October 5, 2026

Single-Shot Error Correction at Optimal Spacetime CostTier 3

Recent work established that, for independent erasures, storing K logical qubits for S time steps with error at most ε requires a spacetime cost of Ω(S(K+log(S/ε))). A matching construction was also given, but assumes ideal error correction. Here we show that the same scaling can be achieved with an explicit noisy error-correction circuit and eff...

Low-Overhead Quantum Error Correction with Boundary-Connected Planar ModulesTier 3

Realizing practical quantum computers requires quantum error correction, but the most widely implemented approach, the planar surface code, demands a substantial physical qubit overhead. Here, we demonstrate that partitioning quantum processors into manageable, flat modules with non-local inter-module connections provides a natural solution. By wiring sparse, static boundary...

The QICK Box: A Modular RF Front-End System for Quantum Control and ReadoutTier 3

The QICK (Quantum Instrumentation Control Kit), first introduced in 2022, is a standalone open source control system for quantum information science. Since then, QICK has been migrated to new hardware platforms, expanded its functionality for applications with different types of qubits (superconducting, atomic, ion-trapped, and spin defects), as well as quantum sensors, and...

Efficient fidelity simulation of high-rate magic distillation circuitsTier 3

Non-Clifford gates are essential for avoiding classical simulability under the Gottesman-Knill theorem. An error-corrected circuit with non-Clifford gates will therefore be an important element for realizing useful fault-tolerant quantum algorithms. However, accurately understanding the fault-tolerant performance of such circuits faces challenges due to the classical hardnes...

Fault-tolerant and fully addressable unitary logical gates via round-robin sparsificationTier 3

For quantum low-density parity check (qLDPC) codes, the ability to manipulate individual logical units of information, i.e. the use of addressable gates, enables maximal utility of their efficient encoding. We introduce explicit low-depth unitary circuits that realize fully addressable and fault-tolerant two-qubit Clifford gates for a subset of two-dimensional translationall...

SpiderCSS: Scalable Fault-Tolerant CSS State PreparationTier 3

Fault-tolerant preparation of logical states is a critical primitive for the realization of large-scale fault-tolerant quantum computing. This paper introduces SpiderCSS, a scalable compilation pipeline that generates highly-optimized, fault-tolerant state preparation circuits for arbitrary Calderbank-Shor-Steane (CSS) codes. Starting from an idealized specification of a CSS...

Efficient Block Encoding of Structured Hamiltonians by Separating Where and WhatTier 3

Fault-tolerant block encodings of structured Hamiltonians can reduce their non-Clifford cost in SELECT by selecting where an operator acts separately from what is applied. We construct permute-act-unpermute circuits whose CSWAP networks exploit the support geometry: a selected support is brought to a fixed target register, a shared local circuit acts there, and the permutati...

Symmetry-preserving quantum compilationTier 3

Fault-tolerant quantum simulation requires compiling symmetry-preserving time evolutions into discrete gate sets. Standard Clifford+T synthesis can break continuous symmetries even when the target evolution preserves them, causing compiled simulations to lose essential features of the original system. We develop a fault-tolerant compilation framework that preserves SU(2) sym...

How I Learned to Stop Worrying and Love the Redfield Equation: Completely-Positive Resummation of Non-Markovian DynamicsTier 3

The Redfield equation with time-dependent coefficients (or TCL2 equation) gives one of the simplest perturbative descriptions of open quantum system dynamics derived from a microscopic theory. Its status as an approximation is nevertheless subtle: while perturbation theory fixes the reduced dynamics order-by-order in the system-environment coupling, solving the Redfield equa...

Rapid mixing of Gibbs samplers via quantum Dobrushin--Shlosman conditionsTier 3

Classical Dobrushin--Shlosman theory uses heat-bath block updates to identify sharp temperature thresholds for establishing the rapid mixing of Gibbs samplers. In particular, these mixing guarantees often hold even when stricter single-site conditions fail. We extend this approach to noncommuting quantum lattice systems through a quantum Dobrushin--Shlosman condition for fin...

Quantum simulation of field-tunable spin spectroscopy of the quantum magnet Cs2CoCl4 on a trapped-ion quantum computerTier 3

The collective excitation spectra of quantum magnets provide a direct test of quantum computers as tools for studying strongly correlated matter. Hardware noise limits the circuit depths available for these calculations, leaving the question of how much magnetic information can survive circuit compression. Using IonQ's 36 qubit Forte Enterprise processor, we simulate the mag...

