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Friday 6 February

Quantum Physics

Measurement-Based Preparation of Higher-Dimensional AKLT States and Their Quantum Computational Power

We investigate a constant-time, fusion measurement-based scheme to create AKLT states beyond one dimension. We show that it is possible to prepare such states on a given graph up to random spin-1 `decorations', each corresponding to a probabilistic insertion of a vertex along an edge. In investigating their utility in measurement-based quantum computation, we demonstrate that any such randomly decorated AKLT state possesses at least the same computational power as non-random ones, such as those on trivalent planar lattices. For AKLT states on Bethe lattices and their decorated versions we show that there exists a deterministic, constant-time scheme for their preparation. In addition to randomly decorated AKLT states, we also consider random-bond AKLT states, whose construction involves any of the canonical Bell states in the bond degrees of freedom instead of just the singlet in the original construction. Such states naturally emerge upon measuring all the decorative spin-1 sites in the randomly decorated AKLT states. We show that those random-bond AKLT states on trivalent lattices can be converted to encoded random graph states after acting with the same POVM on all sites. We also argue that these random-bond AKLT states possess similar quantum computational power as the original singlet-bond AKLT states via the percolation perspective.
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All authors Wenhan Guo, Mikhail Litvinov, Tzu-Chieh Wei, Abid Khan, Kevin C. Smith

Quantum Physics · Cross-listed from cs.OS

HALO: A Fine-Grained Resource Sharing Quantum Operating System

As quantum computing enters the cloud era, thousands of users must share access to a small number of quantum processors. Users need to wait minutes to days to start their jobs, which only takes a few seconds for execution. Current quantum cloud platforms employ a fair-share scheduler, as there is no way to multiplex a quantum computer among multiple programs at the same time, leaving many qubits idle and significantly under-utilizing the hardware. This imbalance between high user demand and scarce quantum resources has become a key barrier to scalable and cost-effective quantum computing. We present HALO, the first quantum operating system design that supports fine-grained resource-sharing. HALO introduces two complementary mechanisms. First, a hardware-aware qubit-sharing algorithm that places shared helper qubits on regions of the quantum computer that minimize routing overhead and avoid cross-talk noise between different users' processes. Second, a shot-adaptive scheduler that allocates execution windows according to each job's sampling requirements, improving throughput and reducing latency. Together, these mechanisms transform the way quantum hardware is scheduled and achieve more fine-grained parallelism. We evaluate HALO on the IBM Torino quantum computer on helper qubit intense benchmarks. Compared to state-of-the-art systems such as HyperQ, HALO improves overall hardware utilization by up to 2.44x, increasing throughput by 4.44x, and maintains fidelity loss within 33%, demonstrating the practicality of resource-sharing in quantum computing.
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All authors John Zhuoyang Ye, Jiyuan Wang, Yifan Qiao, Jens Palsberg

Quantum Physics · Cross-listed from physics.atom-ph

Two-photon-excited fluorescence spectroscopy of Rb atoms in a magneto-optical trap

We report the results of two-photon-excited fluorescence (TPEF) measurements of the $5\mathrm{S}_{1/2} \rightarrow 5\mathrm{D}_{5/2}$ transition of $^{85}$Rb and $^{87}$Rb cooled in a magneto-optical trap (MOT). We observe TPEF at excitation powers as low as 1 $\mu$W or fluxes as low as $2.98_{-0.94}^{+1.37} \times 10^{18}\ \text{photons}\,\text{cm}^{-2}\,\text{s}^{-1}$ ($^{85}$Rb) and $3.31_{-1.33}^{+2.21} \times 10^{18}\ \text{photons}\,\text{cm}^{-2}\,\text{s}^{-1}$ ($^{87}$Rb). Our results demonstrate that optically cooled Rb is a promising platform for observing sensitive two-photon spectral signatures at low photon fluxes.
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All authors Alan McLean, Christian Drago, Daniel Podos, Chengyi Luo, Caleb Brzezinski, Ting-Wei Hsu, John Sipe, Ralph Jimenez

