arXiv Quantum Physics (quant-ph).
In briefQuantum n-coloring is undecidable for every n ≥ 3
SummaryThis paper studies the quantum n-coloring problem, which asks whether a graph G has a perfect quantum strategy for its n-coloring game. Previously, undecidability was known only for n=3. The authors extend this to all n≥3 using one elementary reduction. They define G'=(G□K3)∨K(n−3) and show that G is quantum 3-colorable if and only if G' is quantum n-colorable.
In briefOrigin and classification of fractional dynamical topology in non-Hermitian quantum systems
SummaryThis paper studies dynamical quantum phase transitions (DQPT) in non-Hermitian quantum systems. In a two-band non-Hermitian model, the authors show that half-integer jumps of the dynamical topological order parameter originate from discontinuities in the complex energy spectrum. This behavior is linked to the steady-state phase diagram of the quench Hamiltonian, and the occurrence of DQPT is classified. The analysis is extended to multiband and two-dimensional models.
In briefA numerical, efficient model for thermo-field electron emission calculations (GETELEC-3)
SummaryThis paper presents GETELEC-3, an open-source code that replaces analytical thermo-field emission formulas, such as Fowler-Nordheim and Murphy-Good with their JWKB extensions, with a direct numerical solution of the one-dimensional time-independent Schrödinger equation. The Numerov algorithm was found to be the fastest method, with solution uncertainty below 1%. A neural network trained on the residual of the semiclassical reference reproduces the current density to better than 0.4% at lower computational cost. The code targets metals and semiconductors and accepts arbitrary potentials and densities of states.
In briefComplete classification of reversible entanglement conversions between two-qutrit states
SummaryThis paper studies when two parties, Alice and Bob, can convert two-qutrit mixed states into one another and back using local operations and classical communication (LOCC). It gives a complete classification of such reversible conversions. For mutually convertible entangled states that are not locally unitarily equivalent, it proves a decomposition into a separable part and an entangled block is necessary. Two entangled states are reversibly convertible if and only if their unnormalized smallest retained blocks are locally unitarily equivalent.
WhyGives a complete classification of reversible two-qutrit LOCC conversions with a minimal retained block; strong theory, but impact is confined to a narrow quantum information niche.
In briefExact unitary synthesis with optimal circuit size and depth-ancilla tradeoff
SummaryThe paper studies how to exactly decompose an arbitrary n-qubit unitary into elementary quantum gates. For any ancilla budget m, the authors build a circuit of O(4^n) CNOT and single-qubit gates arranged in O(n+4^n/(n+m)) layers. This matches the asymptotically optimal depth-ancilla tradeoff and optimal circuit size at once. The method encodes basis states as wavefunctions with a shared Gaussian envelope, discretizes the result, and removes approximation errors through exact residual correction.
WhyEncodes basis states as Gaussian wavefunctions to make unitary action shallow, achieving optimal size and depth-ancilla tradeoff with exact synthesis, a substantial theoretical improvement.
In briefEfficient Fidelity Estimation of Quantum Resource States Using Only Clifford Circuits
SummaryThis paper introduces Bell-coherence fidelity estimation (BCFE), a method for estimating the fidelity of quantum resource states using only Clifford gates and Pauli measurements. The authors claim it achieves optimal sample complexity of Θ(1/ε) and applies to arbitrary pure targets, including multi-qubit states. In simulations of the STAR early fault-tolerant architecture using surface codes, BCFE is estimated to need up to about 27 times fewer resource states than direct fidelity estimation for the same standard error.
WhyAchieves optimal Θ(1/ε) sample complexity using only Clifford gates and Pauli measurements, with surface-code simulations showing about 27x fewer resource states than DFE.
In briefCups and Gates II: Operads, twists, and addressable intra-code gates
SummaryThis paper extends the authors' earlier framework for building logical gates in quantum CSS codes from cohomology invariants, covering topological and qLDPC codes. It introduces twists using code automorphisms, higher twisted cohomology operations, and folding in relative cohomology, enabling diagonal gates such as single-qubit rotations and addressable T gates. It also defines new operations, such as higher twisted Pontryagin powers, constructed from E∞-structures on cochains using cyclic resolutions and the surjection operad.