QNu Labs, BISAG-N and IIT Gandhinagar Demonstrate 5 km-Scale Free-Space QKD LinkTier 2

Insider Brief PRESS RELEASE — QNu Labs (www.qnulabs.com), a global leader in end-to-end hybrid quantum cybersecurity solutions, along with the Bhaskaracharya National Institute for Space Applications and Geo-informatics (BISAG-N), and IIT Gandhinagar, have successfully completed a 5.56 km free-space Quantum Key Distribution (QKD) field trial, marking India’s first free...

Three-Dimensional Light Fields Could Help to Control ElectronsTier 2

Insider Brief PRESS RELEASE — By superimposing two ultrashort laser pulses that converge from different directions, a team of physicists at the University of Oldenburg in Germany has succeeded in generating three-dimensional light fields. Using the same method, they were also able to excite electrons into quantum states that had been previously inaccessible in experime...

Scalable Passive QRAMTier 3

We present a blueprint for implementing the physical QRAM operation in a scalable, passive manner, proving its feasibility in principle. Our construction is passive because for an n-qubit address over N = 2^n memory bits the energy cost per QRAM query is O(log N), and to our knowledge it is the first construction to use polylog N energy. The runtime f...

On The Complexity of Redundancy-Free Quantum HamiltoniansTier 3

We consider a class of redudancy-free Hamiltonians, which are those where the trace vanishes for any product of Hamiltonian terms in which at least one term appears an odd number of times. This includes Hamiltonians which are sums of products of Paulis with no relations between them. These Hamiltonians arise naturally in Hamiltonian Decoded Quantum Interferometry, which sepa...

Maximum-Entropy Extension of Quantum Correlation Functions from Short Real-Time DynamicsTier 3

Correlation functions are central to understanding the behavior of quantum systems, with their spectra giving direct access to quasiparticle and collective excitations. However, the long-time evolution required for sufficient spectral resolution is challenging to reach for both classical and quantum simulations. In this work, we introduce a principled approach for extending...

Universal Bounds for Out-of-Distribution Unitary LearningTier 3

How much can the action of an unknown unitary dynamics on one family of states reveal about its action elsewhere? We develop a general framework for out-of-distribution quantum dynamics learning based on the first and second moments of the input distribution. These define a notion of the bias and expressivity of an ensemble, which together control generalization between arbi...

Modified logarithmic Sobolev inequality for 1D non-commuting HamiltoniansTier 3

Given a finite-range, non-commuting Hamiltonian on a 1D chain of finite spins, we prove a modified logarithmic Sobolev inequality with constant Ω(1/log n) for the heat-bath dynamics, a quasi-local Gibbs sampler, and for the regularised heat-bath dynamics. This implies rapid mixing of both samplers towards the Gibbs state in relative entropy and trace norm, with mixing tim...

Efficient capacity-achieving entanglement generation with application to pure-loss Bosonic channelsTier 3

While the ultimate limits on the rate of quantum communication over noisy channels are well-established, very few efficient coding schemes achieving these limits are known. This contrasts sharply with transmitting classical data over classical channels, where polar codes and low-density parity-check codes both efficiently achieve the classical capacity of any noisy channel....

Quantum Simulation on Riemannian ManifoldsTier 3

We investigate algorithms for the quantum simulation of the Schrödinger equation on a Riemannian manifold, where the kinetic operator is defined by the Laplace--Beltrami operator corresponding to the metric. Our first algorithms are based on a global spectral method based on the identification of an efficient transform to the eigenbasis of the Laplace--Beltrami operator. We...

Ordering-Aware Theory of Trotter ErrorTier 3

Product formulas are a basic tool for simulating quantum many-body dynamics, but a fundamental degree of freedom in their application---the ordering of Hamiltonian terms---has remained largely unexplored. Here we systematically study such ordering effects, quantifying their impact on simulation accuracy and exploiting them to design more accurate product-formula simulations....

Classical Algorithms for Bipartite Quantum Max-Cut on Dense ExpandersTier 3

Quantum Max-Cut is a well-studied local Hamiltonian problem for which the classical complexity of the bipartite case remains open. We give a polynomial-time classical randomized algorithm for the spin-1/2 antiferromagnetic Heisenberg model, equivalently Quantum Max-Cut, on dense balanced bipartite expanders. This class includes G(n,n,p) with high probability for eve...

Optimal kernel functions for linear combination of Hamiltonian simulationTier 3

Linear combination of Hamiltonian simulation (LCHS) provides an asymptotically optimal framework for the quantum simulation of linear non-unitary dynamics. The LCHS approach approximately implements the Peano--Baker propagator through an integral representation determined by a kernel function. We establish an LCHS approximation theorem for a broadened class of admissible ker...