Quantum Physics

Putting fermions onto a digital quantum computer

Quantum computers are expected to become a powerful tool for studying physical quantum systems. Consequently, a number of quantum algorithms for studying the physical properties of such systems have been developed. While qubit-based quantum computers are naturally suited to the study of spin-1/2 systems, systems containing other degrees of freedom must first be encoded into qubits. Transformations to and from fermionic degrees of freedom have long been an important tool in physics and, now the simulation of fermionic systems on quantum computers based on qubits provides yet another application. In this perspective, we review methods for encoding fermionic degrees of freedom into qubits and attempt to dispel the persistent notion that fermionic systems beyond one dimension are fundamentally more difficult to deal with.
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All authors Riley W. Chien, Mitchell L. Chiew, Brent Harrison, Jason Necaise, Weishi Wang, Maryam Mudassar, Campbell McLauchlan, Thomas M. Henderson, Gustavo E. Scuseria, Sergii Strelchuk, James D. Whitfield

Quantum Physics

Performance limits of a quantum receiver for detecting phase-modulated communication signals

Quantum sensors are an ideal candidate for detecting weak electromagnetic signals because of their exceptional sensitivity and compact form factor. In this work, we analyze the performance of a quantum-sensor-based receive chain for demodulating information encoded in phase-modulated electromagnetic waves. We introduce a generalized cumulant expansion to model a noisy quantum receiver and use it to compare the performance of various quantum demodulation protocols. Employing bit error probability (BEP) and channel capacity as quantitative performance metrics, we compare the capabilities of ensembles of quantum sensors - both unentangled and entangled - using Binary Phase-Shift Keying (BPSK) as a representative example of phase modulation. We identify conditions when the channel capacity of an ensemble of quantum sensors may surpass the limits of a classical electrically small antenna. Additionally, we discuss modifications to the quantum protocol that enables high-fidelity data recovery even in the presence of sensor noise and channel distortions. Finally, we explore practical performance limits of such a quantum receive chain, with a focus on NV-diamond as the quantum sensor platform.
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Quantum Physics · Cross-listed from cond-mat.mes-hall

Revealing quantum geometry of solids through vacuum fluctuations

We show that quantum fluctuations of electromagnetic fields induce an extensive zero-point energy in insulators. The zero-point energy density is proportional to the quantum fluctuation of electric polarization in the many-body ground state, a fundamental quantum geometric property of solids known as the quantum weight. For fields produced by a superconducting LC circuit, the zero-point energy results in a repulsive force between the circuit and the material and an attractive force on the capacitor plates, which we call quantum geometric force. The force on the capacitor plates is proportional to the quantum weight, inversely proportional to the plate area, and independent of the plate distance. The proposed effects provide direct experimental access to the many-body quantum geometry and reveal a new quantum geometric effect in macroscopic solids induced by vacuum fluctuation.
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Quantum Physics

Entanglement harvesting in conformal field theory

We study entanglement harvesting in general $d$-dimensional conformal field theories using pointlike Unruh-DeWitt detectors coupled to scalar primary operators. This extends standard harvesting protocols beyond free fields to interacting conformal theories and arbitrary spatial dimensions. We find that increasing the operator scaling dimension suppresses both negativity and mutual information, reflecting the faster decay of correlations. For holographic CFTs, we show that bulk effective field theory enables a separation between field-harvested and communication-mediated entanglement. We also derive asymptotic, closed-form approximations that agree well with numerical results.
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All authors Kelly Wurtz, Caroline Lima, Robert C. Myers, Eduardo Martín-Martínez

Quantum Physics

Beyond Wigner: Non-Invertible Symmetries Preserve Probabilities

In recent years, the traditional notion of symmetry in quantum theory was expanded to so-called generalised or categorical symmetries, which, unlike ordinary group symmetries, may be non-invertible. This appears to be at odds with Wigner's theorem, which requires quantum symmetries to be implemented by (anti)unitary – and hence invertible – operators in order to preserve probabilities. We resolve this puzzle for (higher) fusion category symmetries $\mathcal{C}$ by proposing that, instead of acting by unitary operators on a fixed Hilbert space, symmetry defects in $\mathcal{C}$ act as isometries between distinct Hilbert spaces constructed from twisted sectors. As a result, we find that non-invertible symmetries naturally act as trace-preserving quantum channels. Crucially, our construction relies on the symmetry category $\mathcal{C}$ being unitary. We illustrate our proposal through several examples that include Tambara-Yamagami, Fibonacci, and Yang-Lee as well as higher categorical symmetries.
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Quantum Physics