In briefTheoretical Explanation of the Edge-of-Chaos Advantage in Quantum Reservoir Computing
SummaryNumerical studies indicate that quantum reservoir computing performs best near the edge of chaos. This paper builds an explicit quantum reservoir model and analyzes its chaotic regime and integrable point. In the chaotic regime, memory decays rapidly, which limits learning of long-range temporal dependencies. At the integrable point, a linear readout cannot learn nonlinear tasks coupling present and past inputs, and introducing interactions alleviates this limitation.
WhyRigorously proves exponential memory decay in chaotic regime and learning failure at integrable point, offering a new theoretical explanation of the edge-of-chaos advantage.
In briefRadio-frequency electron cascade in semiconductor quantum dots
SummaryThis paper studies radio-frequency (rf)-driven correlated cotunneling in semiconductor quantum dots, where electrons move simultaneously across multiple tunnel junctions via virtual intermediate states. Using theory and experiments on two foundry-fabricated silicon devices, the authors show that an rf drive induces correlated cyclic single-electron transitions that enhance charge polarization. They present a general framework for rf electron-cascade readout using three or more dots and demonstrate it in a gate-sensed architecture, aiming to improve dispersive readout fidelity.
In briefUniversal convergence of standard quantum electrodynamics to the modified Langevin noise formalism
SummaryThe paper examines whether the standard Langevin noise formalism (LNF), which adds fictitious vacuum absorption that vanishes in a limit, reproduces every prediction of the modified Langevin noise formalism (MLNF), including quantum scattering processes. The authors define a mapping that links each MLNF state and observable to a counterpart in the LNF with fictitious absorption. They prove that as this absorption tends to zero, every LNF matrix element converges exactly to its MLNF original.
WhyRigorously proves LNF with vanishing fictitious absorption converges exactly to MLNF predictions, including quantum scattering, advancing the theoretical foundation.
In briefError-transparent switching between two permutation-invariant quantum codes
SummaryThe paper studies moving an encoded qubit between two permutation-invariant codes on the same qubits, one supporting transversal Clifford gates and the other transversal gates from the binary icosahedral group. The authors show that distance-three codes with the required properties are cut out by at most two quadratic equations, and that a single-fault-tolerant switch exists exactly when the codes are joined by a continuous path of such codes. On thirteen qubits they construct the switching unitary along an explicit circle and numerically verify machine-precision recovery infidelity per fault.
In briefCompleteness of Quantum Mechanics: Sources, Records, and Probabilities
SummaryThe paper asks whether quantum mechanics can be complete without being fundamental. Under stated source, detector, and statistical conditions, the authors reconstruct the quantum representation and Born probabilities and prove algebraic closure of the admitted predictions without a collapse postulate. They argue that many familiar quantum paradoxes arise from conflating coherent processes with retained records, or probabilities with realized outcomes.
In briefDiffusive spectral gap of the qubit eSWAP chain
SummaryThe paper proves that the second-moment Hamiltonian of the open qubit eSWAP chain has a diffusive spectral gap, resolving a conjecture by Li, Zheng, Liu, Jiang, and Liu. It characterises the ground space on connected interaction graphs with at least five vertices and gives a uniform lower bound for the unnormalised complete-graph gap. These results are applied to sequential and brickwork circuits, and the periodic chain is shown to also have a diffusive spectral gap.
WhyProves a diffusive spectral gap for the open qubit eSWAP chain's second-moment Hamiltonian, resolving a conjecture with rigorous mathematical argument.
In briefTraining variational quantum algorithms is NP-hard, even just to find local minima
SummaryThis paper studies the computational difficulty of training variational quantum algorithms, which tune parameterized quantum circuits with classical optimization. The authors prove that finding a local minimum of these training objectives is strongly NP-hard, and that finding a parameter vector within ℓp-distance less than π/2 of a local minimizer is also hard for every p≥1. The key step is showing that approximating a local minimizer of a Hermitian trigonometric polynomial is strongly NP-hard, which they reduce to VQA training by constructing suitable quantum circuits.