BARC codes: general polynomial framework for coherent-state superposition codesTier 3

We introduce an algebraic framework for constructing a new class of quantum error-correcting codes, bosonic algebraically-restricted constellation (BARC) codes. The code states are finite superpositions of coherent-state constellations constrained by symmetries of solution sets to multivariate complex polynomial systems. The constraints are associated with photon-gain and ph...

Lie Algebraic Uncertainty RelationsTier 3

Uncertainty relations express the fundamental incompatibility of quantum observables, yet optimal state-independent bounds are known analytically in only a few cases. Here we introduce a covariance-based formulation of quantum uncertainty that yields an infinite family of relations for both Hermitian observables and non-Hermitian operators. For operators forming a basis of a...

Quantum Codeword SensingTier 3

Consider the task of identifying an unknown classical codeword delivered to a register of quantum sensor qubits through a sequence of weak phase interactions that each encode one bit. Suppose that each received bit controls an identical Z-rotation of angle θ applied to every sensor qubit; receivers may also use auxiliary qubits within a total qubit budget Q, with arbit...

Super-Exponential Advantage of Squeezed Light in Phase Estimation under Discrete Phase RandomisationTier 3

A paradigmatic task in quantum metrology is that of phase estimation. For this task it is commonly believed, under reasonable assumptions about prior information, that a quadratic advantage in sensitivity is the maximum possible advantage offered by the use of quantum resources. In this work, we consider a channel that implements a phase shift by an angle 2πk/M, where k...

Breaking the cubic barrier for the inverse-free Solovay-Kitaev algorithmTier 3

The Solovay-Kitaev algorithm states that it is possible to efficiently compile unitaries to accuracy ε in polylog(1/ε) time using any universal gate set. While Solovay and Kitaev's initial work addressed inverse-closed gate sets, recently Bouland and Giurgicǎ-Tiron showed it is possible to efficiently compile arbitrary gate sets without inverses. However, the expo...

A Basis-Aware Approach to Quantum Sampling of the Fermi-Hubbard LadderTier 3

Sample-based quantum diagonalization can reduce the classical cost of many-body calculations by restricting diagonalization to configurations identified through quantum sampling, but its efficiency depends on how compactly the target state is represented in the sampled basis. We investigate this dependence for sample-based Krylov quantum diagonalization of the half-filled Fe...

Reaching the Limits of Ground-State Metrology with Many-Body ProbesTier 3

We investigate the physical requirements to reach the ultimate limits of ground-state metrology -- i.e., the estimation of an unknown parameter θ of an N-body Hamiltonian via measurements on its ground state -- using many-body probes featuring either short-range or long-range interactions. We characterize the conditions to saturate two fundamental limits of the quantum F...

Clarifications on the Experimental Status of Real Quantum TheoryTier 3

In our previous paper on the matter, we proved that real quantum theory (RQT) cannot be experimentally falsified without the experiment falsifying quantum theory (QT) as well. Our result is in direct contradiction to the central claim of an article by Renou et al., published in Nature [600, 625--629 (2021)], reporting a bilocal network experiment that could falsify real quan...

Polynomial-Time Algorithms for Nuclear Tensor Norms and Multipartite SeparabilityTier 3

We study a multilinear optimization problem for tensors with bounded Frobenius norm, where each of the k local factors of dimension d is chosen from a convex set. We give a deterministic algorithm running in time d^(O(k)) for constant additive approximations. As applications, we obtain the first polynomial-time algorithms at constant accuracy for weak membership in the...

Entanglement of purification for Werner states: canonical purification and nonadditivityTier 3

We prove that the von Neumann entanglement of purification is nonadditive. For two-qubit Werner states W(f) with singlet weight 1/64≤f≤1/4, we show that canonical purification is globally optimal: its entanglement, also known as reflected entropy, gives the exact entanglement of purification. In contrast, the regularized entanglement of purification lies strictly be...

Discriminating Lindbladian DynamicsTier 3

The fundamental task of quantum hypothesis testing, which consists in discriminating quantum objects, has been extensively studied for quantum states and channels. Here, we investigate the fundamental limits of discriminating an arbitrary pair of quantum Markovian dynamics generated by Lindbladians, focusing on the roles of total exposure time and temporal resolution. We all...

Decohering Kitaev's Sixteenfold Way: A Holographic ApproachTier 3

We study anyon decoherence in 2D topological order, with the fermion decoherence in Kitaev's sixteenfold way as explicit examples. The sixteenfold way includes chiral topological phases, which are believed to admit no commuting projector Hamiltonian, making it challenging to study their decoherence directly. We further conjecture the density matrix produced by this decoheren...

