Elliptic flow in heavy-ion collisions departs from linear response

Fluctuations of the elliptic flow v₂ in heavy-ion collisions are usually explained as a linear response to the initial anisotropy ε₂, with a small cubic correction. Now, an analysis based on event-by-event hydrodynamic simulations reveals significant deviations from this scheme. The authors decompose the response order by order in the cumulant expansion of the ε₂ and v₂ fluctuations, working in the intrinsic frame where the magnitude and direction of the impact parameter are fixed. The results show two unexpected effects. On the one hand, the asymmetry of the v₂ fluctuations turns out to be opposite to that of the ε₂ fluctuations. On the other, the response to ε₂ fluctuations is approximately 10-20% larger than the response to the average geometry. This second effect is not explained by the cubic response and increases the splitting between the cumulants v₂{2} and v₂{4}, two observables measured with precision in Pb+Pb collisions at the LHC. Collective flow is the most prominent signature of the hydrodynamic evolution of the quark-gluon plasma created at the LHC, and its event-by-event fluctuations encode information about the early stages of the collision. Understanding precisely the relation between the initial geometry and the final flow is essential to extract properties of the medium, such as its viscosity, from the data. The described deviation suggests that the framework of linear response plus cubic correction is not sufficient to describe the full range of observed fluctuations. The work paves the way to refine the hydrodynamic models and analysis chains used to interpret flow measurements at the LHC. If the effect is confirmed, it could affect the determination of quark-gluon plasma parameters and the comparison between theory and data from ATLAS, CMS and ALICE.

Two-loop renormalization constants of Feynman gauge QED computed analytically

A new analytic calculation solves the renormalization constants Z_m and Z₁ = Z₂ of quantum electrodynamics in Feynman gauge, within the on-shell scheme and using dimensional regularization. The results are obtained in analytic form at two loops, closing a technical problem that until now was handled only numerically or through partial approximations. In addition to the main values, the work provides analytic expressions for the expansion coefficients in the dimensional regularization parameter ε, from order 1/ε² through order ε⁵. This expansion range is far wider than what single-loop calculations usually require, and responds to the demands of future higher-order corrections in QED. The value of these high-order coefficients is twofold. On the one hand, they will be needed for more precise multi-loop calculations in quantum electrodynamics, where the ε expansions must be carried several terms beyond the simple pole. On the other, they provide a useful check for multi-loop calculations in quantum chromodynamics (QCD), whose renormalization structure shares techniques and divergences with those of QED. The availability of closed-form analytic results also facilitates the detection of errors in future automatic implementations of Feynman integrals, a field in which independent checks are increasingly necessary as the number of loops considered at colliders grows.

General master equation for two accelerated detectors and their entanglement

A team of theoretical physicists has derived a general Gorini-Kossakowski-Lindblad-Sudarshan (GKLS) master equation describing the joint behaviour of two co-located two-level detectors undergoing uniform acceleration and weakly coupled to scalar fields in the Minkowski vacuum. The formulation retains both coherent and dissipative contributions, and holds for arbitrary detector orientations, any field mass, and any spacetime dimension. The work belongs to a research line investigating how accelerated motion and the structure of the quantum vacuum affect coherence and entanglement between detectors, a terrain where the Unruh effect and quantum thermalisation phenomena have been hotly debated. Until now, available descriptions were largely restricted to simplified configurations —specific orientations, massless fields, or particular dimensions— preventing a unified analysis of the role of geometry and field mass in the system dynamics. For the specific case of two detectors with equal orientation coupled to massless scalar fields in four dimensions, the authors explicitly determine the stationary states and quantify their entanglement via concurrence. The result depends markedly on the type of coupling considered: with symmetric coupling, concurrence varies with acceleration, whereas with rotating-wave coupling the concurrence turns out to be independent of acceleration. The discrepancy between the two couplings underscores that entanglement dynamics in the vacuum is not universal but depends critically on how the detectors are bound to the field. The general master equation paves the way to exploring more realistic configurations —arbitrary relative orientations, massive fields, and spacetime dimensions other than four— as well as to examining whether entanglement can be robustly extracted from the vacuum in experimental scenarios of accelerated detectors.

First observation of weak magnetism in neutron decay

Weak magnetism, predicted by Murray Gell-Mann nearly seven decades ago as a direct consequence of the conserved vector current (CVC) hypothesis, has been observed for the first time in the decay of the free neutron. This is the simplest β-decay system that exists, yet until now the key observable, the β asymmetry, could not disentangle the weak-magnetism form factor g_M from the axial-to-vector ratio λ. By combining β-asymmetry measurements with the neutron lifetime and the Ft value of superallowed 0⁺→0⁺ transitions, the authors resolve that degeneracy. Current data yield g_M = 4.47(87), in good agreement with the CVC prediction of 4.71. The hypothesis of no weak magnetism at all is excluded at the 5.1σ level. The robustness of the result is striking: the significance remains between 5.0σ and 6.1σ across all considered variations of the neutron dataset, despite the well-known internal tensions that these measurements carry. The work also assesses how far a detailed analysis of the energy dependence of the β asymmetry could refine the extraction of g_M. The conclusion is that, under present experimental conditions, that route would improve the determination by only a small margin, just a few percent. The more promising path forward, therefore, lies not in dissecting the shape of the spectrum but in improving the precision of the quantities that feed the global combination. The result matters on two counts. On the one hand, it confirms a pillar of the Standard Model —the structure of the hadronic weak current and its conservation— in the cleanest possible nuclear setting. On the other, it closes a chapter open since 1958, when Gell-Mann formulated the prediction. The measurement of g_M also becomes a useful tool to cross-check future determinations of the ratio g_A/g_V and of the neutron lifetime, two quantities that continue to fuel debate in nuclear and particle physics.

The water cycle already bears the fingerprint of human activity

Human activities are increasingly intertwined with the way water is distributed across the continents, according to a new study that brings the extent of that influence into sharper focus. The work addresses a long-standing open question: to what extent the continental water cycle has ceased to be a purely natural process and how far human intervention is reshaping its distribution. The study follows a line of research aimed at separating natural variability in the hydrological cycle from anthropogenic perturbations, a difficult problem because both effects overlap in the same observable variables. The novelty, according to the available description, lies in the greater precision with which the human fingerprint on continental water distribution can be isolated and visualised. The work thus provides a sharper framework for assessing how water is shared out among the continents and the degree to which that distribution already responds to human activity. Delimiting that influence is relevant for water resource management and for interpreting observed trends in the water cycle. The methodological detail and quantitative results of the study, as well as their contrast with independent observations, remain to be seen in order to gauge the exact scope of the human fingerprint described.

IBM suppresses logical errors in a surface-code memory on the cloud

A team has demonstrated logical error suppression in a rotated surface-code memory implemented on a cloud-accessible IBM Nighthawk r2 superconducting processor. Quantum error correction can reduce the logical error rate exponentially with code distance when physical noise is below threshold, but turning that promise into hardware requires coordinating gates, mid-circuit measurements and qubit coherence in the face of device variation, asymmetric noise and spatiotemporal correlations. The authors optimized the syndrome extraction circuit by combining short readout, fused measurement and reset, and an XX echo on data qubits during ancilla readout. Compared with standard readout without reset or echo, the recipe cuts the median per-round logical error rate by a factor of 3.8 in the X basis and 1.4 in the Z basis. Performance also varies across the tested qubit placements on the processor, which they attribute to differences in qubit quality. In memory experiments of up to 150 rounds, the advantage associated with code distance weakens and long-lived correlations appear that may reflect a persistent error component. Under the applied optimization, increasing the protected distance from 3 to 6 — while holding the other fixed — lowers the per-round logical error rate from 0.53 % to 0.24 % in Z memory and from 0.52 % to 0.22 % in X memory, with maximum-likelihood-decoding error-suppression factors of 1.68 and 1.85, respectively. The results link circuit compilation and optimization to surface-code memory performance — and its limitations — and point toward quantum-processor and error-correction-circuit codesign as a route to fault-tolerant quantum computing primitives on current cloud-accessible quantum hardware.

Localized constraints fragment Hilbert space without slowing transport

An analysis of several many-body models shows that spatially localized conserved quantities uniformly fragment the Hilbert or configuration space, yet leave energy transport properties unchanged. The study covers the quantum Volterra model, its nonintegrable classical limit, and two cellular automata derived from the latter. Although such constraints are often associated with lattice gauge theories, the models considered here admit no such interpretation. In the quantum Volterra model, the authors derive an exact mapping of the dynamics within each Hilbert-space fragment onto a quantum chiral Potts chain. With this tool in hand, they show that energy transport is ballistic and that the transport coefficients are identical across all fragments: the fragmentation does not create sectors with slower or faster dynamics. In the nonintegrable classical limit, energy transport becomes diffusive, but the conclusion is analogous: the diffusion constant turns out to be independent of whether the constraints are fixed or resolved. The cellular automata further confirm that, while transport remains intact, localized constraints lead to a strong breakdown of local ergodicity. The upshot is a picture in which fragmentation and transport decouple: the presence of many local conserved quantities suffices to prevent ergodic exploration of configuration space, but not to modify how energy propagates. Whether this decoupling survives in systems with continuous degrees of freedom or in the presence of long-range interactions, where fragmentation might cease to be uniform, remains an open question.

A balanced parametric amplifier doubles its useful bandwidth without interference

A team has experimentally demonstrated a balanced architecture for superconducting travelling-wave parametric amplifiers (TWPAs) that doubles the instantaneous useful bandwidth while rejecting the image frequency. TWPAs offer broadband gain, low added noise and high dynamic range, qualities that make them attractive for amplifying multiplexed signals spread across a wide frequency range, such as qubit resonator arrays, or inherently wideband signals, such as those from spectrometers and radiometers. In many cases, however, the useful bandwidth of a TWPA is limited by the production of an idler tone at an image frequency. In TWPAs based on three-wave mixing (3WM), an input signal at frequency fₛ is indistinguishable from one at fₚ − fₛ, where fₚ is the pump frequency. This sideband degeneracy introduces spectral ambiguity when the input spectrum is broadband or not known in advance, forcing the use of complex filtering to clean the output. Resolving this ambiguity without sacrificing bandwidth has been one of the practical brakes on using these amplifiers in wideband applications. The proposed architecture spatially separates the amplified signal from the output pump and the generated idler, resolving the degeneracy. It consists of two amplifiers connected between a pair of 180° hybrid couplers, with the signal and pump injected and output through the sum and difference ports, respectively. Using two kinetic-inductance TWPAs, the authors measured approximately 20 dB of signal gain across nearly an octave of bandwidth, with pump rejection below −20 dB and an average image rejection of about −15 dB over the operating band. The results establish the balanced TWPA as a practical route to broadband parametric amplification, with a larger fraction of contamination-free bandwidth and no need for complex filtering. The improvement is especially relevant for reading out superconducting qubits and for sensitive microwave instrumentation, where spectral cleanliness directly at the amplification stage reduces downstream hardware and simplifies the detection chain.

Neural networks describe anyon gases of up to 20 particles

The macroscopic behavior of a non-interacting gas of Abelian anyons has remained unresolved for decades. Previous numerical methods, based on exact diagonalization in the continuum and on the plane, were limited to very small systems of at most N = 4 anyons. A new study explores neural network wavefunction technology to build variational ansätze suited to continuum models of the Abelian anyon gas. The proposed architecture, dubbed "AnyNet", exactly enforces the non-analytic short-distance behavior induced by anyonic statistics. The method reproduces exact diagonalization results with excellent agreement for N = 4 anyons, reaches accurate results up to N = 16 for intermediate statistics, and extends to N = 20 for almost-fermionic anyons, with a 0.4% discrepancy relative to perturbative calculations. Using this methodology, the authors study anyons in a harmonic trap and anyonic atoms as a model for anyon binding to impurities. They analyze the structure of anyonic wavefunctions as the statistics parameter is varied, focusing on the single-particle density, the pair density, the angular momentum, and the scaling of the wavefunction as multiple anyons coalesce. The work opens the door to understanding the macroscopic behavior of anyons, for which rich phases have been predicted theoretically. Next steps will involve extending the method to larger system sizes and exploring those predicted phase transitions.

20-metre multimode fiber synthesises programmable high-dimensional quantum states

A team has demonstrated programmable synthesis of high-dimensional quantum states transmitted through 20 metres of multimode optical fibre. The work, published in Nature, combines photons generated in atoms with long coherence times and control techniques that allow complex states to be prepared and recovered despite the modal dispersion characteristic of this type of fibre. Multimode fibre is an attractive channel for quantum information because it can carry many spatial modes at once, multiplying capacity compared with single-mode fibre. Its drawback is that modes couple and mix as they propagate, deforming the state. Earlier work had recovered individual modes or controlled classical transmission through long multimode fibres, but active, programmable synthesis of high-dimensional states in a quantum channel remained a challenge. The method relies on atomic photons with long coherence, which reduce spectral broadening and ease state control. On that basis, the researchers use a modulation and calibration scheme that compensates the fibre's dispersion and allows the desired state to be written into the output modes. The 20-metre length is an order of magnitude longer than previous demonstrations of this kind while maintaining sufficient fidelity to operate with high-dimensional states. The result opens the door to quantum communication and computing protocols that exploit the internal dimensionality of spatial modes as a resource, rather than limiting themselves to qubits. It also raises questions about scalability: whether control can be maintained in longer fibres, with more modes and in less controlled conditions than in the laboratory. The next phase plans to extend the distance and explore applications in metropolitan quantum networks.

Agentic AI compiles quantum circuits more efficiently with classical verification

A team proposes a new paradigm for compiling more efficient quantum circuits using artificial intelligence agents, provided that the critical subroutines of the circuit exhibit a classically verifiable structure. When these subroutines consist exclusively of reversible classical logic, phasing operations, and measurement-based uncomputation, their execution can be efficiently simulated for classical inputs. This property allows the approximate synthesis of the circuit to be certified through a form of fuzz testing, under suitable assumptions about the input distribution. The pattern was recently exploited to improve quantum cryptanalysis circuits. Now, the authors highlight the flexibility of the approach with three notable applications: the computation of 1/√x, which bottlenecks real-space simulations of the Coulomb operator relevant to modelling inertial confinement fusion reactors; the decoding of Reed-Solomon codes, which hinders the decoded quantum interferometry algorithm when approximately solving the optimal polynomial intersection problem; and quantum look-up tables together with the controlled application of Givens rotations, which constrain the most performant quantum algorithms for simulating chemistry in second quantization. The researchers present reference implementations of these algorithms and explain how they can be constrained and verified to seed a Kaggle competition designed to accelerate AI-based research into the optimization of their compilation.

Dark matter model predicts electron and monopole properties

A new study proposes a generalization of the electroweak theory that incorporates a dark sector with massive photons and monopoles. This theoretical model, following a previous line of research that generalized quantum electrodynamics (QED), suggests that the interaction between the visible and dark sectors could transform leptons and monopoles into dyons, hypothetical particles with both electric and magnetic charge. The existence of magnetic monopoles, predicted to symmetrize Maxwell's equations, has not been experimentally confirmed to date, making any theoretical avenue for their study of great interest. The previous work on QED with massless dark photons and dark monopoles already explored the idea of a mixing interaction resulting in the formation of dyons from leptons and monopoles. The novelty of this research lies in extending this concept to the electroweak theory, which describes electromagnetic and weak interactions, and in considering massive dark photons. The mass of these dark photons is a crucial parameter that could determine the detectability of the effects predicted by the model. The researchers analyze the experimental consequences of these generalizations for electrons in both models (QED and electroweak). Although the study is theoretical in nature, it lays the groundwork for future experimental investigations that could search for signatures of these interactions and exotic particles. The possibility that electrons acquire dyon properties through interaction with a dark sector offers a new perspective for addressing the search for magnetic monopoles and the understanding of dark matter.

K⁻d Interaction Models Refined with Kaonic Atom Data

A new study has addressed the low-energy interaction between negative kaons (K⁻) and deuterons (d), a fundamental three-body system for understanding nuclear forces. Using chiral K̄N (kaon-nucleon) interaction models as input, researchers employed the Fixed-Center Approximation to the Faddeev equations. This framework incorporates coherent multiple-scattering processes, implementing unitary constraints often omitted in conventional approaches. The methodology was extended to include coupled channels and isospin-breaking effects through the use of physical particle masses, providing a consistent description of the K⁻d amplitude at low energies. The work focused on assessing the compatibility of the BCN K̄N interaction model with recent measurements from the SIDDHARTA-2 experiment on kaonic hydrogen and kaonic deuterium. Through a bootstrap analysis, the team achieved a readjustment of the BCN parametrization. This adjustment is crucial, as low-energy K̄N interactions are complex and linked to the potential existence of exotic states, such as the Λ(1405), whose nature is still debated. The results of the BCN parametrization readjustment have visible effects on several isospin-sensitive K̄N scattering observables. This highlights the importance of the new kaonic-atom measurements for imposing tighter constraints on the poorly known isovector component of the K̄N interaction. A better understanding of this interaction is fundamental for nuclear and particle physics, as it influences our ability to model hadronic matter and the structure of neutron stars.