WhyProves finding local minima of variational quantum training objectives is strongly NP-hard via reduction from trigonometric polynomials, a significant theoretical barrier result.
In briefHow much non-Gaussian resource fermionic randomness needs for quantum sampling
SummaryThis paper asks how much non-Gaussianity is needed for sampling-based quantum advantage to reproduce random-matrix statistics linked to quantum chaos. It shows that passive fermionic linear optics, even with Haar-random mode mixing, fails to anticoncentrate for Slater inputs. Overcoming this requires an input resource that scales linearly with system size, and the paper relates this to sampling statistics, preparation cost, and an experimentally accessible witness quantity.
WhyProves fermionic randomness needs extensive non-Gaussian resources, giving exact moment hierarchy and a measurable witness; strong theory with impact mainly within a subfield.
In briefJAQSI: A Differentiable Quantum Simulator Unifying Gate- and Pulse-Level Simulation
SummaryThe paper presents JAQSI, a quantum simulator built on JAX that combines gate-level and pulse-level simulation in one differentiable circuit model. It allows matrix-defined operations, gates synthesised from Hamiltonian evolution, and noise channels to be composed freely. In benchmarks against PennyLane, Qiskit, Qibo, Qulacs, QuTiP and dynamiqs, the authors report performance on par with or up to an order of magnitude faster at the gate level, and at least 5 times faster in pulse-level benchmarks. The software is released as open source on GitHub and PyPI.
In briefA two-part quantum-classical universe in which ħ emerges from classical time
SummaryThe paper reinterprets quantum dynamics within the standard von Neumann formalism, proposing a logical split between a classical observing environment governed by Boolean logic and a quantum observed system governed by non-Boolean logic. It models an ideal classical clock as an open system with uniformly discretized energy levels, described by the GKSL master equation. The author derives the reduced Planck constant ħ as a scaling parameter from the clock's action operator and claims an exact mapping to 't Hooft's cellular automaton formalism.
In briefPhase-controlled recoverable entanglement in accelerated detector registers
SummaryThis paper studies recoverable entanglement in a four-qubit cluster-state register whose two detector qubits couple to a scalar field. A local preparation phase controls protection under collective noise, and at the optimal phase a fixed decoder recovers a Bell pair after channels preserving singlet and triplet sectors. The authors derive an exact trace-norm formula for uniformly accelerated detectors and the decay rate at small separation.
In briefQuantum Fisher-Selke sequence in a transverse-field Ising model and its bilayer
SummaryThe paper studies a transverse-field Ising model with next-nearest-neighbor coupling on a square lattice and a bilayer. Perturbation theory in the transverse field, carried to eighth order, shows that quantum fluctuations select a three-up-three-down phase first, followed by further phases in the order of the classical Fisher-Selke sequence. Quantum Monte Carlo simulations of the two widest phases largely support these results.
In briefSimulating non-Markovian open quantum systems with large uncoupled subspaces
SummaryThis paper develops process tensor methods in matrix product operator form to compute the time evolution and multi-time correlations of open quantum systems. It shows these methods can be used efficiently when only some system states couple to the environment, and combining them with a Taylor expansion reduces numerical cost to about O(d^3). The authors report simulations of systems with several hundred states, relevant to multimode solid-state cavity-QED systems.
In briefArbitrary-phase CZ gate and remote readout between separated superconducting nodes
SummaryThe paper demonstrates an entangling gate between two superconducting quantum nodes connected by a 30-meter cryogenic quantum link. By tuning the frequency of communication photons, the conditional phase acquired on reflection is adjusted, implementing an arbitrary-phase CZ gate. This operation is then used to read out a qubit in the remote node from the other node in a single shot, without active participation by the remote node.
In briefTime reversal's role in Hamiltonian property testing: a paper showing time reversal is needed to close the gap in evolution-time costs for quantum measurement tasks.
