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Tuesday 21 July

Quantum Physics

Quantum sensing of low-frequency electric signal enabled by modulated auxiliary field in Rydberg atoms

Xiayang Fan, Shenchao Jin, Jiatian Liu et al. (+3 more)

Rydberg atoms have emerged as a versatile and efficient platform for high-sensitivity quantum sensing of free-space electric fields, with remarkable progress in detecting low-frequency signals. To date, low-frequency Rydberg receivers have relied on a constant bias field, typically realized via intra-cell electrodes or Rydberg plasmas generated by photoelectric effects or inter-atomic interactions. While these approaches improve sensitivity, they suffer from inherent challenges in calibration, long-term stability, and robustness, hindering practical deployment. Here, we propose, design, and experimentally demonstrate a quantum sensing scheme for low-frequency electric signals using modulated auxiliary fields in Rydberg atoms. Unlike conventional methods that employ external DC electric fields that are often fully shielded by adsorbed atom layers on the cell walls, we introduce an AC-field modulation strategy. The incoming low-frequency signal mixes with the auxiliary field, and together they induce Stark shifts of the Rydberg level. These shifts are mapped onto the probe laser via electromagnetically induced transparency (EIT), in a manner analogous to heterodyne detection. We demonstrate a sensitivity of $7.5 \pm 2.6~\mathrm{\mu V/(cm\cdot Hz^{1/2})}$ at 5 kHz and a minimal detectable field of $0.26 \pm 0.04~\mathrm{\mu V/cm}$ with an integration time of 1000 s. Furthermore, we extend this approach to systematically analyze the performance of generalized auxiliary fields containing multiple frequency components. By virtue of modulated auxiliary field and quantum frequency mixing, our results establish a robust and systematic framework for quantum sensing of low-frequency electric fields with Rydberg atoms, offering improved sensitivity, stability, and immunity to environmental drifts.
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All authors Xiayang Fan, Shenchao Jin, Jiatian Liu, Jialiang Zhang, Qichao Qi, Yuan Sun

Quantum Physics

Trade-off between predictability and quantum coherence for multi-path interferometry and its operational interpretation

Ezra Acalapati

The complementarity principle is a cornerstone of quantum mechanics. In this work, we investigate a complementarity relation between the predictability and quantum coherence, respectively, representing the particle-like and wave-like behaviour in wave–particle duality, in a multi-path setup. We introduce a basis-dependent predictability defined by the Bures distance between the state dephased in the chosen basis and the maximally mixed state. The predictability depends only on the observed basis statistics and admits a closed form in terms of the Bhattacharyya overlap. We derive a trade-off relation between the predictability and a coherence measure defined based on the nonclassicality of the Kirkwood–Dirac quasiprobability. For pure states, the trade-off relation is an exact equality. Remarkably, this wave–particle duality relation endows the coherence measure with an operational interpretation as the classically irreducible part of measurement randomness, yielding a tight worst-case bound on the guessing probability in source-independent QRNG.
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Quantum Physics

A Reversible Continuous-Variable Photonic Memory Architecture Based on Displacement-Evolved Coherent-State Dynamics

Sanjit Krishna G M

Modern information systems increasingly require memory architectures capable of storing not only static data but also the evolution of information over time. While continuous-variable photonic platforms have been extensively studied for quantum communication and computation, their potential as dynamically evolving memory systems remains largely unexplored. In this work, we introduce a metadata-assisted continuous-variable photonic memory architecture in which information is represented by latent vectors encoded across multimode coherent states and updated through reversible displacement operations. The framework combines multimode storage with metadata-based indexing, enabling navigation and reconstruction of historical memory states through rollback retrieval. Unlike conventional approaches that focus on preserving a single quantum state, the proposed framework treats memory as a continuously evolving trajectory in continuous-variable phase space. This perspective naturally extends beyond image storage to temporal data streams, machine-learning latent representations, scientific simulations, and other forms of dynamically changing information. Although presented as a theoretical proof-of-concept, the results suggest that continuous-variable photonic systems may provide a promising foundation for future metadata-aware memory architectures capable of storing, tracking, and reconstructing the history of evolving information.
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Quantum Physics

Monogamy inequalities of entanglement of assistance in $2\otimes 2\otimes d$ systems

Xue-Na Zhu, Gui Bao, Zhi-Xiang Jin et al. (+2 more)

The monogamy relations characterize the distribution of quantum correlations among the multipartite quantum systems. We study the monogamy relations of the entanglement of assistance in $2\otimes 2\otimes d$ systems. We present explicitly the relations satisfied by the concurrence, the tangle and the concurrence of assistance, which can be used to derive rigorous monogamy relations. Detailed examples are given to illustrate our results.
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All authors Xue-Na Zhu, Gui Bao, Zhi-Xiang Jin, Shao-Ming Fei, Tao Li

