In briefAn efficient quantum algorithm for computing the geometry of mixed quantum states
SummaryThis paper addresses the difficulty of computing mixed-state quantum geometry, which is needed for quantum metrology and parameter estimation. The authors build a quantum algorithm that constructs the symmetric logarithmic derivative efficiently using a link between density matrices and Gauss-Laguerre quadrature. They provide a complexity analysis and, under a bounded spectral-gap assumption, construct block encodings of the full quantum Fisher information matrix and quantum geometric tensor.
WhyLinks Gauss-Laguerre quadrature with density-matrix positivity to build SLDs efficiently, with rigorous complexity analysis.
In briefCollective strong quantum Mpemba effect in accelerated quantum detectors
SummaryThe paper studies uniformly accelerated quantum detectors that couple collectively to the Minkowski vacuum. For small transverse separation, a strong quantum Mpemba effect occurs, where a state farther from equilibrium relaxes faster than a closer one. The effect arises from two reflection sectors whose populations are conserved, with weak intersector transfer producing a slow relaxation mode. The same mechanism is said to extend to detectors coupled to a common thermal field.
G. D. de Moraes Neto, Richard Q. Matos, Norton G. de Almeida
In briefContinuous-drive protection of spin-qubit two-qubit gates using concatenated driving
SummaryThis paper extends concatenated continuous driving (CCD), which uses phase modulation of a global microwave drive, to exchange-based SWAP and √SWAP gates between double-dressed spin qubits. Compared with conventional continuous dressing, CCD better resists Rabi-frequency errors and qubit-to-qubit Rabi-frequency mismatch while still protecting against common-mode detuning errors. The authors also find that spatial correlation of qubit-frequency fluctuations strongly affects average gate fidelity, with positive correlations enhancing CCD protection.
Takuma Kuno, Takeru Utsugi, Andrew J. Ramsay et al.
In briefWork extractable from steady states beyond full secular approximation
SummaryThis paper studies the ergotropy, the maximum work extractable by cyclic unitary operations, of the nonequilibrium steady state of two exchange-coupled qubits, each connected to an independent thermal reservoir. It uses a Bloch-Redfield master equation that goes beyond the full secular approximation. For qubits of equal frequency, the steady-state ergotropy is entirely coherent and grows quadratically with temperature bias near equilibrium. For detuned qubits, incoherent ergotropy can appear, and the paper derives an exact analytical criterion for its activation.
In briefComment on a paper about the polarization hierarchy for polynomial optimization over convex bodies
SummaryThis comment re-examines the feasibility and convergence claims of the polarization hierarchy for polynomial optimization over convex bodies, proposed by Plávala, Ligthart, and Gross. Counterexamples show that the original assumption is insufficient for the feasibility characterization and for convergence with the polarization constraint. The authors establish corrected results under a stronger algebraic condition and supply a missing compactness argument.
In briefExact quantum query complexity of the five-bit function EXACT^5_{3,4}
SummaryThis paper studies the Boolean function that outputs 1 exactly when the input Hamming weight is 3 or 4, and proves its exact quantum query complexity is 4. The lower bound uses a fixed rational dual certificate that rules out any exact algorithm with at most three queries. The upper bound follows from a known four-query algorithm for a seven-bit function combined with a padding reduction. The same certificate also gives a positive lower bound on the worst-case error of three-query algorithms, consistent with earlier near-unit success numerical results.
In briefLogical-space theory for ordered-statistics decoding of quantum LDPC codes
SummaryThis paper links the quantum decoding goal of matching an error's logical class with the candidate-list search of ordered-statistics decoding (OSD), using a rank-k logical-signature map. It proves that the logical error rate of minimum-cost list decoding splits into absent-class failures and wrong-selection failures. On the [[144,12,12]] gross code, 672 coverage-aware candidates outperform 899 combination-sweep candidates, showing that logical coverage and within-class selection are complementary.
WhyProposes a framework separating logical-class coverage from representative-selection failure, validated on the gross code with paired statistical tests.
In briefLocal response theory for multiparameter sudden-quench quantum Otto engines
SummaryThis paper develops a local response theory for quantum Otto engines with sudden quenches across several control parameters. It splits the local quadratic work into a thermal-bias gain and a quadratic response cost, whose balance selects the quench vector that maximizes work at a fixed amplitude. The theory is illustrated on a two-level working medium and a finite interacting XYZ spin chain. It offers a practical local criterion for designing small multiparameter quenches.