SummaryThe paper studies total evolution-time lower bounds in Hamiltonian property testing. Without time reversal, the best known algorithms need O(1/ε²) total evolution time, while many quantum tasks reach O(1/ε). The authors prove time reversal is necessary to close this gap, giving lower bounds for Hamiltonian certification and locality testing, plus matching upper bounds for certification.
WhyProves time reversal is necessary for Hamiltonian testing, with a new Fourier-analytic lower bound method matched by upper bounds.
In briefBlind remote implementation of any two-qubit unitary using shared entanglement
SummaryThis paper proposes a protocol that uses pre-shared entanglement to remotely implement an arbitrary two-qubit unitary operation. Bob keeps his unknown two-qubit state in his lab, while Alice applies the operations using calibrated black-box devices without knowing their parameters. The method builds elementary single- and two-qubit operations from Bell states, based on Euler and Cartan decompositions, and combines them. A third party, Charlie, determines the decomposition and supplies the devices beforehand, so neither Alice nor Bob needs a classical description of the unitary.
In briefTighter quantum bounds for estimating the volume of convex bodies
SummaryThis paper studies how many quantum membership-oracle queries are needed to estimate the volume of a high-dimensional convex body. The authors show that relative error ε can be achieved with Õ(d^{5/2}+d^{3/2}/ε) queries, improving the previous quantum upper bound. They also prove an Ω(d) quantum query lower bound for constant relative error, improving the earlier Ω(√d) bound.
WhyImproves quantum upper and lower bounds for convex-body volume estimation via effective spectral-gap analysis and bridge sampling, with rigorous proofs of better polynomial dependence.
In briefHamiltonian Metric Learning: A Dissipative Geometric Framework for Optimization
SummaryThe paper treats machine learning parameter optimization as a dissipative dynamical system, with the loss acting as potential energy and a positive-definite metric defining parameter-space kinetic geometry. It derives Hamiltonian dynamics, a geometric force from a position-dependent metric, and a metric-compatible Rayleigh dissipation law, illustrated on a CIFAR-10 image-reconstruction problem with an analytically known optimum. On a five-seed MNIST MLP benchmark, the proposed HAMLET method achieves the highest mean test accuracy (97.95%) and lowest mean test negative log-likelihood (0.0696) among three evaluated optimizers.
In briefTesting Quantum Control Circuits Simultaneously with Finite Reversible Models
SummaryThe paper studies whether one finite reversible model can reproduce the probabilities of a whole collection of quantum circuits at once, using a shared transition table. It gives an exact feasibility formulation and a 464-frame benchmark showing a sharp repeated-circuit horizon: one controller stays within error 1/3 through 192 blocks, while every allowed controller violates it by block 193. The conclusions apply only to the stated finite reversible model classes and do not establish a universal memory bound or a discrepancy for physical hardware.
In briefGap amplification for local Hamiltonians that preserves locality
SummaryThe quantum PCP conjecture is a major open problem, and classical gap amplification methods lack obvious quantum counterparts. This paper presents the first viable template for locality-preserving gap amplification in the quantum setting. The authors prove that their procedure amplifies the combinatorial gap of local Hamiltonians without increasing locality, instead moderately increasing local qudit dimension.
WhyProposes the first viable template for quantum locality-preserving gap amplification with a proof of combinatorial gap growth; novelty is high, but evidence details are sparse.
In briefPairwise detection and chiral conversion: a resource theory of quantum multistates
SummaryThe paper studies quantum multistates, whose basis-independent coherence depends on whether their states commute, and shows that for qubits this is detected by pairwise overlaps. It gives an exact resource-conversion theory under commutativity-preserving operations. For qubits, a necessary and sufficient criterion for exact conversion is derived, showing that targets with identical pairwise geometry can differ in convertibility by the sign of a cubic invariant.