Quantum Physics

Spontaneous optical emission of a randomly oriented chiral quantum source

Mikael P. Backlund

We derive the form of the one-photon state of the electromagnetic field resulting from spontaneous optical emission of a randomly oriented chiral quantum source, including electric dipole, magnetic dipole, and electric quadrupole contributions to the radiation. We describe how spatial coherence in the emission can be exploited to reveal hidden information about the emitter. This work serves as a prelude and companion piece to our concurrently submitted work \cite{Backlund2026Letter} in which we present the classical and quantum error bounds associated with assigning the handedness of such an emitter.
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Quantum Physics

Dismantling the Stoquastic Dichotomy

Armen Karakashian and Itay Hen

We challenge the notion that a stoquastic binary governs fundamental computational boundaries in quantum computing and classical simulation of quantum systems. We argue that vanishing geometric phase (VGP), a geometric condition on the Hamiltonian's transition graph, more adequately captures these boundaries. To distinguish VGP from stoquasticity, we construct VGP 3-local Hamiltonians that are formally hard to stoquastize, yet belong to a family admitting polynomial-time recognition of the VGP property. Without constructing a stoquastizing unitary, we prove that the local Hamiltonian problem is $\mathsf{StoqMA}$-complete under the promise that the input Hamiltonian has VGP, and that a frustration-free variant is in $\mathsf{MA}$ under the same promise. We use this result to argue that non-VGP is necessary for any claimed adiabatic advantage justified by escaping the $\mathsf{StoqMA}$ regime. Further, we identify natural settings where the VGP property can be recognized in polynomial time. In contrast, we show that recognition of VGP is $\mathsf{PSPACE}$-complete in general for geometrically local Hamiltonians. Our results show that the computational boundaries $\mathsf{MA} \subseteq \mathsf{StoqMA} \subseteq \mathsf{QMA}$ traditionally attributed to stoquasticity are better understood as boundaries between vanishing and non-vanishing geometric phase structure.
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Monday 20 July

Quantum Physics

Motional Kerr-Cat States of an Atom in an Optical Tweezer

Steven K. Pampel, Gur Lubin, Dawson P. Hewatt et al. (+6 more)

Schrödinger cat states - quantum superpositions of classically or macroscopically distinct states - constitute a powerful resource for quantum computing, enhanced metrology, and probing coherence on large scales. Encoding such states in the phase space of an oscillator requires a nonlinearity, typically inherited from an auxiliary degree of freedom such as atomic spin or a Josephson junction. Neutral atoms trapped in reconfigurable optical tweezer arrays - a leading platform for quantum science and computing - provide an intrinsic nonlinearity via the motion of a single atom in a tightly focused trap. However, this self-Kerr mechanism has not previously been exploited for cat-state generation, and remains largely unexplored as a resource for motional-state control. Here we realize Schrödinger cat states in the quantized motion of a single neutral atom trapped in an optical tweezer. By modulating the depth and position, we demonstrate parity control of both Kerr-cat and Fock states alongside tunable nonlinearity, establishing a spin- and species-independent framework for controlling motion. We further show that the cat-state encoding is intrinsically robust against trap-frequency fluctuations that otherwise limit the fidelity of direct Fock-state transitions. These results establish Kerr-based control of neutral-atom motion as a new paradigm for cat-state and bosonic-state engineering in optical tweezers, providing a route toward quantum-error-correcting codes such as grid states, and toward quantum-enhanced sensing with arrays of non-Gaussian states.
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All authors Steven K. Pampel, Gur Lubin, Dawson P. Hewatt, Conall McCabe, Jaeyong Hwang, Sean R. Muleady, Tianrui Xu, Ana Maria Rey, Cindy A. Regal

Quantum Physics

Nonlinear Optics Mediated by Chiral Waveguide QED: Generation of Momentum-anticorrelated Photon Pairs

Wenqi Tong, Spencer J Walsh, H. Alaeian et al. (+1 more)

In recent years, chiral quantum optics has emerged as an active research area due to the promising applications in quantum information processing as well as nonlinear optics. We present results on the properties of the incoherent component of the transmitted light after transmons chirally coupled to a waveguide. The well-known resonance fluorescence of one chiral transmon resembles nonlinear quantum optics as it can convert the coherent light, with photons having spatially extensive coherence and Poisson distribution, into a field with spatially localized coherence and bunching photon statistics. However, another chiral transmon can undo the effect of the first transmon regardless of the Rabi frequency under the idealization involved in this work. A full wavefunction calculation shows that, in the weak driving limit, this incoherent light mainly comes from two-photon processes - one chiral transmon converts two independent photons into a photon pair with opposite momentum shift. In addition to the driving through the waveguide, a local driving with independently tunable amplitude and phase can address each transmon individually. The interplay between the waveguide driving and local driving modulates the contribution of the incoherent transmission and hence enables the engineering of the quantum statistics of the transmitted field, with tunable $g^{(2)}(0)$ spanning anti-bunched, coherent, and strongly bunched regimes.
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All authors Wenqi Tong, Spencer J Walsh, H. Alaeian, F. Robicheaux