Alleviating Post-Linearization Challenges for Solving Nonlinear Systems on a Quantum Computer

The linearity inherent in quantum mechanics limits current quantum hardware from directly solving nonlinear systems governed by nonlinear differential equations. One can opt for linearization frameworks such as Carleman linearization, which provides a high dimensional infinite linear system corresponding to a finite nonlinear system, as an indirect way of solving nonlinear systems using current quantum computers. We provide an efficient data access model to load this infinite linear representation of the nonlinear system, upto truncation order $N$, on a quantum computer by decomposing the Hamiltonian into the weighted sum of non-unitary operators, namely the Sigma basis. We have shown that the Sigma basis provides an exponential reduction in the number of decomposition terms compared to the traditional decomposition, which is usually done in a linear combination of Pauli operators. Once the Hamiltonian is decomposed, we then use the concept of unitary completion to construct the circuit for the implementation of each weighted tensor product component $\mathcal{H}_{j}$ of the decomposition.
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Quantum Physics · Cross-listed from cond-mat.str-el

Charge-$4e$ superconductor with parafermionic vortices: A path to universal topological quantum computation

Topological superconductors (TSCs) provide a promising route to fault-tolerant quantum information processing. However, the canonical Majorana platform based on $2e$ TSCs remains computationally constrained. In this work, we find a $4e$ TSC that overcomes these constraints by combining a charge-$4e$ condensate with an Abelian chiral $\mathbb{Z}_3$ topological order in an intertwined fashion. Remarkably, this $4e$ TSC can be obtained by proliferating vortex-antivortex pairs in a stack of two $2e$ $p+ip$ TSCs, or by melting a $\nu=2/3$ quantum Hall state. Specific to this TSC, the $hc/(4e)$ fluxes act as charge-conjugation defects in the topological order, whose braiding with anyons transmutes anyons into their antiparticles. This symmetry enrichment leads to $\mathbb{Z}_3$ parafermion zero modes trapped in the elementary vortex cores, which naturally encode qutrits. Braiding the parafermion defects alone generates the full many-qutrit Clifford group. We further show that a single-probe interferometric measurement enables topologically protected magic-state preparation, promoting Clifford operations to a universal gate set. Because the non-Abelian modes are bound to flux defects, they can, in principle, be externally controlled using superconducting circuit-based technology. More broadly, our results highlight hierarchical electron aggregation, the formation and condensation of higher-charge electron clusters, as a design principle for topological quantum matter with increased computational capability.
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All authors Zhengyan Darius Shi, Zhaoyu Han, Srinivas Raghu, Ashvin Vishwanath

Quantum Physics

Causally constrained quantum operator dynamics

Finding analytically tractable model systems for quantum ergodicity breaking in interacting systems remains a theoretical challenge. This paper presents an algebraic theory of robust ergodicity breaking in time-periodic unitary circuits under local causal constraints. Our model is based on constructing tripartite unitaries (we dub 'walls') that permanently arrest local operator spreading. We show that the structure of the resulting causally independent subsystems can be understood rigorously through the invariance of embedded operator algebras (i.e. super-operator symmetries). This formalism gives a natural identification of local conserved quantities and permits the generalisation to time-dependent dynamics. Using representation theory, the general form of unitaries exhibiting causal decoupling is derived from the automorphism group of the embedded algebra with links to quantum error-correcting codes. From the point of view of operator spreading, our theory is a minimal model for non-ergodic quantum-circuit dynamics, and we explore its effects on probes of many-body quantum chaos. We prove an entanglement area law due to causal constraints and discuss its stability against local measurements. In a random unitary ensemble with causally independent subsystems, we compare spectral correlations with those of the universal random matrix ensemble using the spectral form factor. Our results offer a rigorous understanding of locally constrained quantum dynamics from a quantum information perspective.
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Quantum Physics