In briefQuantum optimization with custom penalties using Walsh-Hadamard transform
SummaryThe paper proposes a method for handling inequality constraints in QAOA without slack variables, by applying a nonlinear custom penalty directly to the constraint. It uses the Walsh-Hadamard transform to express the penalty as a weighted sum of Pauli-Z strings, which can be truncated at degree k without increasing the qubit count. The authors analyze the optimality of the truncation and test the approach on a 6-qubit knapsack problem.
In briefLong-lived coherent phonons in α-In₂Se₃ reveal competing thermal and many-body dynamics
SummaryThis study uses broadband transient-grating spectroscopy with ~5 fs laser pulses to examine lattice coherence in ferroelectric α-In₂Se₃ after photoexcitation. The dominant ~104 cm⁻¹ oscillation persists for several picoseconds, and its frequency stays nearly constant from 10 to 300 K while its coherence is progressively suppressed. First-principles calculations assign it to a 101.47 cm⁻¹ Γ-point optical phonon, and quantum-dynamical simulations reproduce the main oscillations.
In briefQuantum codes in the Lee metric: a framework for correcting small-shift errors
SummaryThe paper introduces a quantum coding framework for discrete small-shift noise, modeling qudit errors as low-weight X- and Z-type Pauli shifts. It develops stabilizer codes over Z_q using joint and separate Lee metrics. Results include qubit codes via the Gray map, a bound on Lee distance for CSS Lee-LDPC codes, and memory times under local Metropolis dynamics.
In briefEntanglement-of-formation bounds for feedback-assisted communication and randomness extraction
SummaryThe paper develops entanglement-of-formation potentials for classical communication with feedback and one-way randomness distillation. It proves an exact amortization theorem that yields a classical-feedback capacity bound. The authors evaluate the bound for the amplitude damping channel and show that a two-round two-way protocol can exceed the one-way rate for qubit isotropic states.
In briefSelf-shunted superconducting flux qubit with high anharmonicity using epitaxial NbN junctions
SummaryThis paper demonstrates a self-shunted superconducting flux qubit built from fully epitaxial NbN/AlN/NbN Josephson junctions. It reaches an anharmonicity of 797 MHz and a much smaller footprint than capacitively shunted flux qubits. Its lifetime of 13 to 25 microseconds is comparable to or higher than NbN and Al-based comparison qubits.
In briefSidewall-spacer-passivated epitaxial NbN/AlN/NbN Josephson junctions for superconducting qubits
SummaryEpitaxial NbN/AlN/NbN Josephson junctions are considered alternatives to Al/AlOx/Al junctions because of their crystalline barriers and NbN's high transition temperature. This paper presents a sidewall spacer passivation process that reduces amorphous dielectric use and fabrication complexity. The junctions showed a gap voltage of about 5.6 mV and low subgap leakage, and varying the AlN barrier thickness yielded high-quality junctions over critical current densities from 0.3 to 10^4 A/cm². Buffered hydrofluoric acid treatment did not degrade the junctions' electrical characteristics.
In briefCompeting reaction pathways in polaritonic chemistry under strong light-matter coupling
SummaryThis paper studies how strong coupling between molecules and cavity modes can change which chemical reaction pathways are favored. The authors use a tensor-network formulation of the hierarchical equations of motion method to compute probability currents between vibrational states. They find that local cavity-induced changes can shift kinetic bottlenecks and redistribute reaction flux between competing pathways.
In briefActive lattice percolation: a possibly new universality class of percolation transitions
SummaryThe paper studies dynamic lattice percolation models where connections are added and removed by local stochastic rules, forbidding removals that split a cluster. Numerical results show the transition is a hybrid percolation transition, combining diverging critical clusters with first-order-like discontinuities. Adding isotropic local moves removes the absorbing state and suppresses the discontinuity, leaving an apparently continuous transition. Its critical exponents differ significantly from known percolation transitions, suggesting a new universality class.
In briefWhether an instrument keeps its joint records consistent across repeated readouts and composition
SummaryThe paper studies whether a measuring apparatus preserves the joint law of its accessible records under repeated readout, network composition, or reuse, beyond what marginal no-signalling constraints guarantee. It uses a conditional-record criterion and four explicit models to derive results on continuity, angular bandwidth, and finite-resource obstructions. Numerical studies with 8.3 million simulated histories support the model-specific predictions.
