In briefPredictive electronic feedback control for narrow-linewidth lasers
SummaryNarrow-linewidth lasers are key light sources for quantum control, precision measurement, and quantum information. The paper addresses the time delay in the Pound-Drever-Hall (PDH) feedback loop, which causes excess phase noise known as servo bumps. The authors propose a Smith predictor method that handles the delay entirely within the electronic part of the loop. Adding only one FPGA controller, they show it suppresses phase noise and reduces servo bumps.
In briefKrylov Complexity and Entropy in Composite Quantum Systems
SummaryThis paper studies how Krylov complexity changes as a quantum system gains more degrees of freedom. For tensor products of commuting subsystems, the leading excess complexity is governed by a weighted variance of the subsystems' Lanczos coefficients, which measures dynamical heterogeneity and characterizes synchronous evolution. The authors also show that Krylov entropy separates information from subsystem composition from information due to dynamical mismatch, with the same variance controlling the leading mismatch contribution to both quantities.
Jeff Murugan, Victor Van Aswegen, Hendrik J R Van Zyl
In briefQubits Alone Are Not Enough: Limits of Quantum Modal Analysis
SummaryThis paper proposes an evaluation framework for variational quantum eigensolvers applied to finite-element structural dynamics. A Cholesky transformation converts the generalized eigenproblem into a symmetric, mass-orthonormal Hamiltonian, and higher-mode deflation conditions are sharpened with a Davis-Kahan bound for approximate projectors. Numerical studies separate mapping correctness, expressibility, preprocessing, Pauli term growth, shot noise, and damage localization. The authors report resource bottlenecks and explicitly claim no computational speedup.
In briefOrbital-coherence memory shaping electrical transport, linked self-consistently to Maxwell response
SummaryThe paper studies how an internal electronic orbital coherence can alter electrical transport by feeding back on the physical current. Starting from an opposite-parity two-orbital Hamiltonian, it derives under stated closure and collision assumptions a passive three-state model with exponentially damped oscillatory memory. A charge-conserving spatial extension couples this response to Maxwell equations and proposes complex-conductivity and transmission tests. The authors note that conductivity alone does not establish a uniquely quantum mechanism, and material-specific calculations and closure-error assessment are still required.</summary>
In briefFinite Systems in the Space of Projective Measurements: A method for approximating two-level system dynamics using projected quasidistributions
SummaryThe paper studies two-level systems by projecting the quasidistribution Q function onto the space of collective measurements. It builds an approximate analog of the star product and rewrites the Liouville equation to approximate state dynamics, comparing results with exact evolution via the Hellinger distance. The method works well for permutation-invariant pure states and Hamiltonians, but deviates for arbitrary pure states and noninvariant Hamiltonians.
In briefAI agents automate quantum circuit optimization, with every result verified
SummaryThe paper presents a general method that uses AI agents to solve quantum algorithm compilation optimization problems, with every result certified by a frozen, exact verifier. It applies the method to three quantum simulation compilation problems: fermion-to-qubit encoding, Hamiltonian layout, and unitary synthesis. At the largest sizes tested, the agents improved on published or standard constructions, reducing the optimized cost by up to 73%.
WhyCombines autoresearch loops with a frozen exact verifier, improving prior constructions on three quantum simulation compilation problems by up to 73%.
In briefQuantum Fokker-Planck solvers as trainable generative models: a paper using quantum algorithms to solve stochastic-equation probability distributions for sample generation
SummaryThe paper proposes a generative model built on quantum solvers for stochastic differential equations, parameterizing the potential of a Langevin process and solving the associated Fokker-Planck equation. The method uses quantum singular value transformation and allows sampling through computational-basis measurements. The authors report that training to match empirical distributions within the Fokker-Planck family recovers parameters for multivariate distributions, and that the model remains trainable at scale.
Tariq Mahassen, Marcin Jastrzebski, Oleksandr Kyriienko
In briefHighly accurate entanglement between two solid-state emitters with different frequencies
SummaryThis paper presents a method that uses readily available entangled photon pairs to entangle solid-state quantum emitters, such as quantum dots, even when their transition frequencies and linewidths differ. Existing approaches lose entanglement quickly as emission-line separation or linewidth variation grows. The authors demonstrate a concurrence of C = 0.95 for typical quantum dots using narrowband photon inputs.
In briefRank-one loss maximizes response of passive full-order exceptional points
SummaryThe paper studies finite-dimensional non-Hermitian systems in which all modes coalesce at one exceptional point. For a rank-one anti-Hermitian term, the spectral response strength is computed exactly. For passive systems at fixed decay rate, a sharp upper bound is derived and reached only when the decay operator has rank one. Hence rank-one loss is identified as the optimal configuration for enhancing full-order exceptional-point response.
WhyDerives a sharp passive bound on full-order exceptional-point response, proving rank-one loss optimal and improving prior estimates beyond second order.
In briefCan entanglement-based feature maps improve projected quantum kernels?