Quantum Physics

Quantum Reservoir Computing: Recent Advances and Future Directions

Shehbaz Tariq, Muhammad Talha, Arshid Ali et al. (+2 more)

Quantum reservoir computing (QRC) uses the dynamics of a fixed or weakly tuned quantum system to transform temporal and sequential inputs into measured features, while training is typically confined to a classical readout. This separation reduces reliance on repeated quantum parameter updates and avoids the barren plateaus associated with variational circuit training. Its computational power is often attributed to the exponentially large Hilbert space of the quantum system. However, the memory, nonlinearity, and expressivity that determine what a reservoir can actually compute depend jointly on the input encoding, quantum evolution, observables, measurement, and readout, not on Hilbert space dimension alone. On hardware, these capabilities are further constrained by finite sampling, hardware noise, measurement backaction, and the cost of estimating observables, so a large state space alone does not guarantee useful computation. In this survey, we develop a common system model that connects these components and use it to organize QRC foundations, computational properties, reservoir architectures, operating protocols, and physical implementations. We examine spin, photonic, superconducting, bosonic, neutral atom, and other analog platforms, together with applications, software and high performance computing support, benchmarking, and reproducibility. The analysis distinguishes hardware demonstrations from simulations and identifies the assumptions and resources that govern comparisons across implementations. Current results do not establish a broad quantum advantage over well matched classical reservoirs. We therefore specify the resource accounting, benchmark standards, and theoretical criteria needed to evaluate claims of quantum advantage.
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All authors Shehbaz Tariq, Muhammad Talha, Arshid Ali, Muhammad Diyan, Symeon Chatzinotas

Quantum Physics

Simulating the Dicke Model on Qubit-Based and hybrid Qubit-Boson-Based Quantum Computers

Aravind P. Babu, Seongjin Ahn, Jing Sun et al. (+3 more)

The Dicke model provides a fundamental description of collective light-matter interactions and has long served as a testbed for exploring a wide range of physical phenomena in quantum optics and condensed matter physics. In this work, we develop a variational framework for investigating the finite-size Dicke model on both fully qubit-based (digital) and hybrid qubit boson based (digital-analogue) quantum computing platforms. We show that the resulting model reproduces the characteristic critical behavior of the Dicke model in the appropriate large-spin limit while remaining suitable for implementation on both classical emulators of quantum computers and actual trapped ion quantum computers, albeit in the case of latter somewhat limited by noise. Finally, we introduce a complementary hybrid qubit-bosonic variational ansatz that directly exploits the bosonic degree of freedom to reduce quantum resources and discuss its potential implementation on hybrid quantum hardware. Our results establish a scalable, symmetry-aware framework for variational quantum simulations of collective light-matter systems and provide a pathway toward efficient simulations of more general spin-boson models on near-term quantum devices.
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All authors Aravind P. Babu, Seongjin Ahn, Jing Sun, Augustas G. Landsbergis, Andrey S. Moskalenko, Marko J. Rančić

Quantum Physics

From Canonical to Tunable Phase Diagrams in Open Quantum Long-Range Systems

Anish Acharya and Shamik Gupta

We investigate the dissipative dynamics of a generalized Lipkin-Meshkov-Glick (LMG) model coupled to a thermal environment. In this generalized model, in addition to the conventional quadratic interaction, one considers quartic interactions between spin-$1/2$'s coupled all-to-all and evolving in presence of a transverse field. Employing the usual linear Lindblad master equation with thermally-balanced jump processes, we derive magnetisation evolution equations, and demonstrate that the corresponding stationary solution reproduces the canonical equilibrium phase diagram of the model. We then extend our analysis to a nonlinear Lindblad equation that incorporates imperfect quantum-jump processes in terms of jump-retention parameters. Here, remarkably, the system relaxes to a genuine nonequilibrium stationary state whose properties differ qualitatively from those obtained in the linear case. The jump-retention parameters provide tunable knobs that shift the phase boundaries and even modify the nature of the phase transitions with respect to the linear case. Our exact results establish a direct connection between dissipative relaxation dynamics and stationary-state behavior, while identifying controlled quantum-jump retention as a mechanism for engineering nonequilibrium phases in long-range interacting open quantum systems.
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Quantum Physics