In briefQuantum advantage in communication that holds regardless of network layout
SummaryThis paper studies quantum advantage in communication networks that does not depend on how the network is arranged. The authors define topology-independent quantum advantage, where a quantum protocol beats every classical protocol under the same communication limit, even when parties share unlimited classical randomness. They illustrate this with a three-party equality task and fully characterize the corresponding classical correlation polytope. The framework is also extended to four-party networks.
WhyIntroduces topology-independent quantum advantage and fully characterizes the three-party classical correlation polytope, with rigorous theory but no experimental validation.
In briefCompiling matrix-model dynamics into quantum circuits via sum-of-squares factorisation
SummaryThe paper studies compiling the SU(N) matrix-model potential, the interaction in the BFSS/BMN family, into quantum circuits. The authors show that for a generic Trotter step, a Walsh-support lower bound limits the gate count within a given gate class, so improvement requires a different gate type. By factorising the potential into a sum of squared commutator entries, they build circuits for NISQ hardware and for fault-tolerant hardware, and verify both. Reported results show fewer gates than the baseline for most tested sizes.
WhyFactorising the potential into squared commutators yields a non-phase-polynomial gate beating the phase-polynomial lower bound, with gate-count gains on NISQ and fault-tolerant hardware.
In briefMajorana bilinears and how close Bell violations can reach the Tsirelson bound in the free field vacuum
SummaryThe paper studies the Bell-CHSH inequality in the vacuum of a free massive Majorana field in 1+1 dimensions, using even bilinears of smeared fields in the right and left Rindler wedges as dichotomic observables. Test functions are built from seeds in modular-frequency space via the Tomita-Takesaki operator. The authors give a closed-form Bell parameter and show the Tsirelson bound is never attained in the wedge vacuum, while approaching it requires modular spectral weight to concentrate at λ=1.
In briefStabilizer codes on general phase spaces: quantum error-correcting codes mixing discrete and continuous degrees of freedom
SummaryThe paper develops a theory of stabilizer codes whose stabilizer groups contain commuting qudit Paulis, oscillator displacements, and planar-rotor displacements. It constructs oscillator-rotor, rotor-qudit, and oscillator-rotor-qubit codes that cannot be split into separate subsystems by generalized Clifford transformations, and an oscillator-qubit code from a partially lifted Golay code. The authors also derive analytical forms of logical operators, distances, decoding, and weight enumerators.
In briefQuantum-State Oblivious Transfer over Quantum Erasure Channels: a study of a protocol that lets a receiver obtain one of two quantum states while learning nothing about the other
SummaryThis paper studies quantum-state oblivious transfer over the quantum erasure channel with information-theoretic security. Alice holds two quantum systems and Bob wants to recover one of them, using independent channel uses and authenticated public classical communication, without initial shared entanglement. Both honest-but-curious and malicious behavior are considered. The authors give finite-blocklength converse and achievability bounds that match in the honest-but-curious setting, and derive fixed-error asymptotic expansions. They show that the malicious and honest-but-curious settings have the same capacity.
In briefCorrelations determine a shallow-circuit quantum advantage
SummaryThe paper examines whether quantum computers can produce samples that ordinary classical computers cannot reproduce. It proposes a test that certifies such a claim from classical samples alone, showing the test depends on the pattern of single-bit and pair correlations in the classical class. A collapse theorem is machine-checked in Lean 4, and two open problems are left unresolved.
WhyProposes a new criterion where single-bit and pair correlations decide verifiability of shallow-circuit advantage, with Lean 4 machine-checked key theorem; strong evidence, impact limited to quantum a
In briefLower bounds on the average two-way communication needed to simulate quantum correlations
SummaryThe paper studies how much classical communication is needed to reproduce Bell nonlocal correlations exactly, in the average-cost setting. It derives lower bounds on input-average communication cost from nonlocal games and applies them to parallel games to obtain quantum correlations whose exact simulation cost exceeds any finite budget. For n parallel Magic-Square-winning correlations it proves a bound of n·log₂(3/2) bits and gives a one-way protocol that approaches this rate asymptotically.