The Quantum Sieve Tracer: A Hybrid Framework for Layer-Wise Activation Tracing in Large Language Models

Mechanistic interpretability aims to reverse-engineer the internal computations of Large Language Models (LLMs), yet separating sparse semantic signals from high-dimensional polysemantic noise remains a significant challenge. This paper introduces the Quantum Sieve Tracer, a hybrid quantum-classical framework designed to characterize factual recall circuits. We implement a modular pipeline that first localizes critical layers using classical causal tracing, then maps specific attention head activations into an exponentially large quantum Hilbert space. Using open-weight models (Meta Llama-3.2-1B and Alibaba Qwen2.5-1.5B-Instruct), we perform a two-stage analysis that reveals a fundamental architectural divergence. While Qwen's layer 7 circuit functions as a classic Recall Hub, we discover that Llama's layer 9 acts as an Interference Suppression circuit, where ablating the identified heads paradoxically improves factual recall. Our results demonstrate that quantum kernels can distinguish between these constructive (recall) and reductive (suppression) mechanisms, offering a high-resolution tool for analyzing the fine-grained topology of attention.
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Quantum Physics

Consensus Protocols for Entanglement-Aware Scheduling in Distributed Quantum Neural Networks

The realization of distributed quantum neural networks (DQNNs) over quantum internet infrastructures faces fundamental challenges arising from the fragile nature of entanglement and the demanding synchronization requirements of distributed learning. We introduce a Consensus-Entanglement-Aware Scheduling (CEAS) framework that co-designs quantum consensus protocols with adaptive entanglement management to enable robust synchronous training across distributed quantum processors. CEAS integrates fidelity-weighted aggregation, in which parameter updates are weighted by quantum Fisher information to suppress noisy contributions, with decoherence-aware entanglement scheduling that treats Bell pairs as perishable resources subject to exponential decay. The framework incorporates quantum-authenticated Byzantine fault tolerance, ensuring security against malicious nodes while maintaining compatibility with noisy intermediate-scale quantum (NISQ) constraints. Our theoretical analysis establishes convergence guarantees under heterogeneous noise conditions, while numerical simulations demonstrate that CEAS maintains 10-15 percentage points higher accuracy compared to entanglement-oblivious baselines under coordinated Byzantine attacks, achieving 90 percent Bell-pair utilization despite coherence time limitations. This work provides a foundational architecture for scalable distributed quantum machine learning, bridging quantum networking, distributed optimization, and early fault-tolerant quantum computation.
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All authors Kuan-Cheng Chen, Samuel Yen-Chi Chen, Mahdi Chehimi, Felix Burt, Kin K. Leung

Quantum Physics

Comment on "Relativistic covariance and nonlinear quantum mechanics: Tomonaga-Schwinger analysis”

Contrary to the central claim (Hsu, 2026) published in Physics Letters B, the Tomonaga–Schwinger equation remains covariant despite the nonlinear modification of it. The proof of covariance becomes simple after the loopholes and mistakes in Hsu's arguments are identified.

Quantum Physics

Hybrid Coupling Topology with Dynamic ZZ Suppression for Optimizing Circuit Depth during Runtime in Superconducting Quantum Processor

To reduce circuit depth when executing Quantum algorithms, it is necessary to maximize qubit connectivity on a near-term quantum processor. While addressing this, we also need to ensure high gate fidelity, suppression of unwanted ZZ cross-talk, a compact layout footprint, and minimal control hardware complexity to support scalability. In current superconducting quantum chips, fixed coupling is used as it is easier to scale, but it is limited by unwanted static ZZ interaction during single qubit operations, which degrades system performance. To overcome these challenges, we have introduced a first-of-its-kind hybrid tunable-coupling architecture that connects four fixed-frequency transmon qubits using a single coupler. This hybrid coupler uses off-resonant Stark drives to tune ZZ strength between qubit pairs. Experimentally backed simulation results indicate that our proposed hybrid design maximizes the qubit connectivity while reducing control overhead. This design achieves a near 20% reduction in circuit depth compared to IBM's Heavy-Hexagonal layout, showing its potential for scalability.
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All authors Uday Sannigrahi, Amlan Chakrabarti, Swapnil Saha, Shrinjita Biswas