State $k$-designs from Hamiltonian evolution

Shengxian Hou, Zong-Yue Hou and Zhi-Cheng Yang

We study the generation of state $k$-designs from time evolution under a fixed Hamiltonian. Specifically, we consider the ensemble $\mathcal{E}=\left\{e^{-iHt}|\psi_0\rangle | \ t\sim \mathrm{Unif}[0,T],\, |\psi_0\rangle\sim \mathcal{E}'\right\}$, where the initial states are sampled from an ensemble $\mathcal{E}'$. For Hamiltonians drawn from the Gaussian unitary ensemble, we derive a simple relation between the frame potential of the evolved ensemble $\mathcal{E}$ and that of the initial ensemble $\mathcal{E}'$ in the large evolution time limit. This relation shows that $\mathcal{E}$ forms an exact state $k$-design in the thermodynamic limit as long as $\mathcal{E}'$ forms a state 1-design. Remarkably, we further show, both analytically and numerically, that time evolution under a simple nonintegrable mixed-field Ising Hamiltonian can generate approximate state $k$-designs with high precision, starting from product states in an appropriately chosen Pauli basis. We also analyze the finite-$T$ correction and find it scales as $O(1/T)$. To reduce the evolution time, we propose an $M$-step quench protocol that suppresses this correction to $O(1/T^M)$, which is also verified numerically. We then extend our analysis to unitary ensembles, deriving an analogous recursion relation for the unitary frame potential. Our results elucidate the mechanisms underlying recent proposals for generating unitary $k$-designs through sequential quantum quenches in a unified manner.
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Quantum Physics

Observation of a power transfer controlled by the phase of a quantum superposition

Nathanaël Cottet, Sébastien Jezouin, Bogdan Reznychenko et al. (+2 more)

A driven qubit exchanges energy with the propagating modes that drive it. When two spatially separated modes drive a single qubit with opposite amplitudes, their net action on the qubit cancels. Yet the qubit can still transfer power from one mode to the other through stimulated emission. The directionality originates from opposite stimulated emission powers into each line. We realize this situation with a superconducting transmon qubit coupled to two transmission lines and show that the direction of the power flow is set by the phase $\phi$ of the qubit superposition between its ground and excited states, rather than by any classical control parameter. From a time-resolved measurement of the output power in one line, we observe a transfer that varies as $\cos\phi$ and hence changes direction between $\phi=0$ and $\phi=\pi$. The directionality of the total power flow is limited by the phase independent contributions of the reflected drive and of spontaneous emission, which sets a routing efficiency that we measure as a function of the input power. For an equal superposition of ground and excited qubit states (maximal coherence), the efficiency reaches $63\%$, close to the bound of $69\%$ expected from the measured qubit coherence times.
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All authors Nathanaël Cottet, Sébastien Jezouin, Bogdan Reznychenko, Alexia Auffèves, Benjamin Huard

Quantum Physics

Cavity-Mediated Charging of a Graphene Excitonic Quantum Battery

Maryam Hadipour and Soroush Haseli

We study the charging and work-extraction properties of a graphene-based excitonic quantum battery embedded in a driven-dissipative optical microcavity. The system consists of a pair of intervalley excitons in strained graphene, where one exciton acts as the charger and the other as the quantum battery, both coupled to a common cavity mode through a Tavis-Cummings interaction. By solving the open-system dynamics, we analyze the ergotropy as a measure of extractable work and investigate how coherent and incoherent pumping, cavity loss, and the microcavity parameter influence the charging process. Our results show that the battery exhibits a transient ergotropy peak followed by relaxation to a steady state, with the maximum extractable work strongly controlled by the light-matter coupling strength. The study reveals an optimal regime for efficient charging and demonstrates the role of cavity engineering in enhancing work storage in excitonic quantum batteries.
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Quantum Physics

Equivariant Continuous Normalizing Flows with Offline Sampling for Fermionic Ground State Estimation

Sam Cochran, James B. Larsen, James Stokes et al. (+4 more)

We introduce a framework for fermionic variational Monte Carlo in which a continuous normalizing flow refines a fixed antisymmetric base wavefunction. The flow is a permutation-equivariant neural ordinary differential equation (ODE), a smooth, topology-preserving map; equivariance preserves the antisymmetry of the base, so the flow can in principle improve any antisymmetric ansatz that can be sampled efficiently. The flow deforms the nodal surface of the base but cannot change its topology, so the base must supply the correct nodal topology. We use Slater and Jastrow-Slater bases in this work. Exact samples from the flow's Born distribution are obtained by pushing pre-cached base samples through the forward ODE, requiring no Markov chain Monte Carlo at training time. Base samples are generated offline and reused across batches and runs, enabling embarrassingly parallel training across multiple GPUs. We introduce three permutation-equivariant vector field architectures, Pairwise Deep Sets, FermiNet Vector Fields, and Pairwise Deep Sets Gradient, each balancing expressivity and computational cost differently. We further introduce an augmented dynamics formulation that co-evolves the derivatives required for the kinetic energy as ODE state variables, eliminating differentiation through the ODE trajectory and significantly reducing wall-clock time and memory. On harmonically trapped spinless electrons, the flow reaches ground-state energies below configuration interaction with single and double excitations, on par with a FermiNet baseline trained with the same optimizer. Training shows near-ideal strong scaling to 128 NVIDIA A100 GPUs on NERSC's Perlmutter for the largest systems tested, up to $N = 48$ particles in three dimensions.
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All authors Sam Cochran, James B. Larsen, James Stokes, Andrew Wray, Michael J. Keiser, Chad Rigetti, Shravan Veerapaneni

