecture that directly generates measurement-based
quantum computation (MBQC) resources with near-deterministic
photon–atom CZ gates. RHG serves as a natural
benchmark, while the FFCC variants allow us to study whether reduced graph degree improves performance under an architecture-aware
circuit-level loss model with delayed heralding and correlated bond-loss propagation. We construct two generation schemes compatible with the compound hardware and evaluate
circuit-level thresholds under periodic boundary conditions. RHG achieves the highest
circuit-level threshold, 2.75%, and its threshold falls below that of reduced FFCC only for large excess loss on intermodule CZ connections. RHG also achieves the lowest logical error rate in most resource-matched comparisons, but some low-loss windows favor reduced FFCC. Overall, we show that when the hardware supports the native gates and connectivity required for MBQC, the benefits of lower graph degree must be weighed against each code's intrinsic IID loss tolerance, generation-scheme details, and hardware-aware resource overhead.
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All authors Dana Ben Porath, Juval Bechar, Daniel Azses, Yaron Jarach
Quantum Physics
Programming anharmonic potentials in a superconducting harmonic oscillator
Clara Yun Fontaine, Mansi Somani, Kehui Yu et al. (+9 more)
Continuous-variable
quantum systems offer a resource-efficient route to universal
quantum information processing and analogue
quantum simulation of real-world processes, such as molecular physics and chemical reactions. Realising these applications, however, requires non-Gaussian operations that implement anharmonic potentials, which are challenging to engineer on demand. Here, we demonstrate a systematic framework to implement programmable non-Gaussian phase gates $e^{-iV(\hat{X})}$, corresponding to the impulsive action of a potential $V(\hat{X})$, in a superconducting harmonic oscillator coupled to a transmon
qubit. Using modular
circuits derived from bosonic
quantum signal processing, we realise a range of target anharmonic potentials on a single piece of hardware by varying a set of
qubit rotations interleaved with a fixed calibrated control
unitary. We first demonstrate a cubic phase gate, a key ingredient for universal
quantum information processing. The resulting high-fidelity non-Gaussian states and the potential reconstructed using our pointwise force reconstruction method jointly confirm the cubic nature of the target gate. We then engineer a family of double-well potentials, relevant models of tunnelling and biased transfer processes, and experimentally validate the double-well
topology and the tunable asymmetry. Finally, we engineer an approximate Morse gate, a step towards realistic potentials of molecular vibrational systems, and provide a concrete path towards high-quality engineering and reconstruction of the exponential form. Together, these results establish a practical and reconfigurable route towards continuous-variable
quantum information processing and anharmonic
quantum simulation.
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All authors Clara Yun Fontaine, Mansi Somani, Kehui Yu, May Chee Loke, Jonathan Schwinger, Pak-Tik Fong, Ni-Ni Huang, Adrian Copetudo, Mustafa Bakr, Hoi-Kwan Lau, Tanjung Krisnanda, Yvonne Y. Gao