Quantum Physics

Phase-sensitive characterization of a quantum frequency converter by spectral interferometry

We introduce an experimental technique for complete phase-sensitive characterization of arbitrary unitary spectral-temporal transformations of optical modes. Our method recovers the complex spectral transfer function, or Green's function, of a frequency converter by analyzing spectral interference in the response to a tunable bichromatic probe. We perform a proof-of-concept experiment on a frequency conversion module based on Bragg-scattering four-wave mixing in photonic crystal fiber. Our results validate our technique by recovering useful information in the phase of the Green's function, revealing the relative positions of regions of active frequency conversion and passive dispersive propagation within the module. Our work introduces a new approach to characterizing the performance of a variety of active devices with diverse applications in emerging quantum technologies.
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All authors Mateusz J Olszewski, Kasper Hecht Alexander, Michael T M Woodley, Leah R Murphy, Peter J Mosley, Alex O C Davis

Quantum Physics

Heralding efficiency and brightness optimization of a micro-ring resonator via tunable coupling

Efficient and bright single photon sources on photonic chips are key to scaling quantum technologies. Spontaneous four wave mixing in micro-ring resonators creates excellent narrowband and tunable photon sources. We experimentally demonstrate the optimization of heralding efficiency and brightness by tuning the coupling of the pump, signal and idler modes into the resonator whilst operating in a pulsed pump regime. We observe a maximum detected pair rate of over 93,000~coincidences per second in a moderately over-coupled regime, alongside a high intrinsic idler heralding efficiency of 97.87$\pm$8.97% when operating close to maximal over-coupling. The measured dependence on coupling strength is in strong agreement with theoretical predictions, experimentally validating the predicted trade-off between brightness and heralding efficiency.
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All authors Nathan Moses, Marcus J. Clark, Alex S. Clark, Siddarth K. Joshi, Imad I. Faruque

Quantum Physics · Cross-listed from physics.atom-ph

Quasiperiodic nondipole ionization dynamics in the x-ray stabilization regime

Recent advances in strong x-ray laser techniques enable the study of nonlinear multiphoton ionization in extreme high-frequency fields. Although the stabilization regime in such fields is theoretically established, its modified properties in the nondipole regime for long laser pulses remains unknown. Here, we numerically investigate the strong-field ionization of an atom in a long XUV laser pulse in the nondipole regime. Our study of the time-dependent quantum dynamics reveals a quasiperiodic modulation of the ionization yield as a function of pulse duration. We demonstrate that the Coulomb-field-induced slow oscillation of the ionized electron wave packet during the interaction is responsible for the observed modulation of the ionization yield. Furthermore, we scrutinize the unusual photon momentum sharing between the photoelectron and the ion in this extreme regime. These effects are observable in upcoming x-ray free-electron laser facilities.
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Quantum Physics

Bridging Quantum and Semi-Classical Thermodynamics in Cavity QED

In cavity quantum electrodynamics (QED), photons leaving the cavity can be irreversibly lost or reused as a power source. This dichotomy is reflected in two different thermodynamic bookkeepings of the light field, both corresponding to valid thermodynamic frameworks. In this work, we formulate a rigorous semi-classical limit of cavity QED and show that the resulting thermodynamic description may qualitatively differ from that of the fully quantised model. We find that violations of the thermodynamic uncertainty relations are recovered in the semi-classical limit only by one of the two thermodynamic frameworks: the one which treats part of the photon flux as a power source. We illustrate our findings in a three-level system coupled to a driven cavity.
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All authors Marcelo Janovitch, Sander Stammbach, Matteo Brunelli, Patrick P. Potts