Quantum Physics · Cross-listed from cond-mat.mes-hall

Quantum-Enabled Spintronic "Small" Antennas

Supriyo Bandyopadhyay

Antennas transmit information wirelessly from one location to another via electromagnetic waves. Miniaturizing them, however, is challenging since the radiation efficiencies of all traditional antennas, based on the principles of classical electromagnetics, plummet when their dimensions are shrunk to tiny fractions of the radiated wavelength. Lately, a new generation of antennas whose operations are underpinned by non-classical principles have been demonstrated and they can overcome this limitation. This enables embedded applications that were hitherto inaccessible. In addition, beam steering, which normally requires a large phased array (multiple antenna elements each much larger than the wavelength), can now be accomplished with a single element much smaller than the wavelength. Some of these antennas also have stealth attributes for secure and covert communication, which makes this new genre a disruptive new technology.
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Quantum Physics · Cross-listed from hep-ph

Deep inelastic scattering as a probe of entanglement: the complete QCD dipole cascade

Martin Hentschinski, Krzysztof Kutak, Wieslaw Placzek et al. (+1 more)

We study entanglement entropy in Deep Inelastic Scattering (DIS) using the dipole formulation of the high-energy limit of QCD. We argue that a reduced density matrix arises in low $x$ DIS due to a trace over unobserved color degrees of freedom and we obtain entanglement entropy in terms of dipole multiplicities, directly from the von Neumann entropy. Dipole multiplicities are obtained from a solution to low $x$ evolution equations, which we solve numerically. Unlike previous studies, we take into account both transverse-size and azimuthal-angle dependence in the dipole evolution kernel. We study both the exact solution of the equation as well as its double leading-logarithmic approximation (DLLA). We find that for the same initial dipole size, the DLLA solution generates a larger entropy. Finally, we calculate the dipole multiplicities and entanglement entropy and compare our results to the Shannon entropy of hadron multiplicities, as measured by the H1 collaboration.
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All authors Martin Hentschinski, Krzysztof Kutak, Wieslaw Placzek, Martin Rohrmoser

Quantum Physics

Bright Telecom Spin-Photon Interface in Silicon Photonics

Carolina Crosta, Amirehsan Alizadehherfati, Purbita Purkayastha et al. (+5 more)

Silicon is an attractive host for scalable quantum photonics, but the absence of bright telecom-band emitters with optically addressable spin states has limited its use for spin-photon interfaces. Here we demonstrate the Al1-center, an aluminum–carbon defect in silicon, as a bright waveguide-integrated single-photon emitter with a ground-state spin. Using isotopically purified silicon-on-insulator nanophotonic devices, we isolate individual Al1-centers and observe high-purity single-photon emission with $g^{(2)}(0)=0.04$ without background subtraction. Time-resolved photoluminescence spectroscopy reveals a fast excited-state lifetime of 135 ns, nearly an order of magnitude shorter than the benchmark provided by the well-studied T-center. Resonant photoluminescence excitation measurements further resolve the zero-phonon transition and reveal a narrow homogeneous linewidth reaching 47 MHz, threefold narrower than the T-center under comparable temperature. Through magneto-optical spectroscopy, we resolve the spin-dependent transitions of the bound-exciton manifold and achieve spin-selective optical pumping, fulfilling the prerequisite for quantum state initialization and readout. These results establish the Al1-center as a bright telecom-band spin-photon interface in silicon photonics and introduce a promising platform for integrated quantum networks.
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All authors Carolina Crosta, Amirehsan Alizadehherfati, Purbita Purkayastha, Kyu-Young Kim, Jasvith Raj Basani, Chang-Min Lee, Fabio Pezzoli, Edo Waks

Quantum Physics · Cross-listed from physics.optics

Aromatic molecular emitters in a hexagonal boron nitride stack

Tianyu Fang, Ricardo Gioia Alvarez, Babak Behjati et al. (+8 more)

Single polycyclic aromatic hydrocarbon molecules embedded in organic matrices have proven to be an excellent family of narrow-linewidth quantum emitters. Extending this host-guest setting to van der Waals materials offers the opportunity to combine the preeminent properties of molecular emitters with the access to the versatility of two-dimensional hetero-structures and devices. In this work, we incorporate perylene molecules into multi-layered hexagonal boron nitride stacks and observe gigahertz-narrow zero-phonon-line transitions at cryogenic temperatures. We unambiguously verify the origins of photon emission through vibronic spectra analysis. By combining hyperspectral localization measurements with quantum chemistry calculations, we examine the insertion mechanisms of perylene molecules in the hexagonal boron nitride stacks, and conclude that pristine hBN layers tend to expel molecules from the sandwich, while extended morphological defects, hydroxyl groups and unpassivated boron and nitrogen atoms assist to stabilize molecular bindings to hBN. Our work provides valuable insight for future work to deterministically integrate narrow-linewidth molecular emitters into van der Waals devices.
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All authors Tianyu Fang, Ricardo Gioia Alvarez, Babak Behjati, Moritz Scharfstädt, Max Masuhr, Bo Deng, Noah Henseler, Andrea Bergschneider, Stefan Linden, Christian Schäfer, Daqing Wang