SummaryProjected quantum kernels (PQKs) address machine learning tasks using the exponential dimensionality of quantum Hilbert space, but prior studies often depend on heavy classical tuning of RBF and SVM parameters. This paper compares untuned and tuned PQKs against an RBF baseline to assess their native inductive bias. It also proposes new quantum feature maps using two-qubit entanglement to capture multivariate correlations in IoT data. Results show extensive tuning yields competitive performance, while the proposed entanglement topologies exhibit a significantly stronger native inductive bias.
In briefReset-Enhanced Dissipative Quantum Metrology: a simple reset step can improve quantum measurement precision in open systems.
SummaryThe paper studies whether adding a simple reset channel to an open quantum system can improve metrological precision without changing the Hamiltonian or applying continuous coherent driving. The authors derive a unified quantum Fisher information reset-rate response identity that covers both steady-state and transient estimation. They report that the effect differs sharply between static-parameter estimation and elapsed-time estimation in the short-time regime.
In briefCharging quantum batteries without fluctuations using Bell-like entangled and separable charger states
SummaryThis paper studies a three-qubit cascaded collision model in which two qubits act as the charger and one as the battery, with adaptive measurements made on the charger after each collision. The authors derive analytical recurrence relations for the stored energy. They find that certain Bell-like entangled state families and their separable counterparts yield zero energy fluctuations, while other states do not. The results suggest a way to design charger states for uniform charging despite quantum randomness.
In briefHow local dephasing keeps a spin-1/2 ensemble in a continuous time crystal
SummaryThis paper studies a fully connected ferromagnetic spin-1/2 ensemble with local decay and shows that local pure dephasing can sustain continuous time-crystalline order. Within homogeneous mean-field theory, the authors prove that dephasing is necessary for periodic motion. Liouvillian spectra and two-time correlations further show that this temporal order can take part in metastable dynamics.
In briefVerifying entanglement in attosecond light using photoelectron interferometry
SummaryThe paper addresses how to certify entanglement in extreme-ultraviolet light produced by high-harmonic generation. It presents experimentally accessible separability tests and shows that attosecond photoelectron interferometry can serve as a direct method by mapping it to optical correlation functions. The authors derive a negative-partial-transpose witness for infrared-XUV entanglement and a sufficient witness for genuine multipartite entanglement, and discuss extending the approach to XUV-XUV entanglement.
In briefCreating a superleak in a quantum gas to separate superfluid flow
SummaryResearchers used an optical superlattice to build a superleak in an ultracold quantum gas, a device that lets superfluid pass while blocking normal flow. The superfluid moves through coherent Josephson transport, while the normal component responds slowly, giving a flow-rate contrast of 500(49). The team also showed repeated filtering of the normal component and dynamical purification of the superfluid, with coherent oscillations sustained for up to 100 cycles.
In briefQuantum Constrained Hamiltonian Descent: a continuous-time quantum method for optimization with equality constraints
SummaryThe paper proposes Quantum Constrained Hamiltonian Descent (QCHD), a continuous-time quantum framework for optimization under smooth equality constraints. It builds an intrinsic quantum Hamiltonian with a Laplace–Beltrami operator on the constraint manifold, so the dynamics preserve constraints exactly without penalty terms. The authors also analyze how it relates to Quantum Hamiltonian Descent with a strong penalty term, and compare the two numerically on circular, elliptical, and two-constraint problems.
In briefThe often-overlooked role of sound in liquid heat transport
SummaryThis paper revisits the Bridgman formula, proposed a century ago, which links a liquid's thermal conductivity to its sound velocity and thereby suggests phonons carry heat in liquids. The authors note that this empirically successful relation has been underappreciated and is absent from well-known condensed-matter textbooks. The paper reviews its origin, its connection to the hydrodynamic Euler equations and the paradox surrounding the parabolic heat equation, and how it behaves across the quantum-classical crossover.
In briefLimits on how efficiently magic states can be distilled, a key cost in quantum computing
SummaryMagic-state factories are a major cost of fault-tolerant quantum computing, but the minimum number of noisy inputs they need was known only for restricted protocols. The paper shows that, for tolerating input faults, any stabilizer protocol is no better than projecting onto a single stabilizer code. Searches over all codes find that quadratic distillation of one T state needs exactly nine inputs, and that the eight-input CCZ factory is optimal.
WhyProves any stabilizer distillation protocol cannot beat one code projection, and exhaustive searches establish nine-input optimality for quadratic T distillation and eight for CCZ, a strong result.
In briefProduct formulas that use operator norm differences to cut Hamiltonian simulation error
SummaryThis paper proposes first- and second-order product formulas for approximating the time-evolution operator in quantum simulations. The formulas exploit differences in operator norms among Hamiltonian components to suppress the leading error term by a factor alpha less than one, compared with standard Lie-Trotter-Suzuki splitting of the same order. Unlike the THRIFT formula, they keep the general product-formula structure, so they apply to terms acting on any number of qubits. Numerical experiments on two-dimensional spin-1/2 XY models and isolated molecules in the double-factorization framework demonstrate the approach.