Quantum Physics · Cross-listed from cs.LG

Disentanglement by means of action-induced representations

Learning interpretable representations with variational autoencoders (VAEs) is a major goal of representation learning. The main challenge lies in obtaining disentangled representations, where each latent dimension corresponds to a distinct generative factor. This difficulty is fundamentally tied to the inability to perform nonlinear independent component analysis. Here, we introduce the framework of action-induced representations (AIRs) which models representations of physical systems given experiments (or actions) that can be performed on them. We show that, in this framework, we can provably disentangle degrees of freedom w.r.t. their action dependence. We further introduce a variational AIR architecture (VAIR) that can extract AIRs and therefore achieve provable disentanglement where standard VAEs fail. Beyond state representation, VAIR also captures the action dependence of the underlying generative factors, directly linking experiments to the degrees of freedom they influence.
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All authors Gorka Muñoz-Gil, Hendrik Poulsen Nautrup, Arunava Majumder, Paulin de Schoulepnikoff, Florian Fürrutter, Marius Krumm, Hans J. Briegel

Quantum Physics

Geometry of restricted information: the case of quantum thermodynamics

We formulate a geometric framework in which physical laws emerge from restricted access to microscopic information. Measurement constraints are modeled as a gauge symmetry acting on density operators, inducing a gauge-reduced space of physically distinguishable states. In the case of quantum thermodynamics, this construction leads to a gauge-invariant formulation in which the invariant entropy admits a stochastic description and satisfies a general detailed fluctuation theorem. From this result, we derive an integrated fluctuation theorem and a Clausius-like inequality that unifies the first and second laws in terms of invariant work and coherent heat. Entropy production is identified with the relative entropy between forward and backward probability measures on the gauge-reduced space of thermodynamic trajectories, revealing irreversibility as a geometric consequence of limited observability. The third law emerges as a singular zero-temperature limit in which thermodynamic orbits collapse and entropy production vanishes. Since the framework applies to arbitrary information constraints, it encompasses energy-based thermodynamics as a particular case of more general measurement scenarios.
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Quantum Physics

Quantum Attention by Overlap Interference: Predicting Classical and Many-Body Quantum Sequences

We propose a variational quantum implementation of self-attention (QSA)-the core operation in transformers and large language models-which predicts future elements of a sequence by forming overlap-weighted combinations of past data. At variance with previous approaches, our QSA realizes the required nonlinearity through interference of state overlaps and a degree-$k$ polynomial kernel, and estimates a loss based on Rényi-$1/2$ entropic functionals via two observables' expectation values, avoiding the decoding of amplitude-encoded predictions into classical probabilities. QSA also accommodates a constrained, trainable data-embedding tying state overlaps to data-level similarities. Its dominant end-to-end training complexity scales as $O\left(\mu^{-1}k^2Td\right)$, versus $O\left(T d^{k+1}\right)$ of the fairest classical comparison, with $\mu$ a training signal; we show numerically that this allows a complexity advantage in the regime where sequence length $T$ dominates the embedding size $d$. In simulations, our QSA-based quantum transformer learns sequence prediction on classical data and on many-body transverse-field Ising trajectories-establishing trainable attention as a practical primitive for quantum dynamical modeling.
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Quantum Physics · Cross-listed from cond-mat.stat-mech

Second law of thermodynamics in closed quantum many-body systems

The second law of thermodynamics for adiabatic operations – constraints on state transitions in closed systems under external control – is one of the fundamental principles of thermodynamics. On the other hand, it is recently established that even pure quantum states can represent thermal equilibrium. However, pure quantum states do not satisfy the second law in that they are not passive, i.e., work can be extracted from them if arbitrary unitary operations are allowed. It therefore remains unresolved how quantum mechanics can be reconciled with thermodynamics. Here, based on our key quantum-mechanical notions of thermal equilibrium and adiabatic operations, we address the emergence of the second law for adiabatic operations in the thermodynamics limit. We first introduce infinite-observable macroscopic thermal equilibrium (iMATE); a quantum state, including pure states, is in iMATE if the expectation values of all additive observables agree with their equilibrium values. We also introduce a macroscopic operation as unitary evolution generated by a time-dependent additive Hamiltonian, which is regarded as corresponding to adiabatic operations. Employing these concepts, we show that no extensive work can be extracted from any quantum state in iMATE through any macroscopic operations. Furthermore, we introduce a quantum-mechanical form of entropy density such that it agrees with thermodynamic entropy density for any quantum state in iMATE. We then prove that for any initial state in iMATE, this entropy density cannot be decreased by any macroscopic operations, followed by a time-independent relaxation process. Our theory thus proves two different forms of the second law, by adopting macroscopically reasonable classes of observables, equilibrium states, and operations. We also discuss the time scales of macroscopic operations in these results.
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All authors Yuuya Chiba, Yasushi Yoneta, Ryusuke Hamazaki, Akira Shimizu