Quantum Physics

Reducing entanglement with a Hamiltonian derived Clifford transformation

James Brown, Erika Lloyd, Alexandre Fleury et al. (+2 more)

Recently (Physica Scripta, 100(10):105401, 2025), an algorithm was introduced that deterministically generates a Clifford transformation from the Qubit Coupled Cluster (QCC) algorithm which we call Q-Cliff (QCC+Clifford). There, it was shown that Q-Cliff could be utilized to generate a hardware efficient version of the QCC ansatz. Here, we examine and refine these techniques and show that Q-Cliff can be utilized to generate efficient classical and quantum approximations to the ground states of chemical systems. The algorithm generates an efficient variational method that generally has accuracy between MP2 and CISD with $O(N^6)$. Furthermore, we show through DMRG calculations that the entanglement between qubits is reduced significantly and therefore the accuracy for a given bond dimension can be vastly improved (up to an order of magnitude). Finally, we refine the previously reported algorithm to generate low-depth and CNOT efficient circuits that can be optimized with a comparable number of energy evaluations to state-of-the-art VQE algorithms. All these results show that this Hamiltonian derived Clifford transformation should be a tool used for many classical and quantum algorithms.
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All authors James Brown, Erika Lloyd, Alexandre Fleury, Tarini S Hardikar, Kenny Heitritter

Quantum Physics

Monolithic printed-circuit board RF-trap for electrons

Zijue Luo, Jae Eu, Tianyi Wang et al. (+1 more)

Qubits encoded in the spin of trapped electrons have been proposed as a promising novel platform for quantum information processing. While trapping of electrons has been largely carried out in Penning traps for precision measurement purposes, it is desirable to use linear Paul traps instead, leaning on the successes of trapped ion quantum processors. Here we present a Paul trap for electrons made of a single printed circuit board. Our approach requires no assembly and the rigid design minimizes manufacturing intolerances. We characterize the trap performance and observe trapped electron lifetimes of 2.13 ms and secular frequencies of up to 90 MHz.
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All authors Zijue Luo, Jae Eu, Tianyi Wang, Boerge Hemmerling

Quantum Physics

Magic-protected entanglement and Clifford-irreducible structure in magic state space

Alejandro Borda Kuhlmann and Julian Rincon

We develop a Clifford-orbit framework for studying magic-protected entanglement, which we refer to as magical entanglement: the part of bipartite entanglement that remains after optimal stabilizer simplification. This construction leverages residual entanglement under Clifford reduction as a state-level organizing principle for magic state space. It defines canonical representatives, spectra, and ranks that characterize the Clifford-irreducible structure of a state. We identify two regimes of the magic-entanglement interplay. In the $T$-magic regime, local nonstabilizer resources can coexist with entanglement, but the protected component remains weak and state-dependent. In the $W$-magic regime, by contrast, entanglement is Clifford-irreducibly tied to nonstabilizerness, producing typical, strongly self-averaging behavior. Analytical examples and random-circuit numerics support a crossover from broad $T$-magic fluctuations to concentrated, Haar-like $W$-magic behavior. These results identify magical entanglement as an orbit-level diagnostic of how nonstabilizerness protects quantum correlations against Clifford reduction.
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Quantum Physics

Calculus of Robinet: completely positive reconstruction of time-averaged diffusive quantum trajectories

Hector Hutin and Antoine Tilloy

Truly continuous quantum trajectories, obtained from homodyne or heterodyne readouts, can only ever be reconstructed approximately. The continuous measurement signal, needed for exact reconstruction, is averaged over bins of finite time $\Delta t$ during any analog to digital conversion step. The best reconstruction possible, knowing only this discrete record, was introduced recently and dubbed the Robinet state. In this article, we show how the Robinet state can be computed with a numerical discretization scheme that is completely positive, accurate to arbitrarily high order in $\Delta t$, and that does not rely on any other external solver. Our derivation relies on a dilation of the stochastic master equation into a system + transmission line setup, constructed in such a way that measuring what we call the "zero mode" of the line yields the Robinet state. We test the method on a challenging example with random Hamiltonian and jump operator, and verify its accuracy up to order $10$. Apart from its numerical interest, our approach provides a wealth of physical insights, extending in particular recent results on purity obtained by Wonglakhon, Chantasri, and Wiseman, that would be difficult to obtain in any other way.
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Quantum Physics · Cross-listed from cond-mat.stat-mech