In briefNeural correlation learning for quantum-enhanced sensing with time-independently driven Rydberg atom arrays
SummaryThis paper shows that evolving a simple product state under time-independent Hamiltonian dynamics in Rydberg atom arrays creates many-body correlations that could enable quantum-enhanced parameter estimation approaching the Heisenberg limit. The authors propose a neural correlation learning framework combining Bayesian inference with neural networks trained on calibration data, used in a two-stage calibration-and-sensing protocol. Numerical results indicate it saturates the Cramér–Rao bound set by the classical Fisher information, surpassing the standard quantum limit, and remains robust to realistic noise.
In briefExact optimal shadow-measurement circuits for Pauli correlations under two-sided random Clifford layers
SummaryThe paper studies estimating full-support Pauli string correlations on a k-qubit region using classical shadows with random single-qubit Clifford layers on both sides of a controllable unitary, followed by computational-basis readout. It proves that a circuit of two collective Pauli rotations achieves the exact minimum squared shadow norm over all Clifford choices for every k≥1, giving closed forms for odd and even k. For 1≤k≤6, exact rational certificates show the same optimum holds over the full unitary group, and the authors conjecture this optimality for arbitrary k.
WhyProves exact Clifford optimality and an improved prefactor for two-sided randomized shadows, with rigorous certificates but limited to this specific architecture.
In briefNew graph- and matroid-based ways to build classical and quantum error-correcting codes
SummaryThe paper proposes a method for building linear codes from book graphs, using the 2-sum of two equal book graphs to obtain binary codes with the largest possible minimum distance for their length and dimension. It combines matroid and graph theory to study properties of these codes. The authors also show that the incidence matrix of any graph yields a linear code over any finite field with the same parameters. Finally, they translate the condition for two linear codes to form a CSS quantum code into matroid terms and use cycle graphs to generate codes satisfying it.
In briefMulti-Yoked Surface Codes: Denser Quantum Error Correction via Multi-Level Concatenation
SummaryThe surface code is a leading fault-tolerant quantum computing method, but its low encoding rate needs about a thousand physical qubits per logical qubit at the teraquop level. This paper extends yoked surface codes to multiple concatenation levels, introducing chain outer codes and a hierarchical patch allocation. A three-yoked surface code built with a [[256,166,8]] outer code, evaluated numerically under the SI1000 noise model, is about 4 to 4.5 times denser than the unyoked surface code in the teraquop regime.
In briefEfficient Quantum State Identity Testing: Checking Whether Unknown Quantum States Are Nearly Identical or Nearly Orthogonal
SummaryThe paper studies whether a set of unknown pure quantum states are all nearly identical (fidelity at least 1-ε) or contain some pair that is nearly orthogonal (fidelity at most ε). It presents two quantum algorithms: a constant-depth method using parallel swap tests, and a method using the Schur transform that needs fewer copies of each state. The authors also prove a matching lower bound on sample complexity for the second approach.
In briefVariational quantum attention for learning molecular graphs
SummaryThe paper proposes an edge-aware variational quantum attention mechanism for molecular graph learning, where the receiving atom, neighboring atom, and bond jointly determine the quantum attention state. Across five molecular property and bioactivity tasks, QGAT performs competitively with GATv2 and consistently improves BBBP across all six circuit ansatzes. In a BACE1 inhibitor case study, QGAT shows higher Spearman correlation, but broader validation is needed.
In briefDecoy-state quantum information processing using coherent light
SummarySingle-photon sources and photon-number-resolving detectors are hard to build efficiently, so this paper simulates them using phase-randomized coherent states, threshold detectors, and decoy-state post-processing. The authors propose a linear regression method that predicts Fock-state statistics from coherent-state probe observations, combined with random or structured probe sampling. They report that it is more accurate and needs fewer probe settings than traditional linear-programming and grid-sampling approaches, and that it also applies to non-unitary channels.