Quantum Physics

A Complete Equational Theory for Real-Clifford+CH Quantum Circuits

We introduce a complete equational theory for the fragment of quantum circuits generated by the real Clifford gates plus the two-qubit controlled-Hadamard gate. That is, we give a simple set of equalities between circuits of this fragment, and prove that any other true equation can be derived from these. This is the first such completeness result for a finitely-generated, universal fragment of quantum circuits, with no parameterized gates and no need for ancillas.
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Quantum Physics

Normal mode splitting induced synchronization blockade in coupled quantum van der Pol oscillators

We report a normal-mode induced synchronization blockade in coupled quantum van der Pol oscillators under the influence of external drive. In this mechanism, the coupling hybridizes the oscillator modes into spectrally split normal modes. The destructive interference between the transitions to these modes blocks synchronization. We find that this blockade can be controlled simply by tuning the coupling strength and detuning allowing dynamic manipulation of quantum synchronization through collective mode dynamics. We analyze the phase-locking behaviour using perturbation analysis. Further, by deriving steady-state probability amplitudes we show how the energy redistribution and spectral splitting forms the basis of the blockade. Our results might provide new insights into how synchronization can be controlled in quantum systems.
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Quantum Physics

High-speed phase-encoded quantum secure direct communication over 11.4 km heterogeneous free-space and fiber links

Robust quantum transmission is driving a new paradigm in space-ground quantum networking. Although phase encoding has been widely adopted in terrestrial fiber channels, it has long been considered unsuitable for free-space quantum communication. Here, we demonstrate phase-encoded quantum communication over 1400 m of urban free space. The system maintained stable operation for nearly one hour, achieving 99.07% interference visibility and an average quantum bit error rate of 2.38%. The free-space quantum states were directly coupled into the fiber and transmitted over an additional 10 km, confirming seamless interoperability across different media. We further show that turbulence-induced phase drifts between successive picosecond pulses can be effectively compensated. A cascaded-link model and numerical simulations indicate feasibility over free-space distances exceeding 30 km, underscoring the potential for satellite-to-ground quantum links. This work establishes the viability of phase encoding in free-space quantum networks, simplifying cross-medium integration and enabling compatibility with existing classical infrastructures.
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All authors Ze-Zhou Sun, Yuan-Bin Cheng, Yu-Chen Liu, Jianxing Guo, Xiao-Tian Song, Wei Zhang, Dong Pan, Gui-Lu long

Quantum Physics

Extensible universal photonic quantum computing with nonlinearity

Universal quantum computing requires an architecture that supports both linear circuits and, crucially, strong nonlinear resources. For quantum photonic systems, integrating such nonlinearities with scalable linear circuitry has been a major bottleneck, leaving most optical experiments without nonlinear operations and, consequently, incapable of achieving universality. Here, we report an extensible photonic computer that supports a universal gate set by seamlessly combining fully programmable, scalable linear optical networks with integrated nonlinear modules. This platform enables a broad range of quantum computing and simulation tasks. We demonstrate the quasi-deterministic generation of optical Gottesman-Kitaev-Preskill states, which are essential resources for bosonic error correction, yet had previously been realized only probabilistically. Furthermore, we simulate complex many-body quantum dynamics, exemplified by the Bose-Hubbard model. Such quantum simulation tasks have long been considered beyond the reach of photonic hardware limited to linear operations. These capabilities, enabled by our extensible architecture, establish a viable route towards photonic quantum simulation and fault-tolerant quantum computing.
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All authors Shang Yu, Jinzhao Sun, Kuan-Cheng Chen, Zhi-Huai Yang, Zhenghao Li, Ewan Mer, Yazeed K. Alwehaibi, Shana H. Winston, Dayne Marcus D. Lopena, Zi-Cheng Zhang, Guang Yang, Runxia Tao, Mingti Zhou, Gerard J. Machado, Ying Dong, Roberto Bondesan, Vlatko Vedral, M. S. Kim, Ian A. Walmsley, Raj B. Patel