Information Compression at Criticality

Simon Jiricek, Miroslav Hopjan, Boris Altshuler et al. (+2 more)

Highly excited quantum states at the critical boundary of ergodicity are known to deviate from thermal behavior, yet their dynamical properties remain poorly understood. Here, we uncover the complexity of quantum dynamics at criticality through the lens of intrinsic information compression in energy space. We show that the Hamiltonian spectrum can be systematically truncated, yielding a simplified description of the dynamics while preserving its essential features. Specifically, for both interacting and noninteracting systems, we demonstrate that a vanishing fraction of Hamiltonian eigenlevels suffices to reproduce the power-law decay of the survival probability. The resulting truncated spectrum exhibits a fractal structure characterized by a level-spacing distribution with a power-law tail, while its spectral form factor displays the same asymptotic power-law decay as the survival probability.
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All authors Simon Jiricek, Miroslav Hopjan, Boris Altshuler, Vladimir Kravtsov, Lev Vidmar

Quantum Physics

Remote entanglement need not be the bottleneck for modular trapped-ion quantum computing

Felix W. Knollmann, David P. Nadlinger, John Blue et al. (+7 more)

Modularity underpins classical computing; as quantum processors encounter limits on fabrication yield, reliability, and size, they will also need it acutely. The bottleneck to linking modules is producing shared entanglement at sufficient rate, density, and fidelity. Trapped ions hold the best demonstrated photonic links, yet they rely on bulky collection optics that cap how densely links can be packed, and remote entanglement operations trail local gates by two orders of magnitude in rate and fidelity. We synthesize several enabling results $\unicode{x2014}$ single-photon heralding, coherent recoil correction, projective distillation, and trap-integrated photonics $\unicode{x2014}$ into one comprehensive architecture that substantially narrows this gap. Single-photon heralding leads to linear scaling of success probability with detection efficiency, allowing compact integrated photonics to saturate the entanglement rate at a local-operation limit in dense, easy-to-parallelize channels. Addressing its inherent error mechanisms at their source, we project a Bell-pair fidelity of 99.9% at rates and densities compatible with fault-tolerant operations. Remote entanglement then need not remain the bottleneck for modular trapped-ion computing; the limit shifts to the local operations that must improve regardless.
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All authors Felix W. Knollmann, David P. Nadlinger, John Blue, Sabrina M. Corsetti, Sam J. Bishop, Adam R. Martinez, Jelena Notaros, Colin D. Bruzewicz, Robert McConnell, Isaac L. Chuang

Quantum Physics

Adlam's Frame: comment on "Wigner's Frame"

Andrea Di Biagio and Anne-Catherine de la Hamette

Recent no-go theorems based on extended Wigner's Friend scenarios (EWFS) reveal a deep tension between the universal validity of quantum theory and local friendliness (LF), the conjunction of three natural assumptions: the absoluteness of observed events, locality, and no-superdeterminism. In a recent work, Adlam argues that this tension can be dissolved by appealing to quantum reference frames (QRFs). We present and critically assess her proposal. We show that her proposed resolution does not arise from QRFs, but instead relies on three independent modifications: certain degrees of freedom are always definite, observers can flip upon measuring spins, and the outcomes the superobservers record and use to test the inequalities are not their friends' observed results. The proposal and the arguments for the plausibility of these modifications rest on several misconceptions about QRFs and EWFS, which we rectify. We conclude that, within standard quantum theory, quantum reference frames do not evade the EWFS no-go theorems.
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Quantum Physics

Nishimori Threshold Estimation for Bayesian Inference and $\mathbb{Z}_q$ Surface Code Decoding

Rohit Mukherjee and Simon Trebst

In quantum error correction, the error threshold provides essential quantitative guidance for the ability to bring about fault-tolerance through decoding the effects of incoherent noise, weak measurement or inference. However, the numerical value of an error threshold is typically only accessible through large-scale numerical simulations of the underlying noise model. Here we introduce an analytical estimate of error thresholds falling into the Nishimori universality class via a Fourier–Walsh projection scheme that maps the critical point of the underlying disorder-free statistical-mechanics model to the Born-disordered Nishimori critical point. Using a minimal replica theory approach, this closed-form estimate is obtained from a projection of the exact replicated single-bond weight which we find to reproduce (within a percentage point) the known numerical thresholds of random-bond and random-plaquette Ising models / $\mathbb Z_2$ stabilizer codes in spatial dimensions $d=2-5$, and extends to Potts variables with $q\le4$. The main application of our projection scheme is to $\mathbb Z_q$ surface codes, whose decoding problem maps to the disordered $q$-state clock model. For $q\ge5$ the clean clock model has two Berezinskii–Kosterlitz–Thouless transitions, which the projection maps to two Nishimori temperatures that bound an intermediate information-critical phase. The resulting threshold values not only accurately agree with recent decohered-$\mathbb Z_q$-toric-code numerics, but are found to satisfy the Gilbert–Varshamov self-dual entropy relation $\ln q \simeq H_q(T_1^\ast)+H_q(T_2^\ast),$ although no duality condition is imposed in the construction. Our approach thereby points to a deeper connection between the clean and Born-disordered models, while allowing for instant analytical estimates of error thresholds for a variety of stabilizer codes.
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Quantum Physics