The exact LCU sampling overhead of collective diagonal unitaries: resonances and a continued-fraction dichotomyTier 3

We consider the cost of implementing the quadratic collective phase e^(-iγK^2), with K a collective observable of evenly spaced spectrum, such as the permutation-symmetric Hamming weight. This gate is the cardinality-penalty layer of constrained quantum optimization, the one-axis-twisting gate of spin squeezing and the Kerr phase of a bosonic mode. Instead of two-qubit g...

Quantum Fire with Delegated CloningTier 3

Quantum fire is a recently introduced cryptographic primitive consisting of efficiently preparable quantum states, called \emph{flames}, that admit efficient cloning but resist efficient telegraphing, namely reconstruction via classical communication without preshared entanglement. In all prior constructions of quantum fire, cloning is a public operation that requires no sep...

Constant-Rate Certified DeletionTier 3

We present a unified framework for upgrading a broad class of cryptographic primitives to support constant-rate certified deletion. Previous constructions require a linear number of qubits per encrypted bit of certified-deletable plaintext. In contrast, we obtain the first constant-rate constructions in the plain model that achieve certified deletion while preserving everlas...

Lifting Multiplicity in Randomized BenchmarkingTier 3

Randomized benchmarking comprises a suite of standard techniques for assessing the operational fidelity of quantum processors. These techniques can be reliably applied to groups with process matrix representations that are free of multiplicity. However, many groups that are naturally formed from native gates do have multiplicity, and it is difficult to draw robust conclusion...

A Complete Proof of QMA_1-Hardness for Weighted Gapped Clique HomologyTier 3

The computational complexity of simplicial homology has been studied since the question was raised by Kaibel and Pfetsch, and clique homology was later shown to be QMA_1-hard by Crichigno and Kohler. King and Kohler subsequently introduced the weighted, spectrally gapped version considered here and developed a reduction intended to establish the same hardness resu...

Robustness of quantum spectrum estimation: weak Schur sampling under noisy inputsTier 3

We initiate the study of weak Schur sampling (WSS) under noisy input. Many optimal quantum learning algorithms rely on this measurement, for instance in such fundamental tasks as quantum spectrum estimation (QSE) and quantum state tomography (QST). The standard analysis of WSS, QSE and QST assumes that the n input copies of the target state ρ are identical. We study what...

From Symmetry to Secrecy: Covariant Classical--Quantum Wiretap ChannelsTier 3

For classical--quantum wiretap channels with a transitive covariant input action, both output ensembles admit a description as unitary group orbits of fixed seed states. We use this structure to construct finite-block preprocessing schemes whose private information is an entropy difference evaluated at two distributions related by group convolution. Applying the construction...

Quantum algorithms for orthogonal polynomial transformsTier 3

A quantum orthogonal polynomial transform (QOPT) is an algorithmic primitive that maps a superposition of standard basis states Σ_k α_k ketk coherently to a basis of normalized univariate polynomials orthogonal with respect to a probability measure μ(x). We provide new discrete and continuous QOPTs for polynomial families in the Askey scheme extending the results...

How to Build Pseudorandom Unitaries in MicrocryptTier 3

We provide the first evidence relative to a classical oracle that pseudorandom unitaries (PRUs) can exist without quantum-computable one-way functions. To obtain this result, we first prove that five independent random diagonal phase layers interleaved with Hadamard transforms U=F_5HF_4HF_3HF_2HF_1 form a strong PRU with security under adaptive, controlled access to th...

Unitary complexity in polynomial spaceTier 3

We show that if quantum commitments exist, then either there is no polynomial-time solution to the unitary synthesis problem, or BPP ≠NEXP. Thus, showing unconditionally that quantum commitments exist would require answering at least one of two longstanding open questions in complexity theory. We prove our main result as a consequence of a more genera...

Previously: 2026-09-19

An operational characterization of finite-dimensional quantum theoryTier 3

A key goal in the foundations of quantum mechanics is to identify operational constraints characterizing physical theories. Bell inequalities do so for classical probability theory, while Tsirelson's bound provides a first step for quantum mechanics. Here, we address the dual question: Can one certify that all correlations predicted by quantum theory are actually realizable?...

Exponential quantum space advantage in random data streamsTier 3

We show two unconditional quantum space advantages in the random-order streaming model. First, we show the Yamakawa--Zhandry Code Intersection problem admits exponential quantum advantage in the streaming model when its inputs are streamed in random order. This means quantum computers exhibit exponential space advantage even when simply receiving (x,f(x)) pairs for a unifo...