Carlos A. Pi\~neiro Moreno, Pascal Krause, Annika Bande
In briefScale-free localization from one non-Hermitian defect in pseudo-Hermitian quantum walks
SummaryThis paper studies scale-free localization in one-dimensional quantum walks, where a single non-Hermitian defect produces eigenstates whose localization lengths grow linearly with system size. Increasing non-Hermiticity first drives a pseudo-Hermitian phase transition, with scale-free localization appearing alongside pseudo-Hermitian symmetry breaking. A further increase leads to a second transition, where two pairs of bound states emerge. The authors locate both critical points using localization properties and extend the phenomenon to two-dimensional quantum walks.
In briefHow stabilizer codes redistribute noise: a unifying view of concatenated three-qubit codes
SummaryThe paper presents a framework for concatenated quantum error correction in which a stabilizer code's syndrome projection reshapes the noise channel according to the code's structure. Applying it to the bit-flip and phase-flip codes under amplitude damping, it gives a critical damping parameter of about 0.30 and the relation f = 1 − 1.72755γ² between initial channel fidelity and the damping parameter. Under an ideal-recovery assumption, the authors compare the overall gate accuracy of this concatenation scheme with that of the five-qubit code.
In briefA Personal History of Quantum Foundations: Oral History Interview
SummaryThis oral history interview records physicist Yakir Aharonov's personal and conceptual recollections from the 1950s to the 2000s. It traces his academic career across several universities and his work on topics such as the EPR paradox, weak values, and protective measurements. The interview also recounts discussions with many prominent physicists.
In briefA sharp gradient-corrected Lieb–Thirring inequality in one dimension and its use in density functional theory
SummaryThe paper proves a sharp one-dimensional Lieb–Thirring inequality with a semi-classical constant and gradient correction, and identifies its optimizers as a two-dimensional family called the Poincaré density manifold. It applies these results to the Thomas–Fermi–von Weizsäcker (TFW) approximation of kinetic energy in density functional theory, giving explicit error bounds that depend on the distance from the manifold and the particle number. The TFW approximation is shown to be exact on this manifold, and in the high-density limit the Kohn–Sham potential converges to the inverse Thomas–Fermi potential at an optimal rate of order N to the minus two.
WhyProves a sharp one-dimensional gradient-corrected Lieb-Thirring inequality with characterized optimizers and explicit error bounds for TFW approximation, a novel and rigorous theoretical advance.
In briefAll-photonic millimeter-wave and terahertz signal chip working at room and cryogenic temperatures
SummaryThe paper presents a monolithic lithium tantalate photonic chip, called a signal core, that generates, modulates, routes, and detects continuous-wave signals from 100 GHz to 2.5 THz. High-frequency modules are integrated on the chip and connected by micro-coplanar striplines, with optical interfaces using only fiber links. The authors report electro-optic coupling rates and photon-flux conversion efficiencies, and show operation at 4 K in a dilution refrigerator through the same fiber-only links.
WhyA monolithic chip realizes an all-photonic 100 GHz–2.5 THz signal chain operating at 4 K via fiber-only links, with concrete measured conversion efficiencies but limited comparisons.
In briefBoosting axion detection in microwave cavities by pre-exciting the mode
SummaryThis paper addresses axion dark matter detection with microwave cavities in strong magnetic fields, where the conventional signal is extremely weak because axion-photon conversion is a second-order effect. The authors propose pre-exciting the target cavity mode to generate a first-order power signal in situ, whose amplitude scales linearly with the axion-photon coupling. Numerical simulations indicate the pre-excited seed field suppresses thermal noise so sensitivity approaches the lower quantum noise limit, potentially surpassing KSVZ and DFSZ model predictions.
In briefExceptional-point structure and chiral state transfer in a PT-symmetric Bose-Hubbard dimer engineered with rate operators
SummaryThe paper applies the rate operator quantum jumps method to modify the non-Hermitian dynamics of a three-particle Bose-Hubbard dimer under non-local dephasing, yielding a PT-symmetric Hamiltonian with a two-parameter imaginary diagonal part. The authors map the spectral symmetry structure across on-site interaction strengths, identifying exceptional lines and fourth-order exceptional points. They also show chiral state transfer when slowly varying parameters loop around a fourth-order exceptional point, and direction-dependent transfer without encircling such points.
Jaakko Luomala, Federico Settimo, Kimmo Luoma et al.
In briefA simple proof that unitaries can be parallelized via port-based teleportation
SummaryThis paper gives a simpler circuit for port-based teleportation (PBT), a variant of quantum teleportation that needs no corrections on the transmitted state. The authors present an elementary efficient circuit of depth polylog(N,d), avoiding the representation-theory tools used previously. As a corollary, any n-qubit unitary can be parallelized to poly(n) depth, recovering a recent result by Nehoran, Slote, Yuen and OpenAI.
In briefDemand-driven routing space for lattice-surgery compilation
SummaryThis paper presents a co-designed dynamic architecture and compiler that allocates routing space for lattice surgery according to circuit demand. The architecture keeps a fixed virtual qubit floorplan while letting physical spacing change between execution stages. On application benchmarks, the method reduces space-time volume by 55.6% in geometric mean and compiles 75.5 times faster than the evaluated DasCot configuration.