Probing gravitational interaction between milligram optomechanical oscillators operating in the quantum regime

M. Bonaldi, A. Borrielli, G. Di Giuseppe et al. (+12 more)

We present an ongoing optomechanical experiment aimed at detecting gravitational interaction between milligram-scale oscillators, and exploring its interplay with quantum mechanics. The setup employs two microfabricated oscillators coupled via gravity and read out through a high-finesse optical cavity, at ultra-cryogenic temperature. Finite element simulations, noise modelling, and sensitivity estimates demonstrate that gravitational coupling can be detected with integration times ranging from minutes to hours. The experiment establishes a realistic platform for probing gravitational effects in systems approaching the quantum regime.
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All authors M. Bonaldi, A. Borrielli, G. Di Giuseppe, N. Malossi, F. Marin, F. Marino, F. Marzioni, R. Natali, G. Novelli, P. Piergentili, A. Pontin, P. Sberna, E. Serra, G. Sordo, D. Vitali

Quantum Physics · Cross-listed from cond-mat.mes-hall

Non-Abelian Gauge Field Mechanics

Ivan Velkovsky, Carlos Camacho, Tomoki Ozawa et al. (+2 more)

Non-Abelian gauge fields play a key role in describing the behavior of particles whose motion is coupled to internal degrees of freedom, such as their spin. Here, we experimentally realize a tuneable non-Abelian gauge field in an active mechanical lattice by using pairs of oscillators to encode a local pseudo-spin for each site, with inter-site spin-dependent couplings engineered via real-time measurement and feedback. We experimentally extract Wilson-loop observables in our set-up and hence demonstrate that we can create a genuinely non-Abelian gauge field. We then exploit the controllability of our mechanical lattice to engineer non-reciprocal hoppings to explore non-Hermitian non-Abelian gauge potentials. For a two-dimensional (2D) lattice, we demonstrate that the non-Hermiticity can manifest in direction-dependent Wilson loops for a single plaquette, while for a one-dimensional (1D) system, we show that a non-Abelian gauge potential can switch the localization of non-Hermitian skin modes between opposite ends of a chain. Our work establishes active mechanical lattices as a flexible and programmable platform for probing non-Abelian gauge fields and exploring their interplay with non-Hermitian dynamics.
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All authors Ivan Velkovsky, Carlos Camacho, Tomoki Ozawa, Hannah Price, Bryce Gadway

Quantum Physics

QuantiSpect: A Structure-Aware Lightweight 3D CNN Pre-Decoder for Scalable Surface Code Quantum Error Correction

Pan Gao, Xu-Sheng Xu, Ji-Ze Han et al. (+6 more)

Real-time decoding is a critical bottleneck for large-scale fault-tolerant quantum computing. AI-based neural pre-decoders locally correct most physical errors before passing residual syndromes to a global decoder, enabling sub-microsecond latencies. However, existing architectures carry significant overhead from dense 3D convolutions. We present QuantiSpect, a lightweight 3D convolutional neural network (CNN) pre-decoder for the rotated surface code, built on the decoding pipeline of Chamberland et al. The key idea is to replace the dense 3D convolutions with three parallel branches in each residual block: a depthwise spatial branch, a depthwise temporal branch, and a grouped spatio-temporal branch, followed by a squeeze-and-excitation channel gate. This reflects the structure of surface code errors, where spatial and temporal syndrome correlations are partially separable. On a unified 4xA100 GPU benchmark, QuantiSpect matches the receptive field of the Accurate baseline at R=13 while using ~2.71x fewer parameters (0.663M vs 1.80M) and ~2.84x fewer per-voxel convolutional MACs. It matches Accurate's circuit-level threshold and accuracy at moderate and large code distances, reduces the logical error rate by up to ~1.85x relative to uncorrelated PyMatching at d=13, p=0.5%, and speeds up the PyMatching decode by up to 3.11x at d=23. We also explored enlarging the receptive field by adding blocks. Even at R=21, the model uses only 1.18M parameters, fewer than both the R=13 Accurate baseline (1.80M) and the R=17 dense model (4.22M), despite its larger receptive field. This expanded variant significantly outperforms the Accurate model, raising the circuit-level threshold to ~0.80% and further reducing the logical error rate. Together, both variants show that a structure-aware factorized design is an effective, parameter-efficient alternative to a dense one for decoding the surface code.
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All authors Pan Gao, Xu-Sheng Xu, Ji-Ze Han, Jing-Wei Wen, Ling Qian, Xudong Lv, Run-Qing Zhang, Xiao-Xiao Hu, Gui-Lu Long