Quantum Physics

Algebraic Reduction to Improve an Optimally Bounded Quantum State Preparation Algorithm

The preparation of $n$-qubit quantum states is a cross-cutting subroutine for many quantum algorithms, and the effort to reduce its circuit complexity is a significant challenge. In the literature, the quantum state preparation algorithm by Sun et al. is known to be optimally bounded, defining the asymptotically optimal width-depth trade-off bounds with and without ancillary qubits. In this work, a simpler algebraic decomposition is proposed to separate the preparation of the real part of the desired state from the complex one, resulting in a reduction in terms of circuit depth, total gates, and CNOT count when $m$ ancillary qubits are available. The reduction in complexity is due to the use of a single operator $\Lambda$ for each uniformly controlled gate, instead of the three in the original decomposition. Using the PennyLane library, this new algorithm for state preparation has been implemented and tested in a simulated environment for both dense and sparse quantum states, including those that are random and of physical interest. Furthermore, its performance has been compared with that of Möttönen et al.'s algorithm, which is a de facto standard for preparing quantum states in cases where no ancillary qubits are used, highlighting interesting lines of development.
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Quantum Physics

Local Certification of Many-Body Steady States

We present a relaxation-based method to bound expectation values on the steady state of dissipative many-body quantum systems described by master equations of the Lindblad form. Instead of targeting to represent the entire state, we promote the reduced density matrices to our variables and enforce the constraints that are imposed on them by consistency with a global steady state. The resulting constraints have the form of a semidefinite program, which allows us to efficiently bound the values a given expectation value can take in the steady state. Our results show fast convergence of the bounds with the size of the reduced density matrices, giving very competitive predictions for the steady state of several one- and two-dimensional models for an arbitrary number of particles.
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Quantum Physics

Non-Markovian and non-Condon vibrationally assisted electron transfer in an effective ligand–receptor complex

We investigate vibrationally assisted electron transfer in an effective ligand–receptor complex using finite-temperature non-Markovian quantum-state diffusion with explicitly colored environmental noise. The model comprises donor and acceptor electronic states, an explicit bound-ligand vibrational mode, and a Gaussian protein–solvent bath. Linearization of a coordinate-dependent diabatic Hamiltonian yields two distinct system–environment interactions: diagonal coupling, which modulates the donor–acceptor energy gap, and off-diagonal non-Condon coupling, which modulates the electronic tunneling matrix element. In the rapidly equilibrating diagonal-bath limit, the model recovers the expected Marcus–Levich–Jortner vibronic-transfer behavior. Persistent environmental correlations instead generate non-exponential dynamics characterized by transient oscillations, back-transfer, and memory-dependent damping. Under matched weak-tunneling conditions, off-diagonal coupling opens a direct transition pathway and produces a larger finite-time acceptor population throughout the investigated parameter space. Because these dynamics do not generally support a unique kinetic rate, the enhancement is quantified using a time-averaged transfer yield. Comparisons with exact closed-system and commuting pure-dephasing limits validate the deterministic propagator and colored-noise construction, respectively, while the noncommuting dynamics are evaluated within a declared first-order finite-memory closure. These results identify ligand vibration, environmental memory, and non-Condon tunneling modulation as cooperating mechanisms that can dynamically regulate electron transfer in receptor-like environments, while maintaining a clear distinction between a mechanistic coarse-grained model and an atomistically parameterized biological complex.
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