In briefTime-reversal quantum metrology in a central-spin system, from SATIN to butterfly echo
SummaryThis paper studies quantum-enhanced metrology based on time reversal in a central spin model, where one ancilla spin interacts with N bath spins in a star-graph arrangement. The authors show that the SATIN echo protocol can be implemented in this setup, reaching a Fisher information of N²/e with collective ⟨J_y⟩ readout, about 4.34 dB below the Heisenberg limit. They also propose a butterfly echo protocol using random collective bath rotations, which achieves Heisenberg-scaled sensing for an unknown encoding axis. The central spin magnitude is further shown to shorten the optimal protocol timescale.
In briefHigh-fidelity, robust controlled-Z gates for superconducting transmon qubits, Part II: parameter uncertainties and overall errors
SummaryThis paper, the second part of a series, extends an optimal-control framework to account for uncertainties in coupling strength, anharmonicity, and zero-flux qubit frequencies, aiming for controlled-Z gates that tolerate noise and parameter deviations. It reports that coupling errors can be suppressed by dynamical decoupling from strong detuning modulation, and detuning errors are minimized near a symmetric-detuning operating point. The authors also optimize gate controls across six hardware performance stages where intrinsic errors, decoherence, and parameter uncertainties coexist.
In briefCoherence and imaginarity of qubit state sets with vanishing Bargmann invariants
SummaryThis paper studies sets of qubit states whose higher-order Bargmann invariants vanish, examining their set coherence and set imaginarity. The authors show both resources are linked to whether the set contains a pair of orthogonal states. When maximally mixed states are excluded, such a set is imaginarity-free exactly when it contains an orthogonal pair.
In briefStructure of Quantum Finite-Level Systems: Protected Subsystems, Non-Disturbing Variables, and Back-Action-Evading Measurements
SummaryThis paper analyzes the structure of quantum finite-level systems in the Heisenberg picture, where observable evolution is described by bilinear quantum stochastic differential equations. It characterizes coordinate transformations that preserve commutation relations, and constructs decoherence-free subsystems in orthonormal Hermitian bases. It also gives conditions for the existence of quantum nondemolition variables and a sufficient condition for back-action-evading measurements. A four-qubit Heisenberg chain with collective dephasing is used as an illustration.
In briefCompensating micromotion selectively by coherently exciting a ground-state-cooled motional mode
SummaryExcess micromotion from stray DC fields limits control fidelity in trapped-ion experiments. This paper presents a compensation method that uses an auxiliary RF drive to convert ion displacement into coherent excitation of a ground-state-cooled motional mode, detected through the qubit. The optimal compensation field is determined with a 1σ statistical uncertainty of 8 mV/m. The authors state the method is compatible with quantum-enhanced sensing using nonclassical motional states such as Fock, squeezed, and Schrödinger-cat states.
In briefLayered reduction of invariant subspaces, from Hilbert-space fragmentation to many-body scars
SummaryThe paper studies a hierarchy of invariant Hilbert-space structures linking fragmentation, partial solvability, and many-body scars. Using the spin-1 XXC chain as an example, it shows how successive local deformations reduce extensive fragmentation through an implicitly embedded spin-1/2 invariant space to a single scar tower, connecting the XXC, perturbed XXC, and spin-1 XY models. It also gives a rigidity constraint for isolating nonsymmetric towers and shows the reduction can be transferred by a unitary within the embedded subspace, yielding an exact scar tower built on a Néel state.
In briefClustering and entanglement in the superconducting state
SummaryThe paper gives conditions under which the cluster property holds in the superconducting state. Using them, it shows that the cluster property used to derive the superconducting order parameter is valid and identifies the order parameter's form. The authors also examine entanglement between two particles from the BCS maximum geminal and find that larger deviations from the cluster conditions correspond to greater entanglement.
In briefUsing a finite-rate quantum Otto cycle to probe spectral accessibility in quasiperiodic systems
SummaryThe paper proposes a finite-rate quantum Otto cycle as a thermodynamic probe revealing which spectral regions of quasiperiodic systems become dynamically accessible during driven transformations. Studying a generalised Aubry-André-Harper chain, it finds that work fluctuations do not follow a universal ordering from the static regime alone, instead reflecting transition pathways that are dynamically coupled, thermally occupied and energetically significant. The authors argue that quantum Otto cycles offer a fluctuation-resolved complement to spectroscopic diagnostics.
Shujie Cheng, Lucas C. C\'eleri, Norton G. de Almeida et al.
In briefFinite-width altermagnetic quantum rings and their Aharonov-Bohm effects
SummaryThis paper studies a two-dimensional finite-width quantum ring containing a quadratic d-wave altermagnetic term, Tan-Inkson confinement, a perpendicular magnetic field, and an independently tunable Aharonov-Bohm flux. The altermagnetic interaction mixes angular channels m and m±2 while preserving parity, and circular symmetry makes the two spin blocks exactly isospectral. The authors report anticrossings at low filling, multichannel mixing near the Fermi level of highly occupied rings, and converged calculations of the magnetic response and persistent current. The circular model supports an equilibrium charge current but no persistent spin current.
In briefHow problem structure shapes single-layer QAOA performance
SummaryThis paper formalizes problem structure in single-layer QAOA as a joint probability distribution over the coarsest partition of the computational basis, defined by cost values and mixer-dependent walk counts. It proves a mean-max non-separation bound linking parameter-averaged target probability to its variational maximum. An exact matrix inner-product expression is derived and illustrated with transverse field and Grover mixers.
Guilherme Adamatti Bridi, Franklin de Lima Marquezino, and Andris Ambainis
In briefPhoton localization: how local measurements define where a photon is and what causality allows
SummaryThe paper reviews what it means for a relativistic particle to be localized and argues that local measurement is the most useful way to define it, tying localization to relativistic causality. It states that single photons cannot be strictly localized in quantum field theory, while coherent states can, and surveys how such strictly localized states can be constructed. Explicit quantum-optics examples are presented, including states that approximate single photons and entangled Bell states.
In briefDistributed dual-comb spectroscopy using entangled frequency combs for broadband quantum-enhanced sensing
SummaryThis paper proposes distributed dual-comb spectroscopy, a quantum-enhanced method that uses spatial and spectral entanglement to improve broadband spectroscopic measurements. A network of Mach-Zehnder interferometers probes spatially separated samples, and a single two-mode squeezed frequency comb is split by a passive beam-splitter array into K entangled combs. The authors report enhanced signal-to-noise performance compared with schemes requiring K independently engineered nonclassical comb sources.
In briefMemory-transport trade-off in a physical two-bath quantum reset model
SummaryThe paper studies the competition between preserving quantum coherence and renewing the environment as ways to control quantum memory and transport. The authors build a two-reservoir protocol with independent, number-conserving thermal attenuators that guarantees complete positivity and fermionic admissibility. Quadratic dynamics reduces the stroboscopic problem to an exact matrix fixed-point equation without weak-coupling or Markov approximations.
Jishad Kumar, Achilleas Lazarides, Tapio Ala-Nissila
In briefGermanium-vacancy centers in diamond placed in a delta-doped layer, with optical spectra that resolve isotopes
SummaryThe paper reports localizing germanium-vacancy (GeV⁻) centers in a layer under 10 nm thick by timed injection of GeH₄ during microwave plasma chemical vapor deposition. The layer contains 17 ppm Ge and a strain-free environment. After electron irradiation and annealing, the sheet density reached 0.12 GeV⁻/µm², and spectra resolve isotopic clusters with the best cluster showing 2.14 GHz inhomogeneous distribution and 47 ± 11 MHz average linewidth. The authors estimate a vacancy-to-GeV⁻ yield of at least 90% and a spin T₂* lower bound of 730 ns.
Ian N. Hammock (1), Kinfung Ngan (2), Antoni W. Wellisz (1) et al.
In briefStripes versus superconductivity in the hole-doped Hubbard model
SummaryThis paper examines whether the two-dimensional Hubbard model supports uniform d-wave superconductivity, the proposed minimal model for cuprate superconductors, using neural tensor network and neural backflow state simulations on lattices of up to 1024 sites. At 1/8 doping, both in the pure Hubbard model and with next-nearest-neighbor hopping, the authors find near degeneracy among a uniform d-wave state, superconducting stripes, and non-superconducting stripes on the milli-t energy scale. They conclude that the model shows extreme competition between stripes and superconductivity rather than a single dominant order.
WhyNeural tensor and backflow simulations on lattices up to 1024 sites reveal near-degenerate stripe and superconducting states, challenging the single-order narrative for the Hubbard model.
In briefA normal form that maps adaptivity and entanglement in Pauli-based quantum computing
SummaryThis paper studies Pauli-based computation (PBC), a measurement-based model of quantum computation that samples from the output distributions of universal Clifford+T circuits. The authors introduce a normal form that fully characterizes PBC's adaptive and entanglement structure and unifies PBC with one-way quantum computing as dual, time-reversed counterparts. The normal form is used to advance distributed computation strategies and enables ahead-of-time compilation, which reduces CNOT gate count and quantum memory on random circuits by 50% and 40% on average.
Filipa C. R. Peres, Selman Ipek, Daniel Lovsted et al.
In briefFixing low-weight errors that make BP-OSD decoding fail
SummaryThis paper studies low-weight error patterns that cause Belief Propagation with Ordered Statistics Decoding (BP-OSD) to fail on Bivariate Bicycle quantum error-correcting codes. At code capacity, the authors find failures on errors much lighter than d/2, caused by global symmetries of the decoding graph that stop parallel BP from converging. They also identify new issues in rare-event analysis and show that modifying the BP update rules and decoder implementation substantially lowers the logical error rate.
Giacomo Fregona, Matthias C. L\"obl, Benjamin J. Brown et al.
In briefA quantum optimisation method that uses localisation-delocalisation transitions to speed up non-adiabatic search.
SummaryThe paper proposes accelerating continuous reverse quantum annealing across a many-body localisation transition by exploiting the density of Landau-Zener crossings. An effective Fokker-Planck equation is used to interpret the algorithm's success probability, and a Trotterised digital circuit called FRIO is presented. Numerical results on spin-glass problems, particularly the SK model, indicate that FRIO with suitably chosen parameters almost always outperforms QAOA with pre-optimised angles.
Ryan Smith, Sabrina Yue Wang, Filippo Maria Gambetta et al.
In briefProof that pure states are optimal for quantum channel discrimination under a fixed energy budget
SummaryThe paper studies distinguishing two quantum channels acting on bosonic modes, using a probe state with fixed mean photon number and no ancilla. The authors prove that for any mixed probe there is a pure probe with the same mean photon number that performs at least as well. The result covers arbitrary channels and figures of merit based on Chernoff quantities, including the quantum Chernoff exponent, as well as single-shot error probability.
In briefTransversal syndrome extraction: a unified way to measure error syndromes for any stabilizer code
SummaryThis paper addresses fault-tolerant syndrome extraction for quantum error correction based on stabilizer codes. The authors design ancilla stabilizer states for a general transversal circuit. From these they build a family of fault-tolerant extraction gadgets that interpolate between Shor and Steane syndrome extraction for any stabilizer code. The construction offers a configurable trade-off between circuit depth and circuit width.
In briefSymmetry-preserving, scalable variational circuits for the lattice Gross-Neveu model
SummaryThis paper studies variational methods for preparing ground states and sector-resolved excited states of the (1+1)-dimensional lattice Gross-Neveu model, a lattice theory of interacting fermions. The authors build fixed circuits that conserve total fermion number or each flavor number, and a scalable adaptive variational quantum eigensolver (SC-ADAPT-VQE) tailored to this problem. Classical benchmarks up to 30 qubits show relative energy errors of roughly 1×10^-5 to 2×10^-4 for selected massive cases, and matrix-product-state evaluations extend one-flavor circuits to 60 to 400 qubits, compared against DMRG reference values.
Vivan Bhatt, Zhongtian Dong, Kyoungchul Kong et al.
In briefDiscrete time liquids: a state of spins with finite temporal coherence, built from time-solitons
SummaryThe paper studies a two-dimensional spin system coupled to a finite-temperature cavity whose fluctuations give a noisy, approximately periodic drive with only finite temporal coherence. Over an extended parameter range the spins form a metastable state with period-doubled dynamics, yet individual trajectories randomly switch between the two symmetry-related temporal configurations. These switches are identified as time-solitons, and a minimal model is proposed to reproduce the observed temporal correlations.
In briefScalable neural wave functions using mixing and hierarchical networks for quantum many-body systems
SummaryThis paper studies neural networks for representing ground-state wave functions of quantum many-body systems. The authors use a lightweight multi-layer perceptron (MLP) mixer as the wave function ansatz, aiming to capture long-range correlations at low computational cost. On two-dimensional systems in various phases of matter, it performs competitively with recurrent neural networks and transformers. A hierarchical combination with transformers is reported to outperform other state-of-the-art architectures on large systems.
In briefQuantum measurement of axion dark matter: a fully quantum treatment of continuous detection of wave-like dark matter, focused on cavity haloscopes.
SummaryThis paper gives a fully quantum-mechanical account of continuously measuring wave-like dark matter, focusing on cavity haloscopes. The authors treat the axion as a quantum field and show how its quantum state appears in higher-order moments of outgoing transmission-line observables, using a new measurement-record formalism. They compute the exact axion-induced photon number distribution for the first time and find it is not Poisson. They argue virialized axion dark matter is likely in a Gaussian state, distinguishable from coherent states or Bose-Einstein condensates via number or quadrature measurements, while intrinsically quantum effects such as Fock or squeezed states are too weak to detect.
WhyExtends input-output formalism to quantum axion fields and derives the first exact non-Poisson photon-number distribution, a notable theoretical advance.
Yunjia Bao, Dhong Yeon Cheong, Nicholas L. Rodd et al.
In briefUsing membrane vibrations to steer electron recoil and sense forces
SummaryThis theoretical paper proposes that transverse vibrations of an optomechanical membrane scatter cavity light into high-momentum evanescent sidebands, enabling phase-matched interaction with electrons moving in vacuum. The vibration amplitude sets the strength of electron-light interaction and redistributes electron populations among recoil orders. External forces that perturb the vibration are encoded in the electron recoil spectrum. The authors report that, under modeled conditions, electron readout of localized stochastic forces can achieve a higher signal-to-noise ratio than a Gaussian optical homodyne benchmark at equal detected probe rates.
Sunjun Hwang, Lee Wei Wesley Wong, Liang Jie Wong et al.
In briefPure dephasing erases two-photon amplitude, not the nonlinear phase
SummaryThe paper examines whether the π conditional phase that a chirally coupled two-level emitter imprints on a two-photon pulse, a component assumed in quantum photonic neural networks, survives pure dephasing in solid-state emitters. The authors derive the dephased two-photon amplitude in closed form and validate it against an independent Fock-state master equation. They report that the conditional phase stays pinned at exactly π at every dephasing rate examined, so dephasing destroys the amplitude carrying the phase rather than rotating it.}
WhyDerives closed-form dephased two-photon amplitude, validated by independent master equation, showing dephasing reweights rather than rotates the nonlinear phase.
In briefProof of a strengthened quantum Hamming bound by Yu, Lai, and Oh
SummaryThis paper proves a conjecture of Yu, Lai, and Oh, showing that their strengthened quantum Hamming bounds hold for all quantum codes of dimension K>1, including impure and nonadditive codes. The result covers every integer local dimension p≥2 and a range of distances and interpolation parameters. The case e=0 gives the ordinary quantum Hamming bound for general codes. The proof uses the linear-programming method with quantum weight enumerators.
WhyProves a conjecture of Yu, Lai, and Oh, extending their interpolating quantum Hamming bounds to impure, nonadditive codes and general local dimensions.
Valery (Binyamin) Grishin, Aryeh Lev Zabokritskiy (Yohananov)
In briefParallel universal logic in high-rate concatenated self-dual codes
SummaryThe paper proposes a quantum architecture that performs addressable, parallel, fault-tolerant universal logic on high-rate codes built by concatenating small-distance self-dual CSS codes. Logic is injected by fault-tolerantly preparing logical stabilizer states and global magic states, which are consumed by teleportation gadgets that also correct errors. The authors benchmark preparation for several two-level concatenations and report that concatenated cultivation offers at least one order of magnitude spacetime reduction compared with cultivation in the surface code.
In briefQuantum probes cannot reach the cosmological singularity
SummaryThis paper proposes a way to operationally treat the FLRW big bang singularity using quantum probes that obey a curvature-coupled Klein-Gordon equation. Applying the Madelung-Bohm decomposition to such a field on a near-singularity FLRW background, the authors show that the induced conformal geometry keeps curvature invariants finite and makes past-directed causal geodesics complete. The classical big bang surface is thus inaccessible to these probes in finite effective proper time, while the underlying FLRW spacetime remains singular.
Vishnulal Cheriyodathillathu, Saurya Das, Soumen Basak et al.
In briefA random sampling method for testing the weak eigenstate thermalization hypothesis
SummaryThis paper introduces a random sampling method for testing the weak Eigenstate Thermalization Hypothesis (wETH), which is especially suited to large, sparse Hamiltonians. The method is tested on a well-studied Rydberg atom system with both two- and three-body dipole-dipole interactions. The authors report that it enables studies of system sizes well beyond the limits of exact diagonalization. They find that fluctuations in eigenstate expectation values decay as the Hilbert space dimension to the power -1/2, consistent with wETH and earlier numerical results.
Nicolaus A. Chlanda, John Keim, Isabel Martinez-Robles et al.
In briefHydrodynamic description of correlation matrix eigenvalues in a finite-temperature expanding Fermi gas
SummaryThis paper extends earlier work on the entanglement entropy Page curve of a one-dimensional Fermi gas expanding from a finite box into vacuum. It proposes a hydrodynamic description for obtaining correlation matrix eigenvalues in a nonzero-temperature freely expanding gas. For the system, the phase space density gives the eigenvalues directly, yielding von Neumann entropy via the Yang-Yang formula, while the reservoir requires a modified density that incorporates coherences. The authors also derive a Levitov-Lesovik type full-counting-statistics formula for finite systems in the transient regime.
Atharva Naik, Madhumita Saha, David A. Huse et al.
In briefHigh-fidelity, noise-robust controlled-Z gates for superconducting transmon qubits, Part I: intrinsic errors and decoherence
SummaryThis paper presents an optimal-control framework for designing controlled-Z (CZ) gates between directly coupled, frequency-tunable transmon qubits, accounting for intrinsic Hamiltonian errors and decoherence such as energy relaxation and dephasing. Using smooth single-shot flux pulses, the authors obtain a 12-ns CZ gate close to the 11.6-ns theoretical speed limit, with intrinsic errors suppressed to about 2×10^-5. Dephasing noise models calibrated against published experiments are used to identify operating points that reduce sensitivity to dephasing.
In briefTight Randomized Communication Bounds for Vector-in-Subspace
SummaryThis paper proves that the randomized communication complexity of the Vector-in-Subspace problem in R^n, with error at most 1/3, is Θ(√n). Alice holds a unit vector and Bob holds an n/2-dimensional subspace, and they must decide whether the vector lies in the subspace or its orthogonal complement. The lower bound matches Raz's upper bound and improves the previously known lower bound of Klartag and Regev.
WhyProves a tight Theta(sqrt n) randomized communication bound via Gaussian moments and spherical harmonics, improving prior lower bounds and removing restrictions.
In briefAn entanglement band structure for general multiqubit states
SummaryThis paper extends Majorana's geometric picture of symmetric multiqubit states to general multiqubit states. Projecting one qubit leaves the others in a state that varies with direction, and their entanglement invariants are eigenvalues of a non-Hermitian matrix that form bands over the sphere. Exceptional points appear, without gain or loss, where the entanglement family of the remaining qubits changes, and circling one exchanges two bands, so the bands form a Riemann surface. For five qubits, this structure fixes the entanglement class up to finitely many choices.
In briefTwo-particle interference: the Hanbury Brown–Twiss and Hong–Ou–Mandel experiments
SummaryThis paper discusses two important quantum optics experiments that rely on two-particle interference. Since their first demonstrations, many interesting variants have appeared, including ones that use atoms instead of photons as the interfering particles. The chapter gives a pedagogical overview of some of these results and argues that the two effects are closely related.
In briefMeter-scale quantum links connecting two large quantum processors
SummaryThe paper demonstrates four long-distance quantum interconnects (L-couplers) between two 120-qubit IBM Nighthawk R1 superconducting processors. Each coupler uses 80 cm of superconducting aluminum coax, two frequency-tunable L-qubits, and two RF-SQUID couplers. The authors report a mean state-transfer fidelity of 99.00% and a mean distant CZ gate fidelity of 94.75%, and use these links to generate entangled states across the two processors.
WhyDemonstrates four high-fidelity meter-scale couplers between two 120-qubit processors, with distant CZ gates and entanglement; strong novelty and experimental evidence for modular quantum computing.
Neereja M. Sundaresan, Nicholas Cotaj, Yadav Prasad Kandel et al.
In briefOptical clocks in the UK and Germany agree to 5×10⁻¹⁸ uncertainty
SummaryTwo single-ion optical clocks in the UK and Germany were compared remotely over a 2220 km fiber link. The clocks use the electric octupole transition in ¹⁷¹Yb⁺. Over more than 10⁶ seconds of data, the paper reports a fractional frequency difference of 1.0(4.8)×10⁻¹⁸, showing agreement between the systems. The authors also use the large baseline to test Lorentz invariance and improve the constraint on the parameter α by about a factor of five.
WhyRemote comparison of two Yb+ optical clocks over 2220 km reaches 5e-18 uncertainty and improves Lorentz invariance limits about fivefold.
In briefFinite classification of transversal diagonal gates beyond stabilizer codes
SummaryThe paper shows that transversal diagonal gates on exact qubit codes of distance at least two can be exhaustively classified within a finite search space. Any projective logical action from such a gate can be realized by a physical gate of finite order, with an upper bound set by the code parameters. Applied to exact ((6,2,3)) codes, the method finds 66 candidate symmetry classes, of which 15 are realizable. The order-5 case is unique up to local unitaries and qubit permutations, and has the minimum length n=6 among such codes whose transversal group cannot be conjugated into the projective Clifford group.
WhyProves a code-parameter bound on physical gate order enabling finite enumeration of transversal diagonal gates, fully classified for ((6,2,3)) codes; significant but confined to quantum error correcti
In briefTracking Entanglement and Coherence in Quantum Optimization Algorithms via Variational Methods
SummaryThis paper studies how entanglement and coherence, two quantum resources, change while two variational quantum algorithms, QAOA and FALQON, solve the Minimum Vertex Cover problem. The authors use a variational framework based on the Bures metric and Uhlmann's theorem to estimate Bures entanglement and Bures coherence without full quantum state tomography. QAOA's entanglement rises in its initial layers and then falls as the algorithm converges. FALQON reaches a lower entanglement peak at greater circuit depth, so the two algorithms follow distinct resource trajectories.
In briefLean-QuantumAlg: A Framework Enabling Humans to Trust AI-Generated Quantum Algorithms
SummaryThis paper introduces Lean-QuantumAlg, a Lean 4 framework for trustworthy AI-assisted development of quantum algorithms. Its Algorithm-Solves-Problem contract ties correctness, success probability, borrowed-state restoration, and quantum and classical resource bounds to a single execution. The authors demonstrate it on nine AI-formalized quantum algorithms, ranging from Grover search to quantum attacks on RSA-2048 and P-256.
In briefSuperconducting parametric amplifiers using arrays of unequal Josephson junction elements
SummaryThis paper presents a way to achieve impedance matching in parametric amplifiers using arrays of unequal Josephson-junction elements such as rf/dc SQUIDs and SNAILs. These elements act either as flux-tunable inductors or as active gain elements. The authors develop an analytical theory and demonstrate it in devices with flux-biased SNAIL arrays. They report an instantaneous bandwidth of 200 MHz with over 17 dB gain centred near 5.4 GHz, without a static impedance matcher, and bandwidth stability across more than 1.2 GHz of tuning.
In briefField-Free Superconducting Diode Effect from Synthetic Gauge Fields
SummaryThe superconducting diode effect requires different forward and reverse critical currents, which normally needs a magnetic field to break time-reversal symmetry. This paper proposes a spatio-temporally driven chemical potential that breaks both inversion and time-reversal symmetry, using a synthetic gauge flux instead of an effective Zeeman field in a helical Shiba chain. The rectification is odd in the drive wavevector, vanishes at nπ, peaks analytically at 2π/3, and persists even when the exchange coupling is zero. Its strength scales inversely with the pair-breaking gap, and the symmetry breaking can be switched on, tuned, and reversed by the drive alone.
