In briefQPI-DeepONet-MAC: A Hybrid Classical-Quantum Model for Solving Physics-Based Equations
SummaryThis paper proposes QPI-DeepONet-MAC, a hybrid architecture that combines parameterized quantum circuits with physics-informed deep operator networks to learn solutions of parametric partial differential equations. The authors prove the model keeps universal operator approximation ability and derive gradient bounds with conditions for avoiding barren plateaus. They also give a lower bound on the number of sensors needed for a target accuracy and introduce a regularization scheme based on quantum coherence and state fidelity.
Said Lantigua, Jos\'e Valencia, Gilson Giraldi et al.
In briefCollision models: Markovian and non-Markovian impurity models
SummaryThis paper introduces a new class of collision models, called Markovian impurity models, for studying open quantum systems, motivated by scenarios where information backflow is periodically blocked. Using the Breuer-Laine-Piilo (BLP) measure of non-Markovianity, the authors show that if a p-impurity model is non-Markovian, so is the (p+1)-impurity model, implying a critical p where the dynamics transition from Markovian to non-Markovian. Numerical simulations indicate that stronger system-ancilla and ancilla-ancilla interactions generally lower this critical p, though exceptions exist.
In briefHow memory effects change steady states where measurement information balances loss
SummaryInformational steady states occur when information gained from continuous measurements is exactly offset by information from earlier measurements lost to the environment. Previous studies mostly assumed Markovian dynamics, so memory effects were less understood. This paper uses continuously monitored collision models, representing the environment as a sequence of two-qubit ancillas, to study these effects. For this model, the steady-state information gain per measurement is negatively correlated with the degree of non-Markovianity.
In briefEfficient quantum phase estimation using an adaptive entanglement-assisted Hadamard test
SummaryThis paper proposes the adaptive entanglement-assisted Hadamard test (AEHT), which iteratively refines a reference phase so that the amplification of the phase signal grows stronger over successive iterations. It also addresses the systematic bias that arises from imperfect eigenstate preparation, which AEHT can suppress. The authors measure cost by device-restart count rather than shot count. Numerical experiments show AEHT outperforms conventional methods under this measure.
In briefNear-optimal quantum query lower bounds for graph bipartiteness and expansion testing
SummaryThis paper studies two classic graph property testing problems, bipartiteness testing and expansion testing, in the bounded-degree model through quantum query complexity. Classically, about √N queries are known to be necessary and sufficient. Prior quantum work gave Õ(N^{1/3}) algorithms and an Ω̃(N^{1/4}) lower bound for expansion testing. The authors prove essentially tight Ω̃(N^{1/3}) quantum query lower bounds for both problems, fully characterizing their quantum query complexity up to polylogarithmic factors.
WhyTightens prior quantum query lower bounds to near-optimal via rigorous polynomial-method proofs, resolving a long-open question.
In briefLess precise but less noisy: local circuits for momentum-space state preparation and measurement
SummaryThis paper compares two ways to prepare the ground state of a tight-binding chain on a quantum computer: adiabatic evolution and the Fermionic Fourier Transform (FFT). On Quantinuum's H2, the authors find that beyond a certain system size, adiabatic evolution reaches significantly lower energies than FFT with the same gate count and circuit depth. They attribute FFT's higher noise sensitivity to its finer momentum resolution, which requires long-range couplings that spread errors faster. The authors also introduce a less precise but cheaper and less noisy momentum measurement scheme that outperforms FFT for spectral function measurement.
In briefExact first-detection probability in a monitored quantum circuit
SummaryThis paper studies how local stroboscopic measurements interact with unitary dynamics in the deterministic Floquet-quantum East model. Using a renewal equation linked to the subsystem Loschmidt echo, the authors derive exact results for the probability of first detecting monitored qubits in their initial state. The statistics are geometric when monitoring is infrequent and deviate from geometric behavior when monitoring is frequent.
Cecilia De Fazio, Igor Lesanovsky, Gabriele Perfetto
In briefFaster Numerical Optimization of Quantum Bell Inequality Values via Batched Gradient Descent
SummaryThe paper introduces a batched gradient descent (BGD) optimizer for computing the quantum value of Bell inequalities. It performs a differentiable search over quantum strategies without forming the large Bell operator, and runs parallel restarts on a GPU. The authors report a significant speedup over the see-saw method across many Bell inequality families, handling cases with more than a thousand inputs or outputs within minutes.
In briefPredicting electric-field noise in ion traps via fluctuation physics
SummaryElectric-field noise in surface ion traps causes errors in trapped-ion quantum gates, and it has been hard to predict. This paper presents a method that estimates this noise from fluctuations in the bulk of dielectric and metallic materials, valid for any trap geometry and requiring only energy loss as a material property. Applied to a trap with an electrically floating electrode, the model finds dielectrics dominate the noise and predicts the observed level within a factor of four. The authors also analyze how electrode structures shield the noise in typical surface trap designs.
In briefA mentor-led two-way teleportation protocol that handles any qubit state
SummaryThis paper proposes a bidirectional asymmetric quantum teleportation protocol initiated by a mentor. Existing schemes limit the two-qubit state sent from Bob to Alice to a Bell-like form, whereas this protocol allows an arbitrary two-qubit state to be teleported in the same framework. The required entangled resources are built from standard quantum gates, and the protocol is implemented as a quantum circuit and simulated in Qiskit. The simulated probability distributions for the chosen input states are very close to the theoretical values.
In briefA code-agnostic neural decoder that learns quantum error correction from the detection error model
SummaryThe paper presents POLYMECHANON, a graph neural network decoder for quantum error correction that takes only the detection error model of a code under a given noise model as input. On the rotated surface code it yields up to 25% fewer logical failures than correlated MWPM, and on high-rate qLDPC codes it matches or surpasses BP+OSD. Its decoding time does not grow with the physical error rate.
In briefLocal dissipation restores operator-norm locality bounds in Bose-Hubbard systems
SummaryThis paper studies a dissipative Bose-Hubbard model with on-site ℓ-photon loss. It shows that local dissipation restores an operator-norm Lieb-Robinson bound, which closed bosonic lattice systems generally lack for arbitrary initial states, and establishes an almost-ballistic bound. The authors extend the approach to cat-code dissipation and prove a local adiabatic approximation, uniform in total volume, for initial states in the code space.
WhyDissipation yields state-uniform operator-norm Lieb-Robinson bounds for bosons, resolving a known closed-system obstruction with rigorous moment estimates.
In briefElectromagnetic Side-Channel Leakage in Quantum Key Distribution Hardware
SummaryThis paper examines whether quantum key distribution (QKD) equipment can leak information through electromagnetic emissions, which could enable side-channel attacks. The authors measured a prototype time-bin decoy-state BB84 device with Y and Z bases inside an anechoic chamber at distances of 0 m and 3 m. They found the transmitter to be the primary leakage source, with distinct differences between quantum states at the third harmonic of the 1241.6 MHz clock.
In briefAdversarial Robustness in Fake Quantum Simulators: Testing Quantum Machine Learning Models Under Noise
SummaryThis paper studies how quantum machine learning models perform and resist attacks when run on noise-model-based fake simulators. The authors first benchmark the speed of Qiskit's Aer simulator on different hardware, then test Projected Gradient Descent attacks and adversarial retraining under realistic noise. For 4-qubit classifiers, they find that retraining with a 50/50 ratio of adversarial to benign examples is essential for practical robustness.
In briefOn-shell renormalization of sine-Gordon via the quantum inverse scattering method
SummaryThe paper solves the sine-Gordon model on a lattice using the quantum inverse scattering method and rescales the box length to reach continuum and infinite-volume limits. It absorbs all cut-off dependence into a factor Z_L multiplying the box length and uses an on-shell condition to define a renormalization scheme. Soliton and breather masses are obtained from monodromy eigenvalues and reduce to classical and semiclassical results as β goes to zero.
In briefEfficiently certifying time-reversal symmetry requires entanglement
SummaryThe paper proposes a Bell-inequality-like test for time-reversal symmetry that uses only forward access and trusted quantum operations, converting temporal input-output relations into measurable spatial exchange symmetry via entanglement. For n-qubit unitary dynamics, it proves that any classically adaptive protocol needs Ω(min{2^{n/2}, 2^{n-e}}) queries to distinguish time-reversal-symmetric circular ensembles from Haar-random dynamics, where e depends on the probe and measurement logarithmic negativities. Maximally entangled probes and SWAP measurements reduce the query cost to a constant.
WhyProposes a new entanglement-based test for time-reversal symmetry with matching query-complexity lower and upper bounds, giving a substantive theoretical advance.
In briefMolecular dynamics with nuclear quantum effects on quantum computers: a hybrid algorithm for simulating molecules
SummaryThis paper presents a hybrid quantum-classical algorithm for ab-initio molecular dynamics that includes nuclear quantum effects through the nuclear-electronic orbital method. Ground state energies and forces are computed on a quantum computer with a variational quantum eigensolver, while molecular geometries are updated classically. The method is tested on H2, H2O and the Zundel ion, with simulated vibrational spectra compared to experimental data. The authors report that including nuclear quantum effects makes proton shuttling in the Zundel ion effectively barrierless and improves frequency errors, with compact hardware-efficient circuits giving results comparable to UCCSD.
Lukas Ha{\ss}furth, Juliane Heitk\"amper, Elias Walter et al.
In briefLearning quantum interactions from thermal metastable states
SummaryThis paper studies how to learn quantum interactions from metastable states of finite-temperature many-body systems coupled to a heat bath. Previous protocols relied on exact Gibbs states, which are computationally hard to prepare. The authors propose learning from metastable states of detailed-balanced master equations (Lindbladians) and extend the Gibbs-state learning mechanism to these states, achieving nearly optimal sample and computational complexity.
WhyExtends Gibbs-state learning to metastable states of detailed-balanced Lindbladians, with near-optimal sample and computational complexity proofs.
In briefVector vortex beams resist turbulence intrinsically in free-space quantum key distribution
SummaryThis paper studies vector vortex beams (VVBs), hybrid polarization-orbital angular momentum entangled states, for free-space quantum key distribution. Scalar vortex modes decohere severely in atmospheric turbulence, pushing the bit error rate above the security threshold. The authors argue that because turbulence couples equally to both orthogonal circular polarizations, the relative polarization phase cancels the perturbation. Numerical simulations over turbulence strengths D/r0 from 0 to 3.0 show the asymptotic QBER drops from 42.0% to 4.8%, an error-suppression factor of about 11.6, without adaptive optics.
In briefContinuous-time randomized compiling for memory-bearing quantum processes
SummaryThe paper proposes continuous-process randomized compilation (CPRC), which unifies random control trajectories, finite-duration instruments, and environmental dynamics on one time axis to tailor errors correlated across time in memory-bearing environments. It studies four protocols and proves results including pointwise covariance, a total-variation error bound, and Fock norm convergence of the trajectory-averaged process tensor. The theory is numerically validated in a qubit model with non-Gaussian and thermal Drude baths.
WhyExtends randomized compiling to continuous-time non-Markovian processes with a unified framework and rigorous convergence and error bounds, though numerics are limited to a single-qubit model.
In briefComparing zero- and infinite-temperature damping in variational quantum circuits, covering feature scale, sampling cost, and frame gauge
SummaryThis paper compares amplitude damping with its Pauli-twirled version, which is generalized amplitude damping at infinite temperature, to isolate the effect of zero-temperature bias in variational quantum circuits. The bias acts mainly through feature scale, and features shrink under both damping models. A trainable output scale removes most accuracy differences, but the cost reappears as additional measurement shots.
Vu-Quoc-Minh Nguyen, Tuan-Vu Truong, Hoang-Long Nguyen et al.
In briefUnbounded advantage of indefinite causal order over definite order in finite-dimensional quantum metrology
SummaryThis paper studies causal order in quantum metrology for estimating a geometric phase. Indefinite causal order is known to give an unbounded advantage for infinite-dimensional oscillators, but finite-dimensional advantages were thought to be bounded. The authors show that arbitrarily large advantages exist for finite-dimensional systems in the finite-sample regime. They also establish an approximate Weyl relation for discrete Gaussian wavepackets.
WhyProves an unbounded separation between definite and indefinite causal order in finite-dimensional metrology, overturning prior bounded-advantage expectations with rigorous analysis.
In briefUnflattening by Flattening: how input distributions set output variance in angle-encoded circuits
SummaryThe paper studies how the input distribution of angle-encoded quantum circuits affects output variance, which governs barren plateaus in training. It introduces algebraic input purity, the overlap of the input with the circuit's dynamical Lie algebra, to explain output variance scaling. Results are given for matchgate circuits and an off-diagonal family, including zero output on computational-basis inputs. A classical preprocessing check is proposed to certify mean output variance before circuit execution.
WhyLinks input algebraic purity to output variance in angle-encoded circuits, with a computable preprocessing certificate; novel theory with proofs and numerics, though scope is narrow.
In briefBest starting quantum states for measuring coupling in cavity optomechanics
SummaryThis paper identifies which initial quantum states give the most information for estimating the single-photon coupling in a linearized cavity-optomechanical system, using quantum Fisher information. In the red-detuned beam-splitter regime, the problem reduces to maximizing a variance with a closed-form solution. When total excitation number is fixed, the optimum is a two-mode NOON state, reaching a Heisenberg bound quadratic in energy. Extensions cover the blue sideband, cavity loss, and a state-of-the-art experiment.
In briefTransport theory of Majorana fermion and edge vortex tunneling in a generic two-arm co-propagating Majorana interferometer
SummaryThis paper studies electrical transport from interference of co-propagating chiral Majorana edge modes in proximity-coupled quantum anomalous Hall systems. Using bosonization with an extra replica sector, the authors extend earlier work to fully asymmetric setups and analyze a Mach-Zehnder interferometer. They find the DC conductance shows no edge-vortex interference, but at low frequencies the interference appears as an effective capacitance sensitive to non-Abelian anyon statistics. The signatures remain visible if interferometer arm lengths match within the thermal length.|
Domenico Giuliano, Andrea Nava, Fabian Hassler et al.
In briefVacuum entanglement in a time-dependent electric field: how an electric field changes quantum correlations between two regions
SummaryThis paper studies correlations in the vacuum state of a charged scalar field between two disjoint regions, using localized observables. It considers a homogeneous, time-dependent electric field and links the time evolution of these correlations to particle-antiparticle pair creation via the Schwinger effect. The electric field increases state mixedness and total correlations, while the distillable entanglement between the regions decreases and can vanish at a finite time for sufficiently intense pulses. The authors also show that anisotropy from the external background gives the correlations a nontrivial directional dependence.
\'{A}lvaro \'{A}lvarez-Dom\'{\i}nguez, Luis J. Garay, Mercedes Mart\'{\i}n-Benito et al.
In briefLocal thermodynamics and Rényi bounds on black hole mergers
SummaryThe paper studies Rényi entropy inequalities that aim to strengthen the area-theorem bound on mergers of stable black holes in AdS space. It shows that stronger bounds depend on the joint initial state, since masses and temperatures fix only local thermal marginals. Within a canonical two-hole description, the authors derive the smallest compatible entropy and find that for equal Schwarzschild–AdS4 holes it reduces to the area bound at leading semiclassical order.
In briefEqual Stored Energy in Cascaded Collisional Quantum Battery Charging: An Analytical Study
SummaryThis paper studies a two-qubit quantum battery charged through cascaded collision models. After each collision, the charger undergoes an adaptive measurement, producing two conditional battery states with equal expected stored energy. For chargers prepared in the excited state, the authors analytically prove that trajectory-dependent fluctuations in stored energy can be completely suppressed. For general superposition states, the theory shows full suppression is prohibited by battery-phase-dependent recursive dynamics. Numerical simulations agree closely with the theoretical predictions.
In briefPreserving Measurement Incompatibility: Purification, Activation, and a No-Go Result
SummaryThis paper studies how well noisy quantum channels preserve measurement incompatibility, a resource used in quantum information tasks. The authors define robustness-based resource monotones for this purpose. They show that pre-filtering can strengthen preservability but cannot activate a channel that annihilates incompatibility, which is a no-go theorem. They further show that such a channel can be stochastically activated by post-filtering.
In briefQuantum codes nearing the CSS GV bound, encodable in linear time
SummaryThe paper constructs quantum CSS codes whose rate-distance tradeoff approaches the CSS GV bound. The codes can be encoded by a quantum circuit of logarithmic depth using a linear number of gates. The construction is inspired by work of Brehm and Resch and serves as a quantum analogue of classical repeat multiple accumulate codes. The encoder consists of one constant-depth outer quantum circuit followed by several classical accumulation and inverse-accumulation layers.
In briefAdaptive error mitigation for quantum reinforcement learning on noisy quantum hardware
SummaryThis paper proposes Adaptive Policy-Guided Error Mitigation (APGEM), a framework that dynamically selects an error mitigation strategy during quantum reinforcement learning training based on policy-level indicators. It is evaluated on the Capacitated Vehicle Routing Problem under various NISQ noise models and noise levels. The authors report that APGEM outperforms static mitigation methods, reaches about 94% of an oracle strategy's utility, and yields higher state fidelity and more stable learning as noise increases.
Bisma Majid, Shabir Ahmed Sofi, Mir Mohammad Yousuf
In briefImaging by correlating microwave photons and surface acoustic phonons through entanglement
SummaryThis paper proposes a correlation imaging method that uses hybrid entanglement between microwave photons and microwave surface acoustic wave phonons, generated in a superconducting quantum circuit. Because their wavelengths differ by about five orders of magnitude, the field used to probe an object can differ from the field used for readout. The authors report that the approach enables large spatial scaling in both directions and offers a general framework for cross-scale quantum sensing.
In briefWork fluctuation speed limit in boundary conformal field theories: a bound on how fast quantum critical systems can be controlled, using random work fluctuations from external driving.
SummaryThis paper studies the fundamental limits of finite-time driving in quantum critical systems described by boundary conformal field theory. It shows that stochastic work fluctuations can serve as a resource for fast control and derives an exact, saturable fluctuation-based speed limit for weakly driven systems at finite temperature. The bound and its saturating protocol depend only on the universal scaling dimension, interpolating between the Kibble-Zurek regime and an adiabatic regime where linear driving is optimal. The results are said to apply to quantum impurity, fractional quantum Hall, and superconducting-circuit platforms.
In briefExperimental measurement of quantum coherence across a set of quantum states
SummaryThis paper reports an experiment, using a Sagnac interferometer, that directly verifies quantum coherence for a set of quantum states in a basis-independent way. The authors state that the theory matches their results and can be applied to protocols such as BB84. They also introduce set coherence and show that the states used in BB84 exhibit maximal set coherence.
In briefFermionic Gaussian Scrooge ensembles for deep thermalization in free-fermion systems
SummaryThe paper studies the projected ensemble of pure states left on an unmeasured subsystem after measuring part of a many-body wave function. It introduces the fermionic Gaussian Scrooge ensemble to describe deep thermalization in generic free-fermion systems. The authors analytically show that the SYK2 model's projected ensemble follows this ensemble at arbitrary times, and give numerical evidence that it emerges at long times in random Gaussian circuits with charge conservation.
In briefFast mixing of one-dimensional quantum spin chains at any finite temperature
SummaryThe paper proves that a quasi-local quantum Gibbs sampler mixes rapidly to the Gibbs state of any 1D finite-range Hamiltonian at all finite temperatures, in O(log(n/ε)) time. Each update traces out a contiguous block of constant length and reconstructs it with the Petz recovery map. The authors also show that quantum circuits preparing these Gibbs states have depth O(polylog(n/ε)).
WhyProves logarithmic-time rapid mixing for 1D finite-range Hamiltonians at all finite temperatures, extending prior high-temperature results via rigorous analysis.
In briefSolid immersion lenses boost light collection from shallow diamond NV centers
SummaryThis paper studies lenses made from high-k yttria-stabilized zirconia (YSZ) placed on diamond surfaces to improve light extraction from shallow implanted nitrogen-vacancy (NV) centers. The authors investigate photon extraction numerically and fabricate the lenses using room-temperature pulsed laser deposition and photolithography. They report a 63% average increase in saturation intensity and a 33% average reduction in the laser intensity needed for saturation.
In briefExponentially compressed, garbage-free alias sampling for preparing polynomial quantum states
SummaryThis paper studies coherent alias sampling, a subroutine used in quantum lookup tables for block encoding and qubitization-based quantum simulation. It shows that when amplitudes are samples of a polynomial of degree at most d, the alias table can be represented and evaluated coherently at cost polynomial in n and d, exponentially reduced from Θ(2^n). The authors also give a deterministic Clifford+T circuit for preparing polynomial states using reversible integer arithmetic, and a new technique for uncomputing the auxiliary register.
WhyReduces alias sampling cost from exponential to polynomial for polynomial states and adds a new uncomputation technique, supported mainly by circuit-construction analysis.
In briefIsing chain with dimerized anisotropic Gamma interaction: vector chiral order and dynamical quantum phase transitions
SummaryThe paper studies a transverse-field Ising chain with dimerized anisotropic Gamma interactions. The competition between Gamma anisotropy and the transverse field produces vector chiral order and dynamical quantum phase transitions. The phase diagram includes positive and negative vector chiral phases and a paramagnetic phase. Without Ising interactions, a gapless critical line without finite-size drift is found, where spin correlations and entanglement entropy behave differently from Ising criticality.
In briefHow well bipartite quantum correlations survive noisy dynamics
SummaryThis paper develops a robustness framework for Bell nonlocality, EPR steering, and entanglement under noise. It establishes an ordering of the robustness measures for these three correlations and links robustness to noise-induced transition times. The instantaneous robustness decay along Lindblad dynamics is formulated as a directional derivative. Examples with isotropic depolarizing noise and amplitude damping illustrate the results.
In briefProof that classical methods cannot efficiently learn Hamiltonian functions
SummaryThe paper studies predicting quantities of the form Tr[f(H)ρ] from classical descriptions of a Hamiltonian H and a state ρ, where f is unknown. It rigorously proves average-case classical hardness for two specific functions, cos(πλ) and e^{-βλ}, assuming random RSA moduli are hard to factor. Under that assumption, an efficient classical learner for either problem would yield a classical polynomial-time factoring algorithm.
In briefMultivariate Quantum Signal Processing: A Framework for Designing Quantum Transformations of Multiple Noncommuting Matrices
SummaryThis paper introduces Multivariate Quantum Signal Processing, a framework for designing matrix-valued transformations of several noncommuting, nonnormal block-encoded matrices, using a transfer-function approach for quantum query algorithms. The authors provide analytic error bounds that yield circuits with logarithmic error dependence and near-unity success probability. The framework is applied to Hamiltonian simulation, sparse simulation, linear systems, differential equations, ground-state preparation, and generalized eigenvalue problems, with improved query complexity results.
WhyProposes multivariate QSP using IIR filter analysis, improving query complexity for several open multi-oracle problems; largely theoretical with limited experimental validation.
In briefTrade-off between dissipation and sensitivity in dissipative bosonic systems
SummaryThis paper studies a driven single-mode bosonic system in a thermal environment and finds a universal trade-off between the dissipation needed to keep a quantum state more sensitive than a thermal one and the resulting sensitivity gain. For Gaussian states, the authors derive a lower bound on the steady-state entropy production rate, expressed through the quantum Fisher information gain relative to a thermal reference state. Numerical studies of Kerr nonlinearities suggest these bounds may also hold beyond the Gaussian regime.
In briefConstant-depth MPS-pretrained variational circuits for noisy distributed quantum processors
SummaryThe paper compares three circuit realizations of matrix-product-state (MPS) pretraining for variational quantum algorithms. Ideal performance and gradient scales are comparable, but the shallower brick-wall architecture shortens circuit duration and reduces idle-time decoherence and zero-noise-extrapolation overhead, giving better noisy results. The authors extend the approach to modular processors, using a nearest-neighbor QPU-path schedule that keeps per-layer depth constant as more processing units are added.
In briefQuantum synchronization in coupled atom-cavity systems: a study of how driven atoms and cavity light can lock together.
SummaryThe paper studies a cavity quantum electrodynamics setup in which spin-1 atoms, driven by a classical field, interact with two driven cavity field modes. The authors show that the atoms and the two-mode cavity field can display limit-cycle or quantum synchronization depending on the cavity drive strengths and other parameters. They visualize these behaviors with generalized Husimi-Q functions and report that field synchronization and Wehrl mutual information between the two modes are mutually exclusive.
In briefStationarity test: learning quantum many-body systems from thermal states
SummaryThis paper introduces the stationarity test, a learning algorithm that infers the Hamiltonian governing a quantum system from samples of its thermal (Gibbs) states. It guesses a Hamiltonian and measures how local observables change under the associated detailed-balanced quantum Markov chain. The method is applied to learning the interaction graph of lattice Hamiltonians, their coefficients from metastable thermal states, and a refined area law for those states.
WhyIntroduces a new stationarity-test learning algorithm, giving first structure and coefficient learning for lattice Hamiltonians with rigorous, near-optimal complexity guarantees.
In briefLearned Parallel Bit-Flipping Sequential BP Decoder for Quantum LDPC Codes
SummaryQuantum LDPC codes are promising for fault-tolerant quantum computing but need fast, reliable decoders. This paper extends learned sequential belief propagation by running several independent continuations after bit-flip candidates are generated when the syndrome is not satisfied. These continuations can run in parallel, so the method mainly adds hardware rather than sequential latency. Simulations on representative QLDPC codes under the depolarizing channel show improved reliability.
In briefSingle-key generator for reusable families of random-looking quantum operations
SummaryThe paper introduces pseudorandom function-like unitaries (PRFUs), which produce a family of random-looking unitaries indexed by public labels from a single short key. It treats classical-label and coherent-label access separately and proves security against adaptive quantum queries under suitable primitives. The authors also show that PRFUs yield pseudorandom channels and quantum combs, and they list applications such as nonce-resolved authentication and coherently masked QRAM queries.
WhyIntroduces one-key function-like pseudorandom unitaries with security proofs under classical and coherent label access, substantially extending prior pseudorandom unitary work.
In briefExact T-counts of CCZ layers via an isotropy bound
SummaryThis paper proves that a layer of m disjoint CCZ gates requires exactly 6m+1 T gates in any Hadamard-free Clifford+T circuit with clean ancillas, matching a construction by Campbell and Howard. It introduces an isotropy bound: when a phase polynomial has only cubic terms, the qubit-touch vectors span a self-orthogonal subspace over F_2, forcing the T-count to be at least twice its dimension. Applied to PyZX's optimizer output, the bound certifies 193 of 311 phase-polynomial blocks as T-optimal.
WhyIntroduces a novel isotropy lower bound proving exact optimality of 6m+1 T gates for CCZ layers, with empirical validation on PyZX outputs.
In briefSublinear-depth quantum simulation of electrons using atomic orbitals
SummaryThis paper studies how to simulate electronic structure models, discretized into N atomic orbitals, on a quantum computer. The authors combine orbital localization, a hierarchical Hamiltonian decomposition based on the Fast Multipole Method, and shallow quantum Fourier arithmetic circuits to implement a Trotter step in polylogarithmic depth without ancillas. Bounding the Trotter error yields sublinear simulation depth, with a total gate count of N^(5/3+o(1)), matching the best known scaling for any basis.
In briefTATVA: Reinforcement learning for automatically designing quantum circuits
SummaryThis paper presents TATVA, a reinforcement learning system that automatically synthesizes quantum circuits to prepare target quantum states. It treats circuit construction as a sequential process, selecting one gate at a time based on the fidelity achieved, using two agents, DQN and PPO, with Qiskit's statevector simulator. The authors report synthesizing circuits up to 5 qubits with fidelity 0.999999 and reducing circuit depth afterward while preserving fidelity. Target states not seen during training are used to test generalization.
In briefErbium atoms in rare-gas solids emit long-lived telecom-band light
SummaryResearchers isolated neutral erbium atoms in solid neon and argon at 6 K and measured their emission with steady-state and time-resolved photoluminescence spectroscopy. Several near-infrared transitions between 1000 and 1500 nm were observed in both hosts, including inner-shell emission in the telecom O and E bands. In solid neon, the transition near 1437 nm reached a photoluminescence lifetime of up to 78.9 ms, close to the estimated radiative limit. The authors identify rare-gas solids as promising hosts for long-lived telecom-band erbium emission.
In briefA gate-based quantum framework for optimizing codon selection
SummaryThis paper presents a gate-based quantum computing framework for codon optimization, a combinatorial problem relevant to synthetic biology. The objective is formulated as an Ising Hamiltonian expressed in Pauli operators and solved with VQE, SVQE, and QAOA. On a three-amino-acid benchmark the methods reproduce exact ground-state solutions, and hardware runs were performed on IBM Quantum processors.
In briefA quantum phase neural network with 35 inputs and one output for letter recognition.
SummaryThe paper proposes a 35-input, single-output quantum phase neural network to recognize the letters A and B, each formed on a 7×5 pixel grid. Inputs are encoded as quantum phases and processed with adjustable rotation and controlled-NOT gates, with a phase-weight training algorithm using an adaptive learning rate. The authors verify performance on the Qiskit platform and state that expressing probabilities as an analytical polynomial avoids generating all 2^35 complex amplitudes.
In briefFrom Steane to A7: Building Quantum Codes from Invariant States
SummaryThe paper constructs a two-parameter family of single-error-correcting seven-ququart codes with transversal Ã7 symmetry, linked to a two-qubit super-golden gate set. These codes encode two logical qubits and allow non-Clifford operations. The method builds invariant parent states instead of searching for a codespace directly, and puncturing one site yields the target code.
In briefFormal Verification of Quantum Error-Detection Circuits Using Generalised Bit-Vector Abstractions
SummaryThis paper extends a prior bit-vector abstraction for verifying quantum circuits built from H, X, and C-NOT gates using SMT solvers. It generalises correctness properties to repetition and concatenated phase-flip codes of up to 101 logical elements and verifies syndrome extraction for several CSS codes, including the Steane and rotated surface codes. The authors characterise the gate fragment where the abstraction faithfully represents Hilbert-space semantics, and a mutation study with 13 operators detects 622 non-equivalent mutants. A Z3 implementation verifies GHZ circuits with up to 262,144 qubits, while the approach does not cover general stabilizer circuits, Y-type stabilizers, fault-tolerant protocols, or physical noise.
In briefInteractive proofs in arbitrary physical theories beyond IP = PSPACE and QIP = PSPACE
SummaryThis paper studies interactive proofs in general probabilistic theories, including classical and quantum theory, asking how much of their power comes from the underlying physics. With only classical messages, protocols in every theory meeting the standard assumptions decide exactly PSPACE. With a quantum verifier and quantum messages, letting the prover use any theory containing quantum theory does not raise the maximum acceptance probability, so the three-message PSPACE result still holds. When messages can be arbitrary systems, the interactive-proof class can strictly exceed PSPACE.
WhyExtends IP=PSPACE and QIP=PSPACE to arbitrary probabilistic theories, proving exact PSPACE for classical messages and strict excess for general systems.
In briefComplexity of the Hartle-Hawking state in JT gravity, measured by Wigner negativity
SummaryThis paper studies how the negativity of the Wigner function of the Hartle-Hawking state in Jackiw-Teitelboim gravity grows under time evolution. Wigner negativity is presented as a measure of how hard the quantum state is to simulate on a classical computer. At genus zero and high temperature, the leading-order state is a minimum-uncertainty Gaussian wavepacket at rest at the Liouville wall turning point, with a positive Wigner function. Numerical results show late-time negativity approaching a value consistent with 0.08 e^{-4.4/β}, larger at lower temperature.
In briefNeural Fourier Surrogates: A Classical Baseline for Data Reuploading Quantum Neural Networks
SummaryThis paper introduces Neural Fourier Surrogates (NFS), a stochastic classical neural network that learns coefficients over the same finite Fourier series support used by quantum neural networks. On several tabular benchmark datasets, NFS is broadly competitive with established classical baselines, including Random Fourier Features, and performs comparably to data-reuploading quantum neural networks. Analysis of learned Fourier spectra on synthetic data supports the authors' claim that NFS is a natural classical baseline for evaluating quantum neural network performance.
Oliver Knitter, Jonathan Mei, Sang Hyub Kim et al.
In briefRemote preparation of a single electron spin via spin-photon hybrid entanglement
SummaryThe paper proposes a remote state preparation protocol that prepares a stationary electron-spin qubit using hybrid entanglement with a coherent-state light pulse. It avoids coherent-state superposition measurements and photon-number-resolving detection by using an auxiliary spin, homodyne detection, and spin measurement. The authors analyze how nonorthogonality, fiber loss, and spin dephasing affect fidelity, and discuss a possible implementation with diamond nitrogen-vacancy centers.
In briefThermal entanglement transitions from strong SU(2) symmetry: a theoretical study of how mixed-state entanglement in thermal spin systems changes across phase transitions.
SummaryThe paper studies mixed-state entanglement in Gibbs states of locally interacting ferromagnetic spin systems with strong SU(2) symmetry. The authors show that finite-temperature ordering transitions coincide with entanglement transitions, and that in ferromagnetic phases entanglement measures grow faster with system size than in paramagnetic phases. They develop a semiclassical theory and field theory, supported by 3D Monte Carlo simulations and exact numerics for 1D spin-1/2 chains.
In briefAdaptive filtering for qubit readout tests whether extra processing improves state discrimination
SummaryThis paper examines when extra digital processing improves readout fidelity in superconducting qubit systems. It compares a real-time adaptive FIR filter trained with LMS on an FPGA against the standard boxcar averager, using both live hardware and offline analysis of measured shots. For the fixed filters tested, the filtered sum reduces to the boxcar sum times a complex number, so discrimination did not improve, though the frozen LMS filter reduced trace noise. Changing the decision method cut labeled shots per state by 89.8% on a 3D ancilla readout and assigned most shots before the window ended.
In briefConditional Entropy Duality and Quantum Stein's Lemma in von Neumann Algebras
SummaryThis paper establishes duality relations for conditional entropies relative to subalgebras of arbitrary von Neumann algebras, covering the Umegaki relative entropy and the sandwiched and Petz Rényi divergences. The results extend known dualities from tensor-product subsystems to general subalgebras, including in finite dimensions, and apply to the fully infinite-dimensional setting. As an application, it proves an infinite-dimensional quantum Stein's lemma when the alternative hypothesis is the state space of a finite-index subalgebra, interpreting relative entropy to a subalgebra as the optimal asymptotic error exponent in composite quantum hypothesis testing.
WhyExtends conditional entropy duality to subalgebras of arbitrary von Neumann algebras and proves an infinite-dimensional quantum Stein's lemma, a substantial theoretical advance.
In briefesQueranto: A Differentiable Simulator for Structured Quantum Light in Automated Discovery
SummaryThe paper introduces esQueranto, differentiable software that unifies photon-number quantum optics and structured-light propagation in one description, so the spatial evolution of light directly affects non-classical quantum states and their interference. Implemented in JAX, it supports automatic differentiation and hardware-accelerated evaluation for optimization and automated search. The authors demonstrate the framework across a range of quantum-optical applications.
Marcello Armezzani, Tareq Jaouni, Pontus Lindgren et al.
In briefNegative quasiprobabilities only redistribute information in unabsorbed photons, not add to it
SummaryThe paper tests whether negative Kirkwood–Dirac quasiprobabilities provide the quantum advantage claimed for interaction-free measurement, using learning about a weakly absorbing sample as the task. For a single photon, the Fisher information about transmittance per absorbed photon is at most the direct-transmission value. Negativity only concentrates information in weakly lit outputs.
WhyProves negative Kirkwood-Dirac quasiprobabilities confer no advantage for interaction-free measurement, deriving an information bound with rigorous theory; impact concentrated in quantum measurement t
In briefHow light emission synchronizes at room temperature inside virus shells
SummaryOrganic dyes attached to an icosahedral virus shell can synchronize and emit a bright, coordinated burst of light called superradiance. The reason this happens at room temperature despite molecular noise is not understood. The authors experimentally detected underdamped low-frequency vibrations in dye-labeled capsids and used modeling to show how this capsid motion can support the collective polarization needed for superradiant emission.
Tianran Li, Irina Tsvetkova, Matthew Brauser et al.
In briefBeyond Feasibility: Finite-Depth Accessibility in Constrained QAOA
SummaryThis paper studies which feasible solutions a constrained QAOA circuit can actually reach at finite depth. The authors define a finite-depth accessible set and a normalized reachable-quality diagnostic, Q_R. In experiments on constrained portfolio optimization and synthetic block-constrained quadratic problems, Q_R showed the strongest association with realized shallow-QAOA performance, with Pearson and Spearman correlations of about 0.70 and 0.81.
Rushikesh Ubale, Gregory T. Byrd, Yasar Mulani et al.
In briefSmooth perturbation theory and generic Cantor spectrum for multi-dimensional quasiperiodic operators
SummaryThis paper studies multi-dimensional quasiperiodic Schrödinger operators with monotone sawtooth-type potentials in the perturbative localization regime. The authors show that a gap closes exactly when a corresponding eigenfunction, identified by a natural gap labeling, vanishes at the origin. For a generic sampling function f, the spectrum is therefore a Cantor set with all possible gaps open. The paper also gives a class of functions f for which some gaps close.
Ilya Kachkovskiy, Leonid Parnovski, and Roman Shterenberg
In briefPruning Quantum Circuits to Cut Unnecessary Two-Qubit Gates Across Hardware Types
SummaryThis paper proposes a routing-aware pruning method that removes parametric two-qubit gates when their fidelity cost outweighs their computational contribution. The approach is evaluated on both NISQ and fault-tolerant quantum computing architectures. In NISQ simulations on benchmark circuits, gate count fell by up to 48.6% and fidelity improved by up to 47.7%. A feasibility analysis suggests pruning remains relevant on fault-tolerant hardware across a range of code distances and physical error rates.
Pau Escofet, Carmen G. Almud\'ever, Sergi Abadal et al.
In briefFeynman rules in operator language for driven-dissipative quantum systems, covering non-Gaussian photon correlations
SummaryThis paper builds Feynman rules for Lindblad master equations in the operator language of quantum optics, using perturbation theory around an exactly solvable generator that is mostly Gaussian. The expansion splits into a kinematic part, a shift algebra on Wick-ordered operators, and a dynamical part that enters only through the resolvent of the unperturbed generator. The rules are applied to driven Kerr cavities, strongly driven two-level atoms, and Kerr rings and chains, and on a three-site ring they recover the non-Gaussian three-photon correlation missed by Gaussian cumulant closure.
In briefA Hilbert-space analogy for sexual orientation and gender identity
SummaryThe paper introduces a finite-dimensional state-space representation of bisexuality using Dirac notation, with normalized amplitude weights, and analyzes the limits of inference from a single observation. It extends this basis to attraction across genders and defines a separate state space for gender identity with an illustrative normalized time-dependent trajectory. The authors state that the model is an analogy, not a physical theory or a measurement procedure for human sexuality.
In briefDesigning non-ergodic behavior in 2D quantum many-body systems
SummaryThis paper proposes an eigenstate-to-Hamiltonian method for systematically designing two-dimensional quantum spin Hamiltonians on the square lattice with tunable non-ergodic properties. From a common parent model and input eigenstate, the authors build two models differing in interaction geometry. The axial model shows an anisotropy-driven crossover from ergodic to non-ergodic behavior, while the diagonal model exhibits Hilbert-space fragmentation and signatures of quantum many-body scars.
Nyayabanta Swain, Gabriel Lemari\'e, and Shaffique Adam
In briefLate-time power laws in fluorescence decays: quantum decay or other mechanisms?
SummaryQuantum theory predicts that an unstable state's survival probability decays as a power law at late times, but similar behavior can arise in complex systems from other causes. This paper compares alternative explanations, including nonextensive statistical models, continuous lifetime distributions, and energy-migration models, against time-resolved fluorescence data for erythrosine B and eosine Y. The authors find that some features expected from the quantum description, such as a sharp transition from exponential to power-law decay, cannot be reproduced by the fitted statistical and energy-migration parameters.
In briefQuantum key distribution secured by generalized contextuality against post-quantum eavesdroppers
SummaryThe paper asks whether generalized contextuality, a nonclassical property observable in prepare-and-measure experiments, can certify secret keys in a way analogous to Bell nonlocality. Alice and Bob's operational equivalences are treated as constraints that any channel intervention must respect, while the eavesdropper is not assumed to follow quantum theory. The authors show that contextuality is necessary but not sufficient for a positive key rate, since the (3,2)-parity-oblivious random access code yields no certifiable key against a post-quantum eavesdropper.
WhyApplies generalized contextuality to key distribution against post-quantum eavesdroppers via a linear program, proving necessity but not sufficiency with rigorous results.
Daniel Centeno, Roberto D. Baldij\~{a}o, Maria Ciudad Ala\~n\'on et al.
In briefDescribing quantum gates using quasi-process functions
SummaryThe paper shows that every n-qubit unitary can be decomposed into controlled single-qubit unitaries, whose control conditions define a quasi-process function from the study of indefinite causal order. This yields a canonical representation of each unitary as a quasi-process function unitary (qPFU). The authors use it to analyze quantum nonlocality without entanglement and give two sufficient, not necessary, criteria for process function unitaries. They also relate circuit depth to clique-partitioning problems on Hamming graphs.
In briefElectron symmetry and femtosecond coherence of Renner-Teller-mixed vibronic states in CO2
SummaryThis paper studies vibronic states of carbon dioxide that become interleaved because of non-adiabatic coupling. Using vacuum-ultraviolet transient absorption spectroscopy on aligned molecules, the authors separate two electronically distinct classes of states (A' and A'') arising from Renner-Teller splitting. The parallel A' states are measured to dephase faster than the perpendicular A'' states, which the authors attribute to faster departure of the A' wavepacket from the Franck-Condon region.
In briefOptimal two-mode bosonic codes for photon loss, built from finite group symmetry
SummaryPhoton loss is a major noise source in bosonic quantum hardware such as superconducting cavities. The paper studies qubit encodings in two bosonic modes with a fixed total photon number, numerically optimizing encoders and decoders for 4 to 25 photons and finding that every best-found code has finite-group structure. Building on this, the authors derive necessary-and-sufficient Knill-Laflamme conditions for finite-group-invariant codes and construct analytic codes that correct photon loss. Exact MacWilliams-Farkas certificates show that most of these codes reach the maximum loss distance.
WhyDerives Knill-Laflamme conditions for finite-group-invariant codes and constructs codes up to distance 10 with exact certificates, showing strong novelty and solid evidence.
Argyris Giannisis Manes, Mahadevan Subramanian, and Liang Jiang
In briefQuantum Sphere: A Physically Realizable Benchmark with Provable Linear Convergence
SummaryThis paper revisits an established quantum control objective, characterizes its local metric geometry, and presents it as a rigorous benchmark for search and optimization. It shows that the yield gap can be written exactly as a nonnegative linear combination of sin² terms, yielding a two-sided quadratic sandwich with ratio 4/π² on a worst-case quarter-wave region. The paper proves linear convergence of gradient descent in the white-box setting and of an elitist single-parent evolution strategy in the black-box setting, and numerical simulations are reported to corroborate the theory.
In briefA polynomial-time classical sampler for noisy quantum circuits from statistical mechanics
SummaryThis paper studies whether noisy quantum circuits can be efficiently simulated by classical computers, which bears on the limits of quantum advantage. The authors prove that geometrically local circuits of unital operations with single-qubit depolarizing noise of strength p can be approximately sampled in polynomial time once the depth reaches about p^{-1} log p^{-1}, independent of system size. The proof maps the circuit's output state to a polymer model from statistical mechanics and combines a convergent cluster expansion with hypercontractivity of the depolarizing channel.
WhyExtends polynomial-time classical sampling of noisy circuits to general gates without anticoncentration, via a polymer model, cluster expansion, and hypercontractivity proof.
In briefUnified framework extending exact classical simulation of quantum circuits
SummaryThe paper casts exact classical simulation of quantum circuits as functional aggregate queries (FAQs), where gates become factors and wire variables are aggregated. Choices of evaluation plan, semiring, and intermediate representation determine the cost. The framework recovers the treewidth bound for tensor-network contraction and rank-width-based simulation, and gives polynomial-time exact amplitude computation for Clifford circuits. It also introduces a new tractability criterion, tensor layout symmetry width.
Floris Geerts, Rihan Hai, Matthias Lanzinger et al.
In briefQuantum Krylov Learning: Learning a Hidden Quantum System's Spectrum from Probe Measurements Alone
SummaryThe paper introduces Quantum Krylov Learning (QKL), a framework for extracting spectral information about an unknown quantum system when only controllable probes can be measured. It uses single-temperature probe autocorrelators and their operator Krylov structure to reconstruct a Jacobi matrix describing the dynamical modes the probe can access, without requiring symmetry, a parametric model, or controlled-unitary gates. The method assumes the unobserved system is at or near infinite temperature. The authors also provide a Lieb-Robinson bound for the operator Krylov chain and a sample complexity guarantee showing the number of measurement shots grows polynomially with inverse precision.
Shunji Matsuura, Yoji Kawamura, Joseph Salfi et al.
In briefPolynomial-time test for local-unitary equivalence of graph states
SummaryThis paper addresses whether two graph states differ only by independent single-qubit basis changes, a relation called local-unitary (LU) equivalence. The authors present a deterministic algorithm that decides LU equivalence for graphs on n labelled vertices in Õ(n^6.38) bit operations and constructs exact single-qubit unitaries when the states are equivalent. The method builds on an earlier quasipolynomial algorithm by replacing vertex-subset enumeration with constraints from vertex pairs and triples, solved by linear equations over the binary field.
WhyReplaces a quasipolynomial method with a polynomial-time linear-algebra algorithm settling a decade-old open LU-equivalence question for graph states, with exact constructions and proofs.
In briefGood quantum locally testable codes built from lossless cubical complexes
SummaryThe paper studies how high-dimensional lossless cubical complexes could yield asymptotically good quantum locally testable codes. The authors do not construct the required complexes but examine what their existence would imply. Their main result is a local-to-global theorem linking one-dimensional lossless expansion to coboundary expansion of a level chain complex. Consequently, sufficiently imbalanced, two-sided lossless four-dimensional cubical complexes give asymptotically good quantum locally testable codes.
In briefQuantum simulation of non-local potentials using matrix-product-state encoding
SummaryThis paper proposes a matrix product state (MPS) encoding method to efficiently implement non-local pseudopotentials in first-quantized quantum simulations. The authors give a circuit depth expression for the Trotter step operator and test the approach on a model of one-dimensional hydrogen ionization driven by an ultrashort laser pulse. They report that MPS encoding significantly reduces the required circuit depth compared to direct unitary decomposition.
Kazuki Tsuoka, Taichi Kosugi, Masari Watanabe et al.
In briefGround-state preparation without controlled Hamiltonian evolution
SummaryThis paper studies ground-state preparation in quantum simulation and asks whether controlled Hamiltonian evolution is required. The authors give two algorithms that access the Hamiltonian only through queries to U_H and its inverse, one using fresh copies of the input state and one using coherent access to the state-preparation unitary to improve complexity. The framework, based on a spectral descent principle, is also extended to ground-state property and energy estimation.
WhyShows near-optimal ground-state preparation without controlled evolution, matching the known lower bound up to logarithmic factors; a strong theoretical advance.
Dhrumil Patel, Steven T. Flammia, Raul Garcia-Patron
In briefQuantum query complexity with noisy oracles, lower bounds via fractional block sensitivity
SummaryThe paper studies quantum query complexity when oracle access is imperfect, using fractional block sensitivity (fbs) as a common tool for lower bounds. For a negligent oracle that applies the correct query with probability 1-p, it proves a bound in terms of fbs(f,0^n). It also shows that negligence need not eliminate quantum speedups for partial functions, and gives general fbs(f) bounds for hybrid, IID dephasing, and time-varying dephasing models.
WhyUnified fractional block sensitivity framework yields lower bounds across several noisy oracle models, and shows negligence can preserve exponential quantum speedups.
In briefWalshness: An intrinsic metric for whether neural networks can represent quantum states
SummaryThis paper introduces Walshness, a complexity measure that indicates whether a quantum state admits a compact neural-network representation. The authors prove that, for various neural quantum state architectures, a general many-body state can be represented efficiently if and only if its Walshness is bounded. They also give an algorithm that finds an optimal basis by minimizing the energy of a corresponding classical spin model, which improves learnability for states such as transverse-field Ising and toric code ground states.
WhyProposes Walshness, a provably equivalent complexity metric for neural representability, with a basis-selection algorithm and numerical validation.
Nisarga Paul, Kehuang Chen, Jessica K. Jiang et al.
In briefNeural network post-selection to improve quantum error correction reliability
SummaryThis paper introduces graph neural networks that predict decoder failure from syndromes alone, without extra decoder runs, and use belief-propagation posteriors for high-rate quantum LDPC codes. The method is tested on rotated surface and bivariate bicycle codes under uniform depolarising circuit-level noise. Keeping 90% of shots yields about 3700x and 740x logical error rate reductions for the [[72,12,6]] and [[144,12,12]] BB codes at realistic physical error rates. For the distance-5 surface code at p=0.003, it matches the leading complementary gap method without running the decoder.
In briefQuantum algorithms for OPI variants beyond classical decoding limits
SummaryThe paper studies quantum algorithms for finding codewords under nonlinear constraints by decoding the dual code. It solves a problem requiring all coordinates to be nonzero for codes with a two-fold multiplication property, covering parameter regimes beyond known classical guarantees. It also treats histogram-local constraints on symbol frequencies and proves a quantum-classical separation relative to a classical random oracle.
In briefCausal trade-offs in superluminal relativity: a no-go theorem
SummaryThis paper examines a proposal by Dragan and Ekert to extend relativity with superluminal reference frames, using objective uncertainty to reconcile relativistic symmetry with quantum uncertainty. The authors develop a causal modelling framework and prove a no-go theorem: a theory allowing superluminal reference frames can avoid superluminal signalling only if it severely restricts accessible spacetime regions, if the choice of frame has observable consequences, or if free variables are absent or frame-dependent. The authors conclude that introducing objective uncertainty alone does not remove this conflict.
In briefMixed-state phases from power-law quantum channels
SummaryOpen quantum systems can host mixed-state orders such as strong-to-weak spontaneous symmetry breaking (SW-SSB), which have no pure-state counterpart. The paper introduces a long-range quantum channel that applies two-body dephasing to pairs of sites with probability decaying as a power law in separation, circumventing the expectation that finite-depth local channels cannot induce SW-SSB in one dimension. Applied to a family of matrix product states interpolating between trivial and symmetry-protected topological (SPT) phases, it yields a phase diagram with trivial, mixed SPT, and SW-SSB phases, diagnosed by Rényi-2 measures.
Christopher Fechisin, Tsung-Cheng Lu, Zhi-Yuan Wei et al.
In briefAlgebraic structure in holographic tensor networks: testing how regions in a tensor-network model map to subalgebras of observables
SummaryThis paper follows up earlier work on associating subalgebras of observables with generalized entanglement wedges in quantum gravity, using tensor network toy models of holography. In the large bond dimension limit, each such region is argued to have a canonical subalgebra, and the paper studies how algebraic operations such as intersections, joins and commutants correspond to geometric operations. It also examines monotonicity and strong subadditivity of the associated entropies, introduces an alternative entropic quantity, and constructs a type II_1 von Neumann algebra from random tensor networks.
Abhisek Sahu, Jeremy van der Heijden, Mark Van Raamsdonk et al.
In briefSlow algebraic decay of strong-to-weak symmetry-breaking order from charge-sector broadening
SummaryThis paper studies how strong-to-weak symmetry-breaking order in mixed quantum states decays when local charge exchange with the environment breaks strong symmetry. For discrete Z2 symmetry, a local perturbation causes exponential decay of the Rényi-1 long-range correlator. For continuous U(1) symmetry, charge exchange broadens the charge-sector distribution, giving much slower algebraic relaxation. The authors derive an algebraic lower bound on the connected correlation and a two-stage scaling in the thermodynamic limit, and numerical results on fermionic chains agree with the predictions.
In briefFactor Code Networks: a study of when quantum error-correcting code maps can be made consistent across multi-level networks
SummaryThis paper studies factor codes, which are quantum codes paired with unital operator maps that preserve logical operators, and how they combine into code networks. It gives conditions under which the operator maps on a network can be chosen to be mutually consistent. The authors show that Hilbert space dimension divisibility suffices for tree-shaped networks but not in general, and give an exact condition for diamond-shaped networks.
Abhisek Sahu, Jeremy van der Heijden, Mark Van Raamsdonk et al.
In briefQuantum information techniques applied to high-energy physics
SummaryThis conference overview, selected for a Frontiers of Science Award, explains how quantum information techniques can be applied to high-energy physics, with some new insights. It uses the top quark as a case study and examines entanglement in top-antitop pair production at colliders such as the LHC. The authors also describe a quantum tomography protocol for this quark pair, which contributed to ATLAS and CMS observations of entanglement.
In briefA Clifford-hierarchy stabilizer framework for twisted quantum double codes
SummaryThe paper introduces the XC^(k)_d stabilizer formalism, which combines Pauli-X operators with diagonal operators from level k of the Clifford hierarchy on prime-dimensional qudits, extending beyond Pauli stabilizer models. The authors classify many such states as uniform superpositions over affine-linear subspaces with diagonal phases. They show that twisted quantum doubles built from products of type-III cocycles can be realized with only Clifford stabilizers, and conjecture that other cases need higher-level stabilizers. They also show that two qudit toric codes can be deterministically prepared in a logical magic state.
Christopher Fechisin, Kieran Cooney, Mathi Raja et al.
In briefTesting whether a quantum channel is non-mixed-unitary using repeated uses
SummaryThe paper studies how to decide, from multiple uses of an unknown quantum channel, whether it is a specified non-mixed-unitary channel or belongs to the mixed-unitary set. It formulates this as a composite channel hypothesis test and characterizes the achievable Stein exponents for parallel strategies under different restrictions on probe states and auxiliary memory. The authors derive finite-letter expressions for block-i.i.d. probes and establish several strict separations within the resulting hierarchy.
In briefEvery quantum unitary can be built with constant-depth circuits
SummaryAny n-qubit unitary is known to be implementable with a circuit of depth 2^{O(n)} using one- and two-qubit gates, but whether exponential depth is necessary was open. This paper shows that, using 2^{O(n)} ancilla qubits, every unitary can be implemented exactly by a circuit of one- and two-qubit gates with polynomial depth. If unbounded fan-out gates are allowed, the depth can be reduced to a constant.
WhyShows all n-qubit unitaries admit poly-depth circuits, exponentially improving prior bounds via a novel link to locally-decodable codes.
In briefA new quantum method for computing equilibrium correlation functions more efficiently
SummaryMany measurable dynamic quantities of physical systems are expressed as equilibrium correlation functions. The authors store time as a quantum variable using O(log N_t) ancilla qubits, so all time points are computed with a single quantum circuit. This circuit runs O(log N_t) times, giving total resources of O(N_t log N_t). The paper proves this scaling is optimal and demonstrates it on the Green's function of a one-dimensional spinless Hubbard model.
Yizhi Shen, Roel Van Beeumen, Wibe A. de Jong et al.
In briefWhen multiterminal Josephson currents reduce to two-terminal couplings, resonances set the error
SummaryThis paper studies whether currents in multiterminal Josephson junctions, where three or more superconductors share one weak link, can be approximated by pairwise two-terminal couplings. Using a microscopic scattering model, the authors compare exact currents with the best pairwise approximation and find the error is driven by a normal-region mode near the Fermi level that couples to three or more terminals. Detuning this mode with a gate suppresses the error.