In briefLearned-inference method for locating multiple emitters using quantum correlations under limited information
SummaryThis paper addresses locating multiple subwavelength-spaced emitters from quantum correlation measurements, estimating both how many emitters there are and where they are. Instead of the traditional fit-and-select approach, which fits several models and picks the best one, the authors infer the count and positions directly from multimodal data using learned inference. According to the paper, this improves counting accuracy and localization precision, with the largest gains under low photon counts and strong background. The authors suggest it could support real-time biological imaging and single-particle tracking in densely labeled samples.
In briefCertifying nonlocal spin entanglement using local tunneling spectroscopy
SummaryThe paper shows that local tunneling spectra alone can certify quantum entanglement between spins, without coherent control or absolute intensity calibration. An exact spin sum rule and total-spin conservation let two local spectra bound a collective entanglement witness. The analysis includes substrate coupling, instrumental broadening and noise, and the certificate holds at a stated confidence level.
Payman Kazemikhah, Ruolun Zhang, Xiangyu Luo et al.
In briefMargin-Based Generalization Bounds for Quantum Machine Learning with Classical Data Encoding
SummaryThis paper studies how well quantum machine learning models generalize when classical data must first be encoded into quantum states. The authors develop a margin-based framework covering both fixed and trainable encodings, with the trainable case adding a term tied to the number of encoding parameters. In 19,440 simulated training runs, the inverse margin measure performed best among the complexity measures compared.
In briefCheaper Quantum Data Embedding Optimization Using Class Prototypes
SummaryThis paper proposes a prototype-guided framework for optimizing quantum data embeddings used in supervised quantum machine learning. Instead of pairwise fidelity evaluations that scale as O(B²), it builds an objective based on class-prototype separation and sample-to-prototype compactness that needs only O(B) fidelity evaluations. Across five benchmark datasets with up to 20 qubits, the authors report competitive quantum kernel SVM performance with reduced fidelity-evaluation cost.
In briefEntangledEdge: A Distributed System for Orbital Quantum Sensing and Learning
SummaryThis paper presents EntangledEdge, the first system architecture that unifies orbital quantum sensing and learning end-to-end, with a quantum data plane and a classical control plane. It routes each event to a channel-matched model, adapts its detection threshold without labels, and aggregates results across stations by hardware fidelity. Evaluated on satellite telemetry with a hybrid prototype using real quantum processors and a simulated orbit-to-ground link, it reports high-quality inference at bounded per-event cost, better quantum processor utilization, and scaling with fleet size.
Mohammad Abrarul Hasanat, Amit Samanta, Jason Ludmir et al.
In briefFTWeave: A System Simulator Coupling Logic Execution, Decoding, and Magic-State Supply in Quantum-Classical Machines
SummaryFTWeave is a system-level simulator for exploring how fault-tolerant quantum-classical machines should be provisioned. It takes circuits from several formats and models application progress, resource ownership, and error protection on one event timeline. The paper reports that coupling these resources changes provisioning results, such as factory speedup estimates and the effect of decoder workers, and that decoder service variability lengthens some code mappings' runtimes.
In briefIterative Quantum-Classical Method for Pressure-Poisson Equation on Noisy Quantum Hardware
SummaryThis paper proposes a hybrid quantum-classical iterative refinement method for the pressure-Poisson equation in the incompressible Navier-Stokes projection method. The quantum solver is applied repeatedly to the normalized residual, with each correction scaled by a least-squares fit, so solver error acts as a contraction factor rather than an error floor. Experiments on IBM ibm_cleveland and IonQ Forte report relative residuals below 10^-7 and 5.6×10^-8 respectively. The authors state these are the first converged pressure solutions obtained on quantum hardware.
Doyeol "David" Ahn (1, 2), Byeongyong Park (3) et al.
In briefGeneralized Svetlichny Inequalities as a Cross-Platform Quantum Processor Benchmark
SummaryThis paper proposes a cross-platform benchmark protocol based on violations of generalized Svetlichny inequalities to assess collective performance of quantum processors. The same GHZ-state circuits with fixed measurement settings, from 2 to 10 qubits, were run on two trapped-ion processors and one superconducting processor. Both trapped-ion systems maintained violations up to ten qubits, while the superconducting system lost them beyond six qubits without mitigation and recovered to eight qubits with readout-error mitigation.
Andr\'es Camilo Granda Arango, Giuseppe Sergioli, Luigi Iapichino et al.
In briefOptimal robust single-qubit gates on superconducting quantum processors
SummaryThis paper presents energy-optimal control pulses for single-qubit gates that resist both frequency detuning and waveform inhomogeneities while suppressing leakage outside the computational subspace. The authors combine these pulses with an adaptive control scheme and experimentally demonstrate the gates on a superconducting quantum processor. With a 90 ns gate duration, the X gate reaches fidelity above 99.9% and Hadamard-type gates above 99.6%. The X gate keeps fidelity above 99.5% under up to 20% drive-amplitude variation and 4 MHz qubit detuning, which the authors report is an order of magnitude more robust than conventional Gaussian pulses.
In briefReview of symmetry and quantum interference in attosecond physics
SummaryThis review examines how symmetry and quantum interference interact on attosecond timescales, mainly through one- and two-electron photoelectron momentum distributions. It discusses how tailored laser fields control dynamical symmetries and how quantum-orbit methods link momentum-space structures to specific ionisation, propagation, and rescattering pathways. The review also covers how target properties such as binding potential, orbital symmetry, and parity modify this mapping, and considers non-classical light states as additional tools for control.
C. Figueira de Morisson Faria, S. Hashim, J. Rivera-Dean et al.
In briefUsing Nonlocality to Benchmark Quantum Processors
SummaryThis paper proposes a benchmarking method for quantum processors based on violations of the Svetlichny inequality, which tests genuine multipartite nonlocality. The authors compare it with Mermin's inequality and run circuits on three to five qubits on the Garnet superconducting and Marmot ion-trap devices. The results indicate that such tests can characterize collective device performance and guide calibration and optimization.
Andr\'es Camilo Granda Arango, Carla Sophie Rieger, Carlo Cuccu et al.
In briefScheduling concurrent entanglement purification in dynamic quantum networks
SummaryThis paper studies how to schedule entanglement purification for multiple concurrent requests that compete for link capacity and quantum memories in a time-slotted quantum network. The authors propose a mixed-integer linear programming scheduler that maximizes scheduled requests, and reinforcement learning schedulers based on DQN, PPO, and SAC. Simulations show the RL schedulers achieve the highest success rates but use more Bell pairs and defer some requests, while MILP gives the best fairness and lowest Bell pair consumption in most scenarios.
In briefElectric flux as a criterion for string breaking and Schwinger pair production
SummaryThis paper defines string breaking as the point where the midpoint electric field drops to half a flux unit. Using an energy balance in the massive Schwinger model, with only vacuum observables and meson masses as input, it predicts this separation. Numerically, the half-flux point corresponds to equal weights of the two configurations, which explains the observed entanglement entropy peak. The authors also relate the entanglement spectra of the string and the θ-vacuum and construct their wavefunctions with a restricted Boltzmann machine.
In briefUsing a high-Tc superconducting circuit to study how neon thin films wet and freeze
SummaryResearchers monitored neon films near the triple point with a multimode resonator made from the high-Tc superconductor YBCO, which stays superconducting where liquid neon exists. In the liquid state they saw uniform wetting that saturates near 20 nm, drainage of thicker films, and spontaneous thickness oscillations. Solid film thickness varied by about an order of magnitude across freezing trials, and adding neon doubled the resonator quality factor, with dielectric loss ruled out as the limit.
Brennan Dizdar, Yueheng Shi, David I. Schuster et al.
In briefTrajectree: A quantum trajectory-based quantum circuit simulator
SummaryThis paper introduces Trajectree, a quantum circuit simulator based on quantum trajectories that saves partially executed trajectories to efficiently simulate noisy, broad and deep circuits and generate many Monte Carlo samples. The authors report that it simulates standard benchmark noisy circuits over five times faster than Google's QSim in the Cirq package. They also apply its optical backend to simulate three optical experiments: a CHSH Bell test, entanglement swapping, and an optical CNOT gate.
In briefReusable QUBO-derived inverse factors for solving fixed-operator PDEs with finite differences
SummaryThis paper addresses partial differential equation simulations that repeatedly solve linear systems with a fixed matrix but changing right-hand sides. The method splits the discretized operator into small coupled blocks reduced by block Schur complements, and uses quadratic unconstrained binary optimization only offline to build reusable local inverse factors. These factors are stored and then applied in an ordinary classical block solve. The authors analyze the binary representation's accuracy and resources, and test representative finite difference problems, including running the local optimization on quantum optimization hardware.
Arash Fath Lipaei, \"Ozg\"ur E. M\"ustecapl{\i}o\u{g}lu, \"Ozlem Salehi et al.
In briefPure lattice states with unusual half-chain factor types
SummaryThe paper constructs translation-invariant pure quasifree fermionic states on a one-dimensional lattice whose half-chain von Neumann algebras are hyperfinite factors of type II∞ or IIIλ. Through the Jordan-Wigner-Araki transformation, these give spin-chain states with the same factor types, showing such factors need not be type I or III. For gauge-invariant quadratic Hamiltonians with finitely many flavours and exponentially decaying hopping, the authors prove a dichotomy between an area law with type I factors and logarithmic entropy growth with type III1 factors.
WhyConstructs translation-invariant pure states with type II∞ or IIIλ half-chain factors and proves a dichotomy for quadratic Hamiltonians, overturning the expected link between entropy scaling and facto
In briefGate-defined quantum dot and charge noise in a Forksheet transistor
SummaryThe study characterizes a 14-nm nMOS Forksheet transistor at temperatures down to 10 mK. The device keeps working at cryogenic temperatures with improved characteristics and forms a gate-defined quantum dot tunable to the single-electron regime. Measured average charge-noise amplitude is about 10 μeV/√Hz at 1 Hz, with a minimum of 3.3 μeV/√Hz.
Dominic Leclerc, Claude Rohrbacher, Alexis Morel et al.
In briefPerformance Evaluation of a Portable Quantum Memory Based on Hot Rb Vapour
SummaryThis paper evaluates a commercial quantum memory (QMem) built on a warm rubidium atom vapour, which is intended for quantum repeaters. The authors measure its coherence time, absolute efficiency, signal-to-noise ratio and bit error ratio across a range of easily controlled vapour temperatures. They compare these metrics against the conventions used for current optical communication protocols.
Kenneth Gregory, Konrad Socha, Khaled Mnaymneh et al.
In briefTrapped-ion simulation of a 1+1D SU(2) lattice gauge theory
SummaryThis paper reports a quantum simulation of a 1+1 dimensional SU(2) lattice gauge theory with one fermion flavor, run on Quantinuum's trapped-ion System Model H2. It uses the loop-string-hadron formulation, encoding both bosonic gauge and fermionic matter degrees of freedom in qubits. The simulation goes beyond the common j_max=1/2 truncation to access the j=1 electric-flux sector. The authors conclude that current quantum hardware can handle such lattice gauge theories and reproduce the expected Trotterized dynamics.
Toshiaki Kaji, Jesse R. Stryker, Yutaro Iiyama et al.
In briefQuantum error correction well below the color code threshold
SummaryThis paper demonstrates below-threshold operation of triangular color codes with distances 3, 5, and 7 on Quantinuum's Helios trapped-ion processor. Logical error rates reach as low as 5×10⁻⁶, with error suppression of about 12 per step up in code distance. The authors also report results for two-qubit Clifford randomized benchmarking encoded in color codes and compare several decoders, including real-time neural network decoders.
WhyDemonstrates below-threshold color code operation on trapped-ion hardware with Λ≈12 and real-time neural decoding, providing strong experimental evidence with significant field impact.
Ali Lavasani, Adam P. Reed, Joan M. Dreiling et al.
In briefLogical error suppression in a surface code memory on an IBM cloud quantum processor
SummaryThis paper studies a rotated surface-code memory on the cloud-accessible IBM Nighthawk r2 superconducting processor. The authors optimize syndrome extraction with short readout, fused measurement and reset, and an XX echo on data qubits, reducing the median per-round logical error rate by 3.8x in the X basis and 1.4x in the Z basis. They also report that the distance advantage weakens over up to 150 rounds, with long-lived correlations possibly reflecting a persistent error component.
In briefTask-aligned spectral inductive biases in parameterized quantum circuits
SummaryThe paper studies data reuploading quantum circuits that take a varying signal and a fixed, task-defined spectral query as inputs. The authors show that encoding the query changes the circuit's hypothesis class over the signal by modulating how its signal frequencies are combined, without adding new sample-dependent information. In a radar-inspired classification task and small-scale spectral tasks, aligned queries improved learning from limited data compared with parameter-matched circuits using mismatched or absent queries.
Felix Frohnert, Adri\'an P\'erez-Salinas, Sebastian Lange et al.
In briefUsing verifiable structure and agentic AI to find more efficient quantum circuit compilations
SummaryThe paper proposes constraining bottleneck subroutines of quantum circuits to reversible classical logic, phasing operations, and measurement-based uncomputation, which allows efficient classical simulation and fuzz-testing-based certification of approximate synthesis. It discusses three applications: computing 1/√x for Coulomb operator simulations, decoding Reed-Solomon codes for decoded quantum interferometry, and quantum look-up tables with Givens rotations for quantum chemistry. The authors also outline reference implementations and a planned Kaggle competition to spur AI-driven compilation optimization.
Tanuj Khattar, Nicholas C. Rubin, Craig Gidney et al.
In briefEstimating Quantum Network Transmission Quality: Compute, Simulate, or Learn?
SummaryThis paper examines how quantum network controllers can estimate the end-to-end fidelity of entanglement along candidate paths, a key input for routing and scheduling decisions. Drawing on optical-network quality-of-transmission estimation, it proposes a compute, simulate, or learn framework and demonstrates it with a simulation-based proof of concept. The authors report that analytical composition may suffice in simple regimes, simulation is needed when stochastic memory effects shape fidelity, and lightweight learned surrogates can approximate simulator outputs at much lower runtime cost.
In briefAnyonic Neural Network Wavefunctions: Variational Neural Network Study of Non-Interacting Abelian Anyon Gases
SummaryThis paper introduces AnyNet, a neural network wavefunction architecture that exactly enforces the non-analytic short-distance behavior induced by anyonic statistics. It agrees well with exact diagonalization for N=4 anyons and reaches N=16 for intermediate statistics and N=20 for almost-fermionic anyons, matching perturbative results within 0.4%. The methods are applied to anyons in a harmonic trap and to anyonic atoms modeling binding to impurities.
In briefExperimental test of a balanced amplifier that doubles clean bandwidth
SummaryThis paper experimentally demonstrates a balanced superconducting travelling-wave parametric amplifier that separates the signal from the pump and idler outputs. Two 180° hybrid couplers connect two kinetic-inductance TWPAs, which doubles the image-rejected bandwidth of a three-wave-mixing amplifier. The authors report about 20 dB signal gain across nearly an octave, pump rejection below -20 dB, and average image rejection of about -15 dB.
WhyA balanced architecture resolves image-frequency degeneracy in 3WM TWPAs, with experiments showing about 20 dB gain over nearly an octave.
Nikita Klimovich, Peter K. Day, Henry G. Leduc et al.
In briefOperational meaning of the Mandelstam-Tamm time scale in a spin-1/2 clock
SummaryThis paper examines Georgiev's analysis of a freely precessing spin, which argued that the Mandelstam-Tamm quantity ΔT is not a time-operator variance and that its divergence near clock-signal extrema lacks physical meaning. The authors show that, for the same projective σx readout, ΔT equals the inverse square root of the classical Fisher information in one binary outcome at regular operating points. They conclude that divergence at submaximal-coherence extrema reflects reduced local sensitivity of the chosen readout rather than infinite physical time variance, while a complementary σy measurement remains informative.
In briefEnergy transport and thermalization when localized conserved constraints do not commute
SummaryThis paper studies how spatially localized conserved quantities affect transport and thermalization in quantum and classical many-body systems. It examines an integrable quantum Volterra model, its nonintegrable classical limit, and two cellular automata derived from the classical limit. Energy transport is ballistic in the quantum model and diffusive in the classical limit, with the diffusion constant unchanged by whether the constraints are fixed. In the cellular automata, localized constraints leave transport unaffected but strongly break local ergodicity.
In briefDetecting phase transitions by how entanglement scales with distance between subsystems in the t'-π-flux Hubbard model
SummaryThis paper proposes a method to detect phase transitions using the logarithmic derivatives of negativity, which measures entanglement between small clusters at varying distances. It targets strongly correlated electron models where large density matrices or large system-size scaling are not accessible. The authors apply the approach to the t'-π-flux Hubbard model with auxiliary-field quantum Monte Carlo and describe how to reduce Monte Carlo noise at large distances. The model shows three transitions, belonging to the Gaussian, Gross-Neveu, and O(3) Wilson-Fisher universality classes.
Maksim Ulybyshev, Gerg\H{o} Ro\'osz, Gabriele Bellomia et al.
In briefRecursive operator localization and spectral gap bounds via local spectral expansion geometry
SummaryThe paper introduces Recursive Operator Localization (ROL), a framework that links local energy problems in a hierarchy through linear maps between Hilbert spaces. It realizes ROL with nested subspaces indexed by a weighted simplicial complex, using spectral gaps of auxiliary random walks on links to propagate geometric estimates upward and bound the global spectral gap from local gaps. For weighted scalar functions on facets, the decomposition reduces to the law of total variance and recovers known bounds from local link expansion, while for positive binary spin distributions it yields heat-bath gap bounds from spectral independence. The authors note that the framework may help with quantum Gibbs sampling, though applications require model-specific geometric estimates and energy comparisons.
In briefStudy of how IQP quantum feature-map kernels represent functions, using RKHS theory
SummaryThis paper analyzes the IQP feature map used in quantum kernels by deriving a Fourier representation of the generators of its reproducing kernel Hilbert space. The representation is a finite sum of trigonometric functions of polynomially transformed inputs, with frequencies set by the circuit and scaled by a bandwidth hyperparameter. The authors identify three bandwidth regimes (localised, multimodal, periodic). Experiments on synthetic data in classification and anomaly detection, compared with a classical Gaussian kernel, reproduce these regimes and show benefit only when accessible frequencies match the data structure.
In briefHow truncating irreducible representations affects quantum simulations of QCD
SummaryThe paper examines how to choose the basis and truncation for quantum simulations of lattice gauge theories with continuous gauge groups. The authors compute relative probabilities of links occupying different irreducible representations in the SU(3) lattice gauge theory ground state, using Euclidean Monte Carlo and weak-coupling lattice perturbation theory. The results indicate which higher electric flux states must be kept to accurately capture non-abelian gauge dynamics near the continuum limit.
In briefDelta-sigma noise shaping for low-resolution quantum key transmitters
SummaryThis paper studies delta-sigma modulation as a way to reduce the noise caused by low-resolution digital-to-analog conversion in continuous-variable quantum key distribution transmitters. In a 500-MBaud system over 20 km of fiber, 3-bit second-order modulation at an oversampling ratio of 12 achieves an estimated asymptotic key rate of 46.38 Mbps, while uniform quantization yields no positive rate. The authors also analyze a spectral replica from third-order intermodulation between the pilot tone and the quantum signal and its key-rate penalty.
In briefPartially isophase electromagnetic configurations: field setups that behave identically in one conserved-quantum-number sector but differ elsewhere
SummaryThe paper shows that separating variables in the Schrödinger equation allows an infinite-dimensional family of physically distinct field configurations. These are dynamically indistinguishable within one selected sector labeled by a conserved quantum number, yet differ in all others; the authors call this a partial isophase symmetry. Two main examples are axially symmetric magnetic backgrounds (Fock–Darwin oscillator spectral flow and a two-channel extension) and a Cartesian slab with magnetic barriers in two-dimensional electron gases. The work also discusses implications for the inverse spectral problem and a quasiclassical counterpart.
In briefCertifying Information Flow in Multi-Agent Behaviours
SummaryThe paper studies which input-output behaviours of several agents can be produced under a fixed communication schedule with shared randomness, where feasibility reduces to a linear program. Under explicit operational assumptions, it proves that the affine hull of achievable behaviours is exactly described by equalities stating that each joint output depends only on inputs able to reach the relevant agents. One binary message per event already attains this affine hull, so message capacity restricts achievable behaviours without being visible to these equalities. The results are applied to one-bit broadcast and relay hierarchies for two and three agents, and to low-order projections of the local polytope.
M. Haider Akbar, Onur Pusuluk, \"Ozg\"ur E. M\"ustecapl{\i}o\u{g}lu
In briefBetter spin-bath parameter estimation using initial correlations and a two-qubit probe
SummaryThis paper studies whether adding a second qubit can improve estimation of spin-environment parameters when only one of the two qubits is measured. Both qubits couple to a common spin bath via the exactly solvable central-spin model. Without initial correlations or direct inter-qubit coupling, the two-qubit probe performs exactly like a single qubit. Including initial system-environment correlations lets the measured qubit read out a parameter-dependent imprint on the environment, improving the quantum Fisher information for coupling strength and temperature.
In briefHolographic two-point functions across a PT-breaking transition
SummaryThis paper studies a holographic model of a logarithmic conformal field theory, where two scaling dimensions coalesce at an exceptional point in a PT-symmetric system. The authors deform the dipole model so the dimensions split into real or complex-conjugate pairs, and derive renormalized two-point functions in closed form. These change from power-law combinations to logarithmic and then log-periodic forms. In the BTZ black hole, the linear response shifts from overdamped to underdamped across the transition.
