In briefQuantum machine learning that expresses uncertainty using Dempster-Shafer theory
SummaryThis paper introduces a quantum classification framework that represents prediction uncertainty using Dempster-Shafer theory, assigning weights to individual labels and to sets of labels. The authors propose a training strategy that lets parameterized quantum circuits learn these weight assignments directly from data. The approach supports quantum Dempster-Shafer fusion with polynomial resource requirements.
Marie Kempkes, Jakob Spiegelberg, Evert van Nieuwenburg et al.
In briefA 2026 update mapping the global quantum technology company landscape
SummaryThis paper updates a 2022 study of 441 quantum technology companies, now covering 1,092 firms founded with a quantum focus, of which 935 remain active. Using public sources and archived websites, it examines company formation, technology fields, geography, acquisitions, closures, valuations, and links to AI, defence, and space. Computing software and applications remain the largest field, with suppliers and supporting businesses accounting for almost a third of active companies.
Zeki Can Seskir, Orcun Kocak, Arsev Umur Aydinoglu
In briefQuantum Keynesian Beauty Contest with Continuous Variables
SummaryThis paper uses quantum game theory to study speculative pricing in the Keynesian Beauty Contest, or p-Beauty Contest (p-BC). The authors implement the game with continuous-variable quantum systems, where increasing multimode squeezing transforms players' bids and reduces the boom-and-bust behavior that rational play produces for p above or below 1. They also show that a similar game can be built with classical channels and a global mediator, and analyze the cases of two, three, and many players.
In briefHong-Ou-Mandel-like interference between two independent narrowband thermal states
SummaryThis paper proposes a method to measure the frequency difference between two narrowband light sources at the single-photon power level, using Hong-Ou-Mandel-like interference between thermal states. The absolute value of the frequency difference can be extracted from temporal coincidence counts, and the results agree well with the authors' theoretical model. The beat signal of this interference resembles that of Hong-Ou-Mandel interference between two heralded single-photon sources, suggesting the method can locate zero detuning between such sources.
Sheng-Hsuan Huang, Thomas Dirmeier, Andrea Aiello et al.
In briefNon-convex optimization using quantum tunneling search on a photonic chip
SummaryThis paper proposes quantum tunneling search (QTS), an algorithm combining quantum tunneling walks with local annealing to explore non-convex optimization landscapes. The authors map objective functions onto the potential term of a waveguide Hamiltonian by controlling refractive indices with femtosecond laser writing. Adding a varying Hamiltonian approach improves search efficiency and enables identification of the global minimum.
In briefLearning to control and predict light-driven electron-phonon dynamics
SummaryThis paper presents a differentiable framework that combines optimization of driving fields with neural-network prediction in a two-site electron-phonon model. It compares piecewise-constant, B-spline, feed-forward, and recurrent control parameterizations, optimized to increase accumulated electronic double occupation. For the dissipation-free model at zero Hubbard interaction, all methods increase double occupation, with B-spline raising the mean total double occupation by about 30%.
In briefKey Relay vs. Secure Network Coding: One-Shot Equivalence and Separation on Undirected Networks
SummaryMotivated by quantum key distribution networks, this paper compares key relay protocols with secure network coding on the same undirected graph. For two terminals and a one-symbol key, arbitrary key relay is shown to be equivalent to scalar linear acyclic network coding, but this equivalence fails in general when several terminals seek one common key. A four-terminal complete graph example over a ternary field separates the two approaches, and the paper also establishes a linear key relay normal form.
In briefFree-space quantum key sharing over 18.9 km between cities through a dense urban area
SummaryThis paper reports an experimental free-space quantum key distribution (QKD) link spanning 18.9 km across a densely populated area of the Barcelona metropolitan region. It uses a decoy-state BB84 system at 785 nm with room-temperature single-photon detectors and no adaptive optics at the receiver. In a 40-minute field test, the average channel attenuation was about 19 dB and the mean QBER was 1.31%, yielding an estimated average finite-size secret key rate above 11 kbit/s.
Ignacio H. L\'opez Grande, Nicolas Linale, Giacomo Paganini et al.
In briefLow-dimensional filtering for continuous quantum error correction via a classical control hierarchy
SummaryThis paper addresses the complex model estimation that limits the practicality of continuous quantum error correction by recasting it as a classical control-theoretic state hierarchy. For any stabilizer code whose generators are all continuously monitored and driven by a single Pauli recovery channel, the authors prove that the exact filter closes at dimension 3·2^(n-k-1), a dimension set by the syndrome space. They also present a committed-recovery controller that needs only 2^(n-k) class probabilities and reports higher entanglement fidelity than the compared methods in the three-qubit bit-flip code and the five-qubit perfect code.
WhyThe authors recast continuous quantum error correction filtering as a classical control hierarchy and report an exact filter dimension of 3·2^(n-k-1), a tight bound set by the syndrome space.
Manuel Guatto, F. A. C\'ardenas-L\'opez, Felix Motzoi
In briefDetecting non-Hermitian topology via long-time dynamics of open systems
SummaryThis paper proposes a method to measure a non-Hermitian topological invariant linked to the Lindbladian spectrum of open Gaussian systems with chiral symmetry, using the relaxation of the covariance matrix. The approach works in the long-time regime without isolating edge modes, so it still applies when the Liouvillian skin effect may hide them. The authors demonstrate the technique on a Kitaev chain with reservoir-induced nonreciprocity, deriving an analytical phase diagram, and show that the protocol reliably distinguishes all the phases.
In briefGround-State Spin Squeezing and Collective Excitations in Z2-Symmetric Systems
SummaryThis paper studies spin squeezing in the ground state of many-body quantum systems, a resource for quantum-enhanced measurements. Using the anisotropic Lipkin-Meshkov-Glick model, the authors show that squeezing is set by the ratio of two transverse fluctuation costs, while the collective gap is set by their geometric mean. Analyses of XXZ chains and power-law interacting systems identify a branch where squeezing and gaps increase together and squeezing persists at higher temperatures.
WhyThe authors derive an exact link between squeezing and collective excitations, identifying a gap-opening branch, checked via exact diagonalization and spin-wave analysis on XXZ chains.
Arkadiusz Kosior, George Mihailescu, and Karol Gietka
In briefConverting superpositions of nonorthogonal states with optimal probability
SummaryThis paper studies how superpositions of nonorthogonal states, such as optical cat states, can be converted into one another using operations that create no superpositions. The authors determine the optimal probability of such a conversion, which depends on how the components of the superposition interfere. The problem is fully solved for two reference states, and the paper also gives results for three or more reference states.
In briefReply to a comment by Hradil and Řeháček on phase-sensing advantage
SummaryThis is a reply to a comment by Hradil and Řeháček. The commenters argued that the quantum metrological advantage in the authors' squeezed-vacuum phase-sensing experiment disappears once measurement repetitions and total photon resources are properly counted. The reply contends that this conclusion is mathematically unfounded and that neither resource accounting nor finite-sample statistics invalidate the demonstrated quantum-enhanced local phase sensitivity.
Jens Arnbak Holb{\o}ll Nielsen, Jonas S. Neergaard-Nielsen, Tobias Gehring et al.
In briefResidual Entanglement vs. Residual Coherence: Neutrino Oscillations in Matter via Wave Packets
SummaryThis paper studies quantum correlations in three-flavor neutrino oscillations in constant matter density using a wave-packet approach rather than the standard plane-wave description. It expresses residual l1-norm coherence, Concurrence Fill, and Three-π entanglement in terms of transition probabilities for an initial muon-flavor state, with calculations for the DUNE setup. Residual l1-norm coherence is found to dominate the entanglement measures. At very long distances, wave-packet decoherence washes out the oscillations and drives all measures toward averaged values.
Lekhashri Konwar, Abhishek Kumar Jha, and Bhavna Yadav
In briefThermodynamically consistent hybrid quantum-classical dynamics via metriplectic methods for generalized Lindblad and Bloch-Torrey equations
SummaryThis paper presents a metriplectic framework for hybrid quantum-classical dynamics that ensures thermodynamic consistency by conserving energy and producing non-negative entropy. It combines a hybrid Poisson bracket with a metriplectic 4-bracket, with quantum and classical subsystems coupled only through dissipative processes. Within this framework, the authors derive thermodynamically consistent generalizations of the Lindblad master equations and Bloch-Torrey equations for open quantum systems. The approach provides a unified description of quantum relaxation and classical diffusion, along with a clear interpretation of entropy production.
In briefPersistent-mediator architecture for universal fault-tolerant quantum computation with color codes
SummaryThis paper proposes a cross-dimensional persistent-mediator architecture for universal fault-tolerant quantum computation using color codes. A 2D data register and a 3D mediator remain distinct encoded modules at each cycle boundary, coupled only through a transient protected subsystem. The authors combine a membrane-like joint-logical measurement with a cross-dimensional CZ gate, string-like surgery and feed-forward to close a deterministic logical T-gate cycle that avoids post-selection and destructive mediator readout, returning the 3D block to its initial logical state.
In briefHilbert's 17th problem for symmetric functions: undecidability and a counterexample
SummaryThe paper studies inequalities between symmetric functions that hold for every number of variables. It proves that deciding whether a rational-coefficient symmetric function is nonnegative in all dimensions is undecidable, answering a question by Acevedo, Blekherman, Debus and Riener. It also presents a degree-8 homogeneous symmetric function that is positive definite in every dimension but has no sum-of-squares multiplier uniform in the number of variables.
WhyThe authors prove dimension-free nonnegativity of symmetric functions is undecidable and give a degree-8 counterexample to uniform sum-of-squares multipliers.
In briefGeometric theory of quasiperiodically driven quantum systems
SummaryThis paper builds a general framework for quantum systems driven quasiperiodically, which lack a theory comparable to Floquet theory for periodic drives. Using counterdiabatic driving, the authors identify dynamically invariant spaces and decompose the evolution with an effective average-energy operator and gauge potential. They classify quasiperiodic drives into four classes by the geometry and topology of these spaces, extend Floquet's theorem to incommensurate drives, and propose ergodicity marked by coalescing invariant eigenspaces.
In briefAutomatic O(a) improvement for Wilson twisted mass fermions in Hamiltonian formalism
SummaryThis paper applies a recently developed Symanzik effective Hamiltonian framework to Wilson twisted mass fermions. The effective Hamiltonian is built only from the symmetries of the lattice Hamiltonian, without using the Euclidean spacetime Symanzik effective action. The authors use it to prove automatic O(a) improvement at maximal twist and O(a) improvement via the mass and Wilson average. They also derive the structure of the O(a) counterterms needed for clover on-shell improvement of Wilson fermions.
WhyThe authors apply a Hamiltonian Symanzik framework to Wilson twisted mass fermions, proving automatic O(a) improvement at maximal twist and finding the clover term splits into two independent coeffici
In briefQuantum tunneling of a measurement-driven active particle
SummaryThis paper studies a quantum active particle whose motion is locked to its spin orientation through spin-orbit coupling and damping, with the orientation kept persistent by continuous measurement via the quantum Zeno effect. Individual quantum trajectories therefore show active run-and-tumble motion. When the particle meets a potential barrier, its persistent velocity causes it to accumulate near the barrier, unlike a free quantum particle, which is reflected. The authors show that quantum tunneling raises this active particle's transmission rate above that of an analogous classical particle.
Tobias Nadolny, Matteo Brunelli, Christoph Bruder et al.
In briefA quantum algorithm for finding all eigenvalues and eigenstates of sparse matrices
SummaryThis paper presents a quantum algorithm that finds all distinct eigenvalues of a sparse Hermitian matrix and prepares a corresponding eigenstate in each eigenspace. Eigenvalues are output as binary strings at a prescribed precision, and eigenvectors are kept as quantum states rather than converted to classical vectors. The stated runtime is Õ(n s² log² n / Δ_min), where Δ_min is the minimum spectral gap, and the authors claim it beats the classical O(n³) approach under certain conditions.
In briefIdentifying beauty and charm jets with a quantum circuit trained end to end on superconducting hardware
SummaryThis paper implements a variational quantum circuit to classify b- and c-jets from simulated LHCb data on a 5-qubit superconducting quantum computer at the Politecnico di Torino. All stages of the machine-learning workflow, including training and testing, run on the real quantum hardware. The authors report that hardware-induced quantum noise does not significantly degrade performance, and they propose a framework for evaluating training time and optimizing the circuit.
Lucio Anderlini, Pietro Asinari, Gianluca Bertaina et al.
In briefGatemon qubits built on in-situ ternary InAsSb/Al nanowire Josephson junctions
SummaryThis paper realizes gatemon qubits using in-situ InAsSb/Al Josephson junctions formed in a single growth process. Gate-dependent microwave spectroscopy shows the qubit transition can be electrostatically tuned over several gigahertz. Rabi oscillations and Ramsey interference demonstrate coherent control, with representative T1 = 0.79 µs and T2* = 0.91 µs.
Zhenhai Sun, David Feldstein-Bofill, Sangeeth Kallatt et al.
In briefWhether the channel RLD Fisher information bound can be reached asymptotically
SummaryThis paper studies the channel right-logarithmic-derivative (RLD) Fisher information, a computable upper bound on the optimal asymptotic SLD Fisher information per channel use. It gives an exact equality criterion, a semidefinite feasibility condition involving only the Choi matrix and its first derivative. For generalized dephasing channels, the condition reduces to linear equations for nonnegative certificate weights. At a completely dephasing point, the authors compute optimal SLD information for every finite number of uses and show a repeated ancilla-free probe attains it, while the ratio of the two quantities can be unbounded.
WhyThe authors give an exact semidefinite criterion for when the channel RLD Fisher bound is attained and compute optimal finite-use SLD information at a dephasing point.
In briefSuperconducting qudits protected from noise using higher-harmonic Josephson junctions
SummaryThis paper studies three-level superconducting qutrits built from a loop of three Josephson junctions, using the second harmonic of the current-phase relation to protect against relaxation. It presents two designs, one based on a small junction asymmetry and one using a small symmetry-breaking term, each with different trade-offs such as charge noise sensitivity. The authors identify altermagnetic Josephson junctions as a candidate platform and propose the sign of the second harmonic as a design principle for protected superconducting qudits.
In briefHow atomic interactions affect quantum communication protocols
SummaryThis paper builds a first-principles framework for analyzing photonic quantum communication in an atomic medium, deriving an effective model for polarization-encoded single photons from a real-space quantization of the field coupled to two-level atoms. It applies the model to superdense coding, showing that the protocol capacity of a fixed Bell-state alphabet under erasure, dephasing, and depolarizing channels decreases monotonically with increasing atom-field coupling. For the BB84 quantum key distribution protocol, it proposes a modified version that discards erasure outcomes and derives the asymptotic key rate. The key rate decreases monotonically with coupling strength and vanishes beyond a finite critical coupling.
In briefQuantum Mpemba effect from logical information exposure in monitored circuits
SummaryThe paper studies the Mpemba effect in a monitored Clifford chain, where a state starting farther from stationarity overtakes one starting closer. An unencoded Bell-pair state ends a cycle with lower mean entanglement entropy than a scrambled reference, with the gap growing linearly in chain length. The authors attribute the reversal to logical information exposure and certify the effect at 128 and 256 qubits using stationary-entropy bounds.
In briefSymmetry and quasi-gauge invariance of Krylov complexity
SummaryThis paper studies Krylov spread complexity, which measures how widely a quantum system explores its Hilbert space during time evolution. The authors identify two classes of unitary transformations that leave this complexity unchanged and link this to the invariance of the initial state's energy distribution. For a localized initial state with a nondegenerate spectrum, the long-time average complexity equals (D-1)/2, where D is the Hilbert-space dimension.
Edoardo Ferretti, Matteo G. A. Paris, Claudia Benedetti
In briefLearning graph states from local Pauli measurements, with tight limits for non-adaptive strategies
SummaryThe paper studies how many copies of an unknown n-qubit graph state are needed to identify its graph when each qubit is measured in the X, Y or Z basis. For nonadaptive strategies with d^3 ln n ≤ n, it shows that Ω(d^2 log n) copies are required, even with an unrestricted classical decoder and a small fraction of misreported edges. It also shows that adaptivity between copies can beat this barrier: on certain connected bipartite regular graphs with hidden bipartition, a breadth-first adaptive learner needs only O(d log^2 n / log log n) copies.
WhyThe authors prove a tight nonadaptive lower bound of Ω(d²log n) and an adaptive separation, answering an open question on graph-state learning.
In briefCircuit-level analysis of quantum error correction in second-generation quantum repeaters
SummaryThis paper presents a circuit-level simulation of second-generation quantum repeaters using stabilizer-based quantum error correction. It models a two-segment repeater link including encoding, syndrome extraction, decoding, and entanglement swapping. The authors compare the Steane and five-qubit codes under incoherent noise (depolarizing and dephasing) and coherent over-rotation errors. The Steane code performs better with coherent errors, while the five-qubit code achieves higher fidelity under low-level incoherent noise, and no code is best at higher noise levels.
Suraj Vishwanath, Jonas Hawellek, Roberto Ferrara et al.
In briefDesigning bosonic steady states via Fock-space current engineering
SummaryThis paper presents a unified framework for autonomously stabilizing selectable Fock states, which are valuable resources in quantum technologies. The authors derive a stationary current-balance law that predicts a population background scaling inversely with Fock number and shows how a local current selectively enhances the target Fock-state population. Full quantum simulations support these predictions and demonstrate target-state populations building up autonomously from the vacuum.
In briefExact quantum algorithms for composed Boolean functions
SummaryThe paper studies exact quantum algorithms for block compositions of total Boolean functions and proves an upper bound on exact quantum query complexity in terms of the outer function's exact query complexity and polynomial degree. Applied to recursive ternary not-all-equal and other fixed gadgets, it yields improved exact rates. The composition technique also transfers separations from approximate query complexity and degree to exact quantum query complexity.
WhyThe authors give a composition construction whose costs are governed by exact query complexity and degree, improving separation exponents.
Gagan Chiruvella, Chandrima Kayal, Rajat Mittal et al.
In briefTesting whether quantum physics can break the Leggett-Garg limit
SummaryThe Leggett-Garg inequalities test the clash between macrorealism and quantum theory, and their maximal quantum violation is called the temporal Tsirelson bound. Some studies have claimed that this bound can be exceeded within quantum theory. This paper critically examines the reasoning behind these claims and finds conceptual shortcomings. Once these issues are addressed, the macrorealist bound reaches its algebraic maximum, so no violation of the bound occurs.
In briefNoise erodes the quadratic speedup of Grover search, shown analytically for local and coherent errors
SummaryThis paper analyzes how noise degrades Grover's quantum search algorithm, which gives a square-root query scaling over unstructured databases. It shows that global depolarizing noise reduces the coherent weight of the state without slowing the Grover rotation. For local noise, the tolerable error per qubit falls faster than the inverse square root of database size. For coherent rotation-angle errors, the success probability is bounded by a band that widens linearly with circuit depth.
In briefLearned eavesdropping detection in BB84 quantum key distribution does not converge
SummaryThis paper studies a game between a reinforcement-learning eavesdropper and a threshold-adapting detector in BB84 quantum key distribution. Using alternating best-response training across three learning rules and three channel-noise levels, the authors find that detection performance keeps fluctuating and does not settle to a fixed point. They conclude that a learned detector cannot be certified from any single round, and that closing the stealth window may require changing the information structure rather than out-adapting the attacker.
In briefCreating and converting vortex light in coherently prepared hydrogen atoms and hydrogen-like ions
SummaryThis paper proposes a theoretical method for generating and converting optical vortices in a coherently prepared three-level Λ-type atomic medium. An azimuthally varying phase difference between two lower states adds phase winding to the generated light, so the output topological charge equals the input charge plus the coherence winding number. Analytical results and numerical examples using hydrogen transition parameters are presented, and under resonant conditions electromagnetically induced transparency is predicted.
In briefTraining quantum neural networks one job at a time: making each cloud job count
SummaryThe paper studies how a classical optimizer can make the most of each cloud job when training a quantum neural network classifier, since every round trip to a quantum processor is costly. The authors compare optimizers given equal job counts and shots per job, differing only in how each job's parameter batch is filled. A batched parameter-shift method needed fewer jobs than SPSA in simulation and reached SPSA's final training loss on IBM's ibm_fez processor, with clearly higher test accuracy at eight qubits.
In briefUnified quantum-network infrastructure using optical frequency combs
SummaryThis paper presents an infrastructure based on optical frequency combs that carries precision timing, classical communication and quantum communication over one shared fibre link. Classical data is encoded on the comb's intensity while its pulse timing is preserved, achieving 100 Mbit/s transmission and 0.85 fs timing stability at 1 s over 100 km. The authors also analyse the excess noise introduced into a co-propagating continuous-variable quantum key distribution channel, reporting 0.0037 shot noise units of added noise and a 6.7% secret-key-rate penalty after optimization.
WhyThe authors unify timing, classical data and CV-QKD on one comb-based fibre link, modeling added noise; the abstract reports 100 Mbit/s and 0.85 fs stability over 100 km.
In briefCollective information backflow inferred from local population records
SummaryThis study examines how information backflow in two identical qubits collectively coupled to a vacuum reservoir with an exponential correlation function can be inferred from population data. An ideal bright-state population trajectory determines the bath-parameter combinations relevant to a finite-time trace-distance backflow diagnostic, which sparse records can predict via learning or physical fitting. However, a two-hypothesis bound shows that finite measurement statistics prevent distinguishing reservoirs with similar short population records but substantially different backflow.
In briefInformation thermodynamics for closed quantum many-body systems
SummaryThis paper builds an information-thermodynamic framework for closed quantum many-body systems that start in microscopic thermal equilibrium, using local measurements and finite-time controls. It proves that in the thermodynamic limit the extractable work matches that of the corresponding canonical ensemble, and expresses it via quantum-classical mutual information and Holevo information. The authors also show that feedback based on distant measurement outcomes can exploit hidden long-range correlations, extensively increasing extracted work relative to the canonical ensemble.
WhyThe authors develop a framework for many-body quantum information thermodynamics, showing local feedback matches canonical extractable work and distant feedback exploits hidden long-range correlations
In briefScaling Pelagos: Multi-GPU Software for Faster Quantum Error-Correction Decoding
SummaryThis paper extends the Pelagos relay belief-propagation (Relay-BP) decoder, a tool for correcting errors in quantum computers, so it runs across multiple GPUs. The changes add process-isolated orchestration, parallel host preprocessing, and shared-memory output to reduce overhead. Tests on two codes, the one-gross and two-gross codes, report end-to-end speedups of about 5.68 to 7.01 times. The system has been deployed on Oak Ridge National Laboratory's Lux supercomputer.
Xulong Tang, Dionysios Diamantopoulos, Tristan M\"uller et al.
In briefRoom-temperature optically detected spin coherence in the negatively charged gallium-vacancy (GaV⁻) in diamond
SummaryThis paper reports the physical properties of GaV⁻, a newly discovered negatively charged gallium-vacancy defect in diamond, created by focused ion beam implantation and high-pressure high-temperature annealing. It shows optically detectable spin coherence at room temperature, with a T₂ of 1.06 microseconds, combining features of both nitrogen-vacancy and group IV vacancy defects. The authors position GaV⁻ as a promising resource for quantum sensing, networking, and computation.
In briefHierarchical decoding for quantum error correction: a review of algorithms, code designs, and hardware trade-offs
SummaryThis paper reviews hierarchical decoding for quantum error correction, covering single-code approaches such as cascaded predecoding and windowed, streaming, and distributed implementations. It also examines how decoding is coordinated across levels of hierarchical and concatenated codes, how qubit modalities affect decoding workloads and timing, and how CPUs, GPUs, FPGAs, ASICs, and cryogenic platforms meet these requirements. The authors identify gaps in performance reporting and system integration.
Xulong Tang, Narges Alavisamani, Joan Camps et al.
In briefNoise-driven anti-Kibble-Zurek behavior with generalized stochastic resonance in a quantum Ising chain
SummaryThis paper studies anti-Kibble-Zurek behavior in a quantum Ising chain under nonequilibrium critical quenches driven by dichotomous Markov noise, a prototypical non-Gaussian colored noise. The noise-induced defect density, used as the response function, is nonmonotonic in the correlation time and shows generalized stochastic resonance. The authors attribute this to sparse realizations of the noise in the slow-noise regime, a feature the power spectrum does not capture. The results reveal a fundamental difference between Gaussian and non-Gaussian noise in this setting.
In briefInfinite or indefinite: query cost of simulating the quantum switch
SummaryThis paper studies how many calls to black-box operations a standard quantum circuit needs to reproduce the quantum switch, a device that applies two operations in a superposition of causal orders. For general channels on a d-dimensional system, the authors prove that no finite number of calls suffices for exact simulation in the worst case. They also show that approximation below error 1/2 requires a number of calls that grows with d. The paper further compares the switch with ordinary circuits in distinguishing two noise models with equal entanglement and gate fidelity.
WhyThe authors prove that circuits cannot exactly simulate the quantum switch with finitely many calls, give a d^{1/4} lower bound, and show a query advantage in noise discrimination.
In briefQuantum Control: methods for precisely steering quantum systems to target states
SummaryThis chapter introduces quantum control, the precise steering of fragile quantum dynamics toward a desired final state or transformation. It covers the control problem, basic error sources, and methods for suppressing those errors. It then discusses state-of-the-art approaches such as adiabatic methods, quantum optimal control, and machine learning.
Matthias M. M\"uller, Tommaso Calarco, Felix Motzoi
In briefHow phase interference and transverse catalysts shape noise fragility in long-range variational quantum circuits
SummaryThis paper studies how sensitive variational quantum circuits with long-range entangling gates are to a single fault. It argues that fragility depends on how fault phases interfere on the target Hamiltonian, not on how far faults spread. For Heisenberg chains, the authors prove that two faults with the same extensive support can cause either bounded or extensive energy damage, in the exact ratio N/2.
In briefTesting Quantum Advantage in High-Frequency Trading Using Market-Derived Nonlocal Games
SummaryThis paper proposes market-derived nonlocal games, where the game's inputs and payoffs come from historical order-book data and backtests of joint actions rather than an idealized model. The method is demonstrated on a two-agent, three-stock cross-venue arbitrage setting. For the measured interaction scales, the game derived from observed market data shows no resolved quantum-classical gap. Controlled variations in liquidity competition and cross-stock repricing are used to identify regimes that support quantum advantage and to test whether it survives end-to-end backtesting on modified historical data.
Daiwei Zhu, Brendan Pankovich, Alireza Marefat Khah et al.
In briefTGen-SQD: A Physics-Informed Transferable Configuration Generator for Sample-Based Quantum Diagonalization
SummarySample-based quantum diagonalization (SQD) estimates a molecule's ground-state energy by selecting a compact subset of determinants. TGen-SQD is a generator trained jointly across multiple Hamiltonians, using configuration-interaction-weighted data and orbital- and system-level descriptors to guide selection. Its weights stay frozen after training and are applied to both seen and zero-shot geometries. On a 20-qubit H10 chain, it achieves mean energy errors of 40.18 ± 1.52 and 43.97 ± 1.20 mHa, 12.7% and 11.7% lower than the PIGen-SQD baseline under matched budgets.
Niloy Deb Roy Mishu, Lei Jiang, Mohsen Heidari et al.
In briefTesting superconducting single-photon detectors in very strong magnetic fields
SummaryThis study tests a NbTiN superconducting nanowire single-photon detector in magnetic fields up to 14 tesla, adjusting the detector's orientation in situ and comparing results with normal operation at lower fields. Performance depends strongly on angle: perpendicular fields quickly shrink the usable bias-current range, while near-parallel alignment allows operation at higher total fields, keeping a finite margin up to 12 tesla and measurable dark counts at 14 tesla. The angular dependence is mainly governed by the perpendicular field component.
Harshit Agarwal, Yueh-Chun Wu, Katyayani Seal et al.
In briefQuantum search for new materials with target properties
SummaryThe paper introduces QMage, a quantum algorithm that searches over crystal compositions, unit cell structures, and atomic positions for configurations whose calculated properties meet target values within a set error tolerance. It uses a single coherent quantum circuit that block-encodes each configuration's Hamiltonian, isolates the target eigenstate, and amplifies satisfying configurations with fixed-point search. The authors design it for fault-tolerant quantum computers and estimate its cost in logical qubits and Toffoli depth. The approach generates candidates from first principles rather than generalizing from known materials.
In briefShear strain's role in narrow Si/SiGe quantum wells for spin qubits
SummaryThe paper studies the long-wavelength k1 term that couples conduction-band valleys in Si/SiGe quantum wells when paired with shear strain, and which governs Dresselhaus spin-orbit coupling. It shows that shear strain from metal top gates may be enough to activate this valley-coupling mechanism, reducing the need for ultrasharp interfaces. The authors propose a narrow quantum well scheme that avoids ultrasharp interfaces while still giving large valley splittings, spin-orbit couplings, and g-factor corrections.
In briefCommunication inequalities for telling quantum from classical statistics with minimal setups
SummaryThis paper derives dimensional inequalities that separate classical and quantum statistics in communication scenarios, a setting where no general separation method was previously known. The inequalities depend only on four parameters of the input-output statistics, corresponding to correlated and anticorrelated events. Under dimensional assumptions, saturating one inequality certifies SIC-POVMs with analytically derived noise robustness. The inequalities also serve as dimension witnesses for both correlated and anticorrelated statistics without requiring measurement incompatibility.
Albert Rico, Javier Fern\'andez, Mani Zartab et al.
In briefQuantum geometric bounds and uncertainty relations for velocity variance and spread of Wannier functions
SummaryThis paper uses a derivation of the quantum Cramér-Rao bound and an argument by Efimov and Dodonov to derive uncertainty relations for mixed states. These constrain the diagonal product, trace, and individual diagonal elements of the covariance matrix of several observables. Choosing the symmetric logarithmic derivative as the observables yields geometric bounds on the quantum Fisher information matrix set by mean Uhlmann curvature, which become bounds on the quantum metric set by Berry curvature for pure states. The work also reveals optical sum rules for the velocity variance and spread of valence band Wannier functions, and verifies these bounds using spin canonical ensembles, topological materials, common semiconductors, and transition metal dichalcogenides.
Felipe P. Abreu, Luis F. C\'{a}rdenas-Castillo, David Porlles et al.
In briefA comprehensive quantum electrodynamics study of fast electrons interacting with lossy nanostructures and quantum light
SummaryThis paper develops a quantum electrodynamical theory of how fast electrons interact with dissipative nanostructures. The authors use a modified Langevin noise formalism that quantizes scattering modes and material-generated fields independently, and derive a transverse-momentum-resolved energy exchange formula. They examine finite-temperature effects and report an intrinsic Stokes–anti-Stokes asymmetry in the resulting diffraction patterns.
In briefQuasi-cyclic qudit lifted product codes with near-linear minimum distance
SummaryThe paper introduces a qudit generalization of the lifted product construction over cyclic group algebras, extending it to arbitrary prime-power dimensions and yielding quasi-cyclic qudit CSS codes. In the fully stacked case, families built from lifted Tanner codes on expander graphs have Θ(log N) logical qudits and almost linear minimum distance Θ(N/log N). Monte Carlo simulations under a depolarizing channel show that the qudit codes achieve lower frame error rates than comparable qubit codes, with a linear merged-syndrome decoder giving further gains.
Shantom K. Borah, Abhinav Vaishya, V. Lalitha et al.
In briefSuperradiant photon bunching in nanolasers: how a hierarchy of polarization orders leads to Dicke Markov chains
SummaryThis paper studies how a few to several hundred quantum emitters sharing one light mode in nanolasers produce superradiant bursts and strongly bunched photon statistics below threshold. The authors derive a hierarchy of models from the laser master equation, classified by polarization order, and show that eliminating all coherences yields a Dicke Markov chain predicting superthermal bunching and an even-odd dependence on emitter number. A population-polarization model that keeps the optical polarization but eliminates two-photon coherences reproduces the full master equation's bunching almost perfectly.
In briefU(1)-Symmetric Spin SYK Models: Ordering, Chaos, and Transport
SummaryThis paper studies a U(1)-symmetric spin version of the complex SYK model, built from random charge-conserving multi-spin interactions native to qubit systems, which avoids fermionic encoding overhead. In the large-N, large-q limit, it reproduces the leading saddle-point structure of complex fermionic SYK. Using exact diagonalization at finite size, the authors examine ordering susceptibility, level statistics, and domain-wall dynamics in coupled-cluster chains.
Shiyong Guo, Enrico Rinaldi, Jad C. Halimeh et al.
In briefBell nonlocality can be revealed without repeating any measurement direction
SummaryThis paper asks whether Bell inequality violations still occur when measurement directions never repeat. The authors ran an experiment with polarization-entangled photons, detecting only one particle pair per randomly chosen analyzer direction. They report that violations remain possible and keep their operational significance even without repeated measurements.
Pawe{\l} Cie\'sli\'nski, Carlotta Versmold, Jan W\'ojcik et al.
In briefLearning Quantum Hamiltonians from Dynamical Structure Factor Data for Neutron Scattering Analysis
SummaryThe paper studies how to learn a quantum Hamiltonian, which describes microscopic interactions, from the dynamical structure factor (DSF) that captures two-point correlations in quantum magnets. The authors show the DSF contains enough information to efficiently reconstruct broad classes of quantum Hamiltonians. Using simple relations between low-order spectral moments of the DSF and microscopic couplings, they introduce three complementary tomography protocols based on high-temperature expansion, strong-field polarization, and the fluctuation-dissipation theorem. The framework is intended for practical use with common experimental data such as neutron scattering.
In briefCrystal-symmetry anomalies in lattice systems and a crystalline equivalence principle
SummaryThis paper gives a microscopic description of crystalline symmetry anomalies in bosonic lattice systems in one and two spatial dimensions. The authors construct anomaly indices for symmetry operators and derive their cocycle conditions and coboundary ambiguities. The results establish a crystalline equivalence principle for bosonic anomalies, and show that a nontrivial total anomaly index prevents a symmetric invertible phase.
WhyThe authors build microscopic anomaly indices for crystalline symmetries on bosonic lattices, establish a crystalline equivalence principle, and prove obstructions to symmetric invertible phases.
In briefk-partite quantum homomorphisms: a framework for multiplayer quantum constraint problems
SummaryThe paper extends k-XOR games to k-partite homomorphism games on arbitrary relational structures, providing a framework for multipartite quantum constraint satisfaction and quantum homomorphisms. It characterises perfect strategies in the tensor-product and commuting-operator models as Kleisli morphisms of graded quantum monads. The authors compare synchronous and k-partite quantum advantage, exhibit an NP-complete six-element template with two-player quantum advantage, and prove that no bounded-width template admits k-partite quantum advantage.
In briefBoosting neutral-atom gate fidelity by correlating and synchronizing atomic motion
SummaryThis paper studies two-qubit controlled-phase gates based on Rydberg blockade in neutral-atom tweezer arrays. It shows that engineered collective dissipation can correlate and synchronize atomic motion, which suppresses motion-induced gate errors. At finite temperature, these motional correlations reduce gate infidelity compared with independent thermal atoms.
In briefQuantum search methods for finding paths that meet several metric limits at once
SummaryThe paper studies finding a path in a directed acyclic graph whose totals under several additive metrics all fall within given ranges. The authors propose a Grover search whose state preparation builds a uniform superposition over all source-to-destination paths using one qubit per arc and no ancilla, and they prove its correctness and give its exact gate count. They relate the problem to max-LINSAT and Optimal Polynomial Intersection, verify the translation in simulation, and run two small binary instances on IBM Heron processors.
In briefBuffer-gas-assisted fluorescence spectroscopy of molecular ions
SummaryThis paper presents a nondestructive method for continuous fluorescence detection of strongly rovibronically open molecular ions, specifically 232ThF+. Room-temperature buffer-gas collisions replenish the laser-addressed states, creating effective quasi-cycling transitions without optical repumping or auxiliary atomic readout ions. A signal-to-noise ratio of about 6 is reported for the P(1) line in a 1 s exposure.
In briefComparing quantum and classical thermal equilibrium for probabilistic classical fields
SummaryThe paper describes how the time evolution of probability distributions for classical fields can be captured by quantum formalism, with free quantum field theory appearing as a subsystem for free fields. It compares the momentum dependence of energy and field correlations under classical (Rayleigh-Jeans) and quantum (Planck) thermal equilibrium. Numerical simulations with interactions are used to determine which equilibrium is selected by asymptotic dynamics. For free fields, the authors construct a classical-quantum map between the classical probability distribution and a quantum field theory density matrix.
In briefCritical excited states in quantum metrology: a paper on using phase-transition behavior of excited states to improve precision measurements.
SummaryThis paper studies criticality-enhanced sensing near ground- and excited-state quantum phase transitions in a strongly coupled qubit-boson system. Accounting for finite-size effects, the authors design driving protocols that exploit diverging susceptibilities to boost signal-to-noise ratios in parameter estimation. They show that excited-state criticality offers a metrological advantage through single-mode bosonic squeezing in regimes that avoid the death of adiabaticity.
In briefExact mixed encodings for resource-limited QAOA
SummaryThis paper studies how to choose Hamiltonian encodings for higher-order objectives in the quantum approximate optimisation algorithm (QAOA) on resource-constrained hardware. It proposes a family of pointwise-exact mixed representations that replace selected higher-order interactions with shared Rosenberg auxiliaries while keeping the rest native, with conditions certifying exactness. Compilation experiments on 12-qubit line, ring, and grid targets identify feasible mixed-only regions for many instances, while noiseless depth-one evaluation shows the native encoding achieving the best mean objective.
In briefDefect-stabilized Wigner negativity and parity-protected phase locking in a quantum oscillator
SummaryThe paper shows that a parity-preserving delta-shaped potential, modeling a localized defect, reduces the continuous U(1) symmetry of a quantum van der Pol oscillator to a discrete Z2 symmetry. This protects even-parity steady-state quantum coherences, producing non-classical steady states with negative Wigner functions. The authors also propose an experiment using the motional states of ultracold alkaline-earth-like atoms in optical tweezers.