Universal random matrix behavior in a Fermionic quantum gas

Scientists have observed for the first time universal random matrix behavior in a fermionic quantum gas. This finding is significant because random matrices are mathematical tools that describe complex systems with many degrees of freedom, from the energy levels of atomic nuclei to the zeros of the Riemann zeta function. The observation in a condensed matter system opens new avenues for understanding universality in quantum physics and the relationship between quantum mechanics and number theory. The experiment was conducted by cooling lithium-6 atoms to extremely low temperatures, forming a degenerate fermionic gas. By adjusting a magnetic field, researchers were able to control the interaction between the atoms, leading them to a regime where interactions are very strong. In this state, the system's energy levels showed a distribution consistent with predictions from random matrix theory, specifically with the Wigner-Dyson distribution for the Gaussian Unitary Ensemble (GUE), which is characteristic of quantum systems with broken time-reversal symmetry. This result not only confirms long-standing theoretical predictions about the universality of random matrices in strongly interacting quantum systems but also establishes a new experimental testbed for exploring connections between quantum mechanics, quantum chaos theory, and number theory. The ability to precisely control and measure these degenerate fermionic systems offers a unique platform for investigating complex phenomena that are difficult to study in other contexts, such as atomic nuclei or black holes.

Black hole analogue lightring observed in optical fiber

Scientists have achieved the complete observation of a black-hole lightring resonance in an optical analogue system. This breakthrough generalizes the concept of the photon sphere to non-stationary systems, showing that the lightring is associated with a spatiotemporal caustic. The complex frequency of this caustic, determined by its propagation and divergence, is key to understanding the observed properties. The experiment, conducted in a 5-km-long dispersion-shifted optical fiber, used an optical soliton to act as an analogue black-hole potential. A characteristic spectral plateau was predicted, with its boundaries set by the real part of the lightring frequency and its spectral decay governed by the imaginary part. These spectral signatures were observed for two distinct pump wavelengths using a pump-probe experiment. The measured spectra align with numerical simulations and the predicted lightring-mode profile, providing direct access to both the oscillatory and decay properties of the resonance. These results establish a direct experimental connection between black-hole lightrings, spacetime caustics, and quasinormal-mode spectroscopy in a dispersive analogue system, opening new avenues for studying extreme gravitational phenomena in laboratory settings.

New method for estimating energy spectra in quantum materials

Researchers have developed a new method, named SPICES (Single-Particle Inference via Contour Estimators), to determine the energy weights (Eᵢ) of Hamiltonians in quantum systems. This breakthrough is crucial for materials modeling, where the structure of non-interacting modes in a system is obscured by an unknown global unitary transformation (U). SPICES aims to recover these Eᵢ values without needing to reconstruct the complex U transformation, significantly simplifying the Hamiltonian learning process. The problem of learning these Hamiltonians, even with an underlying non-interacting structure, has been shown to be DQC1-hard, indicating its computational complexity. SPICES addresses this difficulty by combining a Hadamard test, a quantum technique for phase estimation, with a novel classical post-processing procedure. This approach efficiently recovers the spectral distribution of energies in Wasserstein distance, under certain physical assumptions. The key to the SPICES method lies in exploiting the correspondence between the single-particle energies of individual quasiparticles and the Fourier frequencies governing the zeros of the partition function at complex inverse temperature. Given that quasiparticle spectra are fundamental in downstream materials calculations, and that SPICES is a practical and straightforward method to implement, it is expected to have a significant impact on quantum materials simulations. Its application could improve the accuracy and efficiency of classical methods such as mean-field theory, Green's functions, and density functional theory.

