In briefLong-lived light-driven bimagnon pairs at finite momentum in NiO
SummaryResearchers pumped NiO with mid-infrared pulses far below its bandgap to excite phonon-assisted two-magnon absorption, then probed the resulting dynamics with time-resolved resonant inelastic X-ray scattering. The one-magnon linewidth and energy stayed essentially unchanged, indicating preserved long-range magnetic order. A pump-induced two-magnon signal peaks about 4 ps after excitation and decays with a time constant of 28±11 ps, far longer than previously reported two-magnon dephasing times.
Xian P. Yang, Christopher T. Parzyck, Martin Cross et al.
In briefDiscrete time crystals stabilized by anti-unitary periodic driving and dynamical freezing
SummaryThe paper proposes stabilizing a discrete time crystal using anti-unitary periodic driving, through a mechanism called dynamical freezing. Magnetization emerges as a conserved operator, and an exact pairing of two-cycle quasi-energies locks in the subharmonic response. Infinite-chain simulations show no detectable decay for generic states with finite magnetization. An outline of an experimental realization of the anti-unitary drive is also given.
In briefEnsemble phase transitions in measurement-unitary hybrid quantum circuits
SummaryThis paper studies hybrid quantum circuits that alternate unitary evolution with projective measurements, examining an ancillary qubit entangled with the system. It analyses two state ensembles: the trajectory ensemble conditioned on measurement outcomes and the projected ensemble generated by final-boundary measurements. Both ensembles undergo non-analytic ensemble phase transitions at the measurement-induced purification critical point, even though their first moments remain maximally mixed.
In briefQuantum Metrology and Quantum Battery in a Hyperfine-Coupled Spin System
SummaryThis paper studies an effective electron-proton spin system inspired by the hydrogen atom's hyperfine structure, examining both multiparameter quantum metrology and energy storage. The authors treat the external magnetic field and hyperfine coupling strength as unknown parameters, deriving the quantum Fisher information matrix and quantum Cramér-Rao bounds for their joint estimation. They also characterize the system's ergotropy, anti-ergotropy, storage capacity, and energy-basis coherence under a unitary charging protocol.
In briefStatistics of correlation lengths in the many-body localised phase's Fock space
SummaryThis paper studies long-range many-body resonances in the many-body localised phase of disordered quantum systems, which appear as strong inhomogeneities in eigenstate amplitudes on Fock space. The authors develop a microscopic theory based on two-point Fock-space correlation functions and apply exact diagonalisation to the disordered tilted-field Ising chain. They find that typical correlations decay exponentially, while average correlations decay subexponentially with system-size dependence driven by rare resonant nodes.
In briefTraces of vacuum squeezing: a superconducting circuit that mimics particle creation in the early universe
SummaryThe paper proposes a superconducting transmission line loaded with dc-SQUIDs as an analogue of a scalar field in a radiation-dominated expanding universe. Its mode equation reduces to the Weber equation, which has an exact solution in parabolic cylinder functions and an exponentially suppressed particle spectrum. The authors show that the anomalous moment of each mode witnesses Bogoliubov mixing and survives dissipation.
Paula I. Villar, Federico Guglielmucci Nazar, Eli\'an Urtubey et al.
In briefSharp uniform Fisher information bounds from projective designs
SummaryThe paper studies fixed measurements for multiparameter pure-state metrology that stay informative without knowing the input state. It defines the Fisher floor as the smallest fraction of quantum Fisher information retained across all pure states and tangent directions. Bounds on this floor are derived from projective design order alone, and they approach one half, the single-copy quantum limit, as the order increases.
WhyThe authors derive dimension-independent Fisher floor guarantees from projective design order, reporting tightness at order two and odd orders, with applications to pure-state estimation.
In briefNear-optimal multiparameter quantum metrology using a fixed small number of measurement bases
SummaryThis paper studies multiparameter pure-state quantum metrology, where measurements optimized for different parameters are incompatible. It introduces the Fisher floor, the smallest fraction of quantum Fisher information retained across all pure states and parameter directions, as a measure of uniform sensitivity. The authors prove that a constant number of fixed orthonormal bases, independent of the Hilbert-space dimension, achieves a nearly optimal Fisher floor. They also show that d+1 mutually unbiased bases, when they exist, have a Fisher floor of at most 1/(d+1), so optimal average balance does not ensure uniform sensitivity.
WhyThe authors show a constant number of fixed orthonormal bases yields a dimension-independent near-optimal Fisher floor, with tight bounds and a contrast to MUBs.
In briefStiffness in Confined Quantum Walks: how boundaries shape spreading
SummaryThis paper studies discrete-time quantum walks confined to a finite lattice by unitary reflecting boundaries built from coin-flip operations. The authors define a dimensionless spreading stiffness and compare localized, Gaussian, and truncated-uniform initial states. They find that long-time behavior depends strongly on the initial profile, and that dynamical stiffness and spatial homogenization are distinct properties.
Pablo G. M. Dantas, Rafael Vieira, Edgard P. M. Amorim
In briefObserving genuine multi-time quantum memory: a quantum memory effect that only appears when three or more times are examined together
SummaryThis paper studies memory effects in open quantum systems, noting that non-Markovian dynamics alone does not prove quantum memory, since classical-memory models can reproduce it. The authors show that some quantum memory escapes all tests comparing two times and becomes detectable only when three or more times are considered jointly. They derive an analytical three-time witness and experimentally demonstrate this hierarchy on a superconducting-qubit platform with time-independent couplings.
WhyThe authors introduce quantum memory detectable only via three-time witnesses, derive an analytical witness, and report experimental demonstration on superconducting qubits.
Konstantin Beyer, Valentin Link, Surihan Sean Borjigin et al.
In briefA four-city quantum key network in Slovakia that delivers keys in real time and is checked against finite-key security limits
SummaryThis paper reports the entanglement-based part of Slovakia's national quantum communication infrastructure, linking four cities in western Slovakia over deployed telecom fiber. Entangled photons from a central node are routed by wavelength to form a fully meshed network of three cities without trusted relays, with the fourth city connected through the central node. Keys are delivered in real time via ETSI GS QKD 014, and reported rates are measured after full post-processing. The implemented scheme is also benchmarked against finite-key security bounds.
In briefScalable Pauli propagation via F₂-linear partitioning
SummaryThis paper presents a parallelization method for Pauli propagation, which simulates quantum circuits by evolving observables as sparse sums of Pauli strings. The authors partition the strings with an F₂-linear hash function, so each gate application splits into independent tasks with bounded communication. Benchmarks of their open-source implementation show efficient parallel scaling within and across compute nodes, handling over 10^11 Pauli strings.
Alev Orfi, Andrew Kille, J. Clayton Peacock et al.
In briefNon-interferometric tests of gravity-related decoherence using mechanical oscillators
SummaryThis paper studies three phenomenological models of quantum-gravity-induced decoherence, based on deformed relativistic symmetries, stochastic metric fluctuations, and a stochastic generalized uncertainty principle. For a harmonic oscillator, the models cause anomalous heating with qualitatively different energy dynamics. The authors compare the models with existing mechanical platforms and conclude that heating and continuous measurements are complementary non-interferometric probes of gravity-related decoherence.
In briefExperimental 1D Gaussian-modulated CV-QKD with finite-size parameter estimation
SummaryThis paper experimentally demonstrates a one-dimensional Gaussian-modulated continuous-variable quantum key distribution system over 12 km of standard single-mode fiber. It uses a local local oscillator architecture with independent lasers and a full processing chain including coherent detection, reconciliation, parameter estimation, and privacy amplification. The system achieves privacy-amplified rates of 1.421 Mbit/s under asymptotic security and 0.364 Mbit/s under the finite-size scenario.
Christiano M. S. Nascimento, Maron F. Anka, Gabriel A. V. Menezes et al.
In briefUsing large language models to generate quantum circuit parameters
SummaryThis paper studies whether a frozen, general-purpose large language model can generate complete parameter vectors for variational quantum circuits from symbolic circuit and Hamiltonian descriptions, with optional iterative energy feedback. It tests random Hamiltonians, structured parent Hamiltonians, and molecular systems. For general Hamiltonians the generated parameters show larger energy errors than Adam optimization, while for structured Hamiltonians the model can produce promising low-energy configurations.
Tiago de Souza Farias, Caio Moraes Porto, Matheus da Silva Fonseca et al.
In briefQuantum Boltzmann equation for static electron correlation with cumulant closure
SummaryThis paper proposes a Quantum Boltzmann Equation for static electron correlation, mapping the BBGKY hierarchy onto discrete atomic-orbital matrix mechanics. The two-body cumulant is projected onto its lowest unitary invariant and relaxed toward a hyperbolic-tangent attractor via Markovian collisional relaxation, giving a parameter-free cumulant kernel. The method is tested on the H3 exchange reaction, H4 dissociation, BeH2, H2O and N2 bond cleavage, an 11-molecule dissociation set, and benzyne diradical isomers. The authors report that the kinetic relaxation resolves strong static correlation at zero electronic temperature.
In briefExplicit maximally entangled states in dimensions twice a prime power
SummaryThe paper constructs explicit families of absolutely maximally entangled (AME) states for multipartite systems with local dimension 2q, where q is an odd prime power, using a coupling of qubits and q-level systems. It gives AME(N,2q) results for several party numbers and, together with known inputs, completes the classification for nine parties: an AME(9,d) state exists if and only if d is at least 3.
WhyThe authors construct AME states in dimension 2q via a qubit-qudit coupling and report completing the nine-party classification with several explicit parameter families.
In briefLattice response theory for subsystem symmetry-protected topological phases
SummaryThis paper develops a lattice response theory framework for studying subsystem symmetry-protected topological (SSPT) phases. It builds spacetime lattice gauge fields from X-type symmetry actions on qudit systems and shows that a gauge-invariant response action yields an entangler that stacks the corresponding SSPT phase. The authors also construct a new 3+1d SSPT phase protected by two 2-foliated planar subsystem symmetries and a global 0-form symmetry.
WhyThe authors develop a lattice response framework for subsystem SPT phases and construct a new 3+1d phase with a mixed 't Hooft anomaly.
In briefEfficient PEPS gradient computation for optimized quantum state preparation on 2D hardware topologies
SummaryThis paper addresses preparing shallow quantum circuits for complex entangled states. The authors propose a topology-aware method that combines local disentanglement with global fine-tuning via gradient optimization of 2D projected entangled pair states (PEPS). They also describe an efficient procedure for computing PEPS gradients. Numerical simulations are used to test the method's convergence and performance.
In briefGeneralized ladder operators and superintegrable systems built with Painlevé VI
SummaryThe paper studies quantum Hamiltonians whose separated angular part is given by a generic Painlevé VI transcendent. It constructs generalized ladder operators for one-dimensional Schrödinger operators with a quadratic spectral variable. Combining these with radial shift operators yields higher-order integrals and deformed Racah-type polynomial algebras, giving new two-dimensional superintegrable families with Painlevé VI in the potential.
In briefMulti-particle quantum walk search on a two-dimensional lattice
SummaryThis paper studies how to formulate a search problem using multiple particles performing quantum walks and how efficient such search algorithms are. Each particle evolves in discrete time under Grover's quantum walk operator, with a focus on local interaction models where states change only when particles meet on the same grid site. The analysis centers on a two-dimensional grid and examines how the search's asymptotic success time and probability depend on interaction strength and maximal entanglement between particles. The authors report that two- and three-particle searches, with or without interactions, achieve a constant-factor improvement over the single-particle case, and they also propose a new generalized interaction model.
In briefGaussian ergotropy as a probe of spatial correlations between accelerated vacuum and thermal fields
SummaryThis paper studies whether work extraction from two harmonic-oscillator detectors can reveal the spatial correlations of their environment, comparing an accelerated vacuum with an inertial thermal field of matching local spectrum. It links equilibrium Gaussian ergotropy to a response-weighted spatial correlation and proposes a two-setting scheme that reconstructs these correlations from work measurements.
In briefNoise-robust super-Heisenberg scaling in quantum sensing via subspace-twirled dynamical decoupling
SummaryThis paper studies quantum parameter estimation using time-dependent Hamiltonians to achieve precision beyond the Heisenberg limit, known as the super-Heisenberg limit. The authors propose a dynamical decoupling scheme based on subspace twirling, which averages random unitaries over a subspace instead of the full Hilbert space, to preserve the encoding signal while removing environmental noise. They give conditions and performance guarantees for reaching super-Heisenberg scaling without adaptive control, and illustrate the results with a qubit sensor example.
In briefPhysical observables and superselection rules
SummaryPhysical states are usually represented by unit vectors in a Hilbert space, and observables by self-adjoint operators. This paper reviews how superselection rules restrict which self-adjoint operators can represent observables, with consequences for states and the processes that transform them. It considers arguments for superselection rules arising from charge or possibly energy conservation, and discusses effective superselection rules arising from decoherence or the lack of a shared reference frame.
In briefErgodicity breaking in a dissipative Ising model for data classification, with competing two- and four-body interactions
SummaryThe paper examines how data processing relates to the dynamics of interacting many-body systems. It studies an Ising model with competing two- and four-body interactions, whose dissipative evolution is described by a Lindblad master equation. Using mean-field calculations and exact small-scale numerics, the authors show that the stationary state manifold can be designed to enable multi-class classification. They also analyze the resulting basins of attraction, showing that the system can classify quantum states with nonlinear decision boundaries.
In briefDetecting entanglement via moments of the realigned state for continuous-variable systems
SummaryThe paper introduces separability criteria based on singular and weak moments of the realigned density operator. The second-order moment criterion alone is found to detect entanglement in a wide range of quantum optical states. The moments can be measured with a linear interferometer that combines several identical copies of the state. The method's detection ability is illustrated for Gaussian and non-Gaussian states, including lossy NOON states.
In briefImproved low-cost quantum error mitigation using benchmarked-noise zero-noise extrapolation
SummaryThis paper theoretically extends and experimentally tests benchmarked-noise zero-noise extrapolation (bnZNE), a simple, low-cost method for reducing errors in quantum computations. It improves the classical extrapolation step to aim for better results than standard ZNE at the same total shot cost. Tests on IBM and IQM hardware across many large-scale applications found bnZNE consistently more accurate than standard ZNE.
Joseph Harris, Kevin Lively, Peter Ken Schuhmacher
In briefLorentz Boosts and Field Equations of the Quantum Geometric Tensor in 4D Energy-Momentum Space
SummaryThis paper extends the quantum geometric tensor, which combines the quantum metric and Berry curvature in solids, to four-dimensional energy-momentum space with frequency and momentum as independent coordinates. Its Lorentz transformation is fixed by the invariance of the real-spacetime plane-wave phase. Lorentz boosts of static Dirac and Weyl systems generate frequency metrics, shear metrics, and electric-like Berry-Maxwell fields. The authors derive governing field equations and flow conservation laws, and discuss experimental realizations in spacetime crystals or time-varying media.
In briefStatistics of coupled defects on lattices, where higher groups describe how defects of different dimensions interact
SummaryThe paper defines higher-group statistics on lattices as Berry phases of closed sequences of hopping operators. Its central example is a closed Majorana chain embedded in three dimensions, which gives a loop-flip process with phase +i. In two dimensions, charge-wall statistics distinguish two Z4 toric codes that have identical anyon statistics but different gauged theories. The authors also present two methods for generating statistical processes, which detect boundary anomalies of higher-group symmetry-protected topological phases.
In briefA unified master equation for driven multilevel quantum systems, with spectral signatures
SummaryThe paper derives a unified master equation for arbitrarily driven multilevel quantum systems, using Lewis-Riesenfeld dynamical invariants within Born-Markov theory. It applies the equation to constant, periodically modulated, ramped and chirped Rabi-coupling protocols. Fluorescence spectra of a periodically driven three-level Λ system show features that conventional master equations miss. The authors attribute these shortcomings to inaccurate bath-frequency sampling and neglected nonsecular dissipative couplings, and identify parameter regimes that need a unified treatment.
In briefQubit Communication Advantage in the Holevo-Frenkel-Weiner Scenario
SummaryThe paper studies communication tasks in the Holevo-Frenkel-Weiner framework, asking whether a qubit can outperform a classical bit. It shows a qubit advantage without extra quantum resources such as entanglement, with shared classical randomness treated as free. The authors introduce a witness that separates quantum strategies from classical ones and examine the advantage under noise. They also link the advantage to the detection of coherence.
In briefSqueezed terahertz light from electron band nonparabolicity in ultrastrongly coupled 2D electrons
SummaryThis paper studies two-dimensional electron gases in terahertz cavities, showing that electron band nonparabolicity turns the system into a quantum Kerr oscillator that can squeeze terahertz light. The authors use a non-rotating-wave theory with a self-consistent electron heating model, predicting 1.8 to 2.4 dB output squeezing for present GaAs samples. Improved mobility or cavity frequency could raise this to about 9 dB.
In briefWhen can local unitaries complete a state into an orthogonal basis?
SummaryThis paper studies whether a multipartite pure quantum state can be completed to an orthonormal basis using only local unitary rotations, a property called universal local encodability (ULE). The authors prove that two-qutrit systems are universally locally encodable, resolving a previously open conjecture. They also construct universal local encodings for several other dimension combinations and give a necessary condition for universality in finite-dimensional multipartite systems.
Julian Eisfeld, Hermann Kampermann, Nikolai Wyderka
In briefSymmetry-controlled critical states and localization in monitored superconductors
SummaryThe paper studies a continuously monitored model of free fermions with pairing, examining how quasiperiodicity affects localization and symmetry class. Using topological entanglement entropy as a probe of nonlocal correlations in quantum state diffusion simulations, the authors identify a transition between a critical phase and an area-law phase, with no transition in the clean limit. Deep in the Zeno regime, they derive an effective non-Hermitian Bogoliubov-de Gennes description and a closed-form localization length that agrees with numerics.
Pinaki Singha, Marcin Szyniszewski, Nilanjan Roy et al.