New Complexity Measure for Quantum State Preparation in Spin Glasses

Researchers have developed a new method to quantify the complexity of state preparation in dense quantum $p$-spin Hamiltonians, operating on $n$ qubits. This approach extends beyond limitations based solely on quantum circuit light cones, introducing "effective profile complexity" derived from the metric entropy of Pauli profiles. These profiles record the expectations of all Pauli operators supported on exactly $p$ qubits. This advancement allows for a more precise characterization of the computational difficulty in achieving certain quantum states. The study reveals that classes of states with uniformly bounded quadratic effective profile complexity remain separated from the ground-state energy by a positive multiple of $\sqrt{n}$ for sufficiently large fixed $p$. At subquadratic effective profile complexity, the class cannot outperform a suitable benchmark class at leading order, with product states providing a universal benchmark. The proof of these results combines an adaptation of a nonsymmetric quantum de Finetti theorem with Gaussian process entropy bounds. Applying this framework, it is shown that attaining near-ground-state energy requires $\Omega(n^2/\log n)$ one- and two-qubit gates, even with arbitrary discardable ancillas. The work also obtains depth-width tradeoffs, entanglement-depth and matrix product state bond-dimension lower bounds. Furthermore, obstructions are identified for both orientations at every fixed level of Parham's magic hierarchy, with a total circuit width of $O(n)$. For the first-level reverse magic class, where a shallow circuit is followed by an unrestricted Clifford circuit, the bounds also allow arbitrarily many clean ancillas at fixed shallow-circuit depth. A sharper benchmark shows that Clifford+$T$ circuits with $o(n)$ $T$-gates have no leading-order energy advantage over product stabilizer states, even with unrestricted Clifford operations and arbitrary discardable ancillas.

1966 Nobel Prize in Physics: Collaboration in Atomic Structure

The 1966 Nobel Prize in Physics was awarded to Alfred Kastler for the discovery and development of optical methods for studying Hertzian resonances in atoms. His work, which laid the groundwork for the development of lasers and atomic clocks, focused on manipulating atomic energy states by combining light and radiofrequency fields. Kastler pioneered techniques that allowed unprecedented control over nuclear and electron spins, opening new avenues for research in atomic and molecular physics. The main contribution of Kastler was the invention of the "optical pumping" method in the 1950s. This technique uses polarized light to selectively excite atoms to higher energy states, creating an inverted population of spins. Subsequently, by applying radiofrequency fields, transitions between these states are induced, allowing for highly precise study of the magnetic properties of nuclei and electrons. This approach was crucial for understanding the interaction between matter and radiation, and for the development of devices such as masers and lasers. The impact of Kastler's work extends to various areas of modern physics. His methods not only improved the precision of measurements in atomic spectroscopy but also provided fundamental tools for metrology, such as high-precision atomic clocks, essential for global navigation systems like GPS. Furthermore, optical pumping is an underlying principle in laser physics and has found applications in quantum computing and coherent manipulation of quantum states. The ability to accurately control and measure atomic states remains a cornerstone of contemporary research in quantum physics.

Single-Copy Quantum Bandit Problem Collapses to Classical

A new study demonstrates that the single-copy quantum bandit problem, where each "arm" supplies an unknown quantum state and rewards are defined by a known quantum effect, can be optimally solved using classical strategies. Researchers have proven that the measured relative entropy from an arm state to its "confusing reward half-space" is equivalent to the classical Burnetas-Katehakis functional, which is based on the spectral statistics of the quantum effect. This implies that the inherent quantum complexity of the problem collapses to a classical description in this specific scenario. The work establishes that a spectral measurement of the quantum effect, combined with a classical KL-UCB (Kullback-Leibler Upper Confidence Bound) type algorithm, is asymptotically instance-optimal. This means that neither adaptive or randomized measurements, nor storing copies for later measurements, can improve the leading logarithmic regret coefficient. This result holds for any finite dimension and for every reward effect under the stated nondegeneracy assumptions, providing a self-contained finite-time bound for general effects and boundary distributions. The key implication is that any further improvement in solving these types of quantum bandit problems must come from measurements outside the single-copy class. For consistent policies with arbitrary quantum memory, an amortized relative-entropy argument provides a converse with the Umegaki half-space divergence. The authors note that the per-arm regret floors, 1/Kᵢₙ𝖿 and 1/Dᵢₙ𝖿, coincide if and only if the arm commutes with the effect F. Whether the Umegaki floor is attainable remains an open problem, with two-copy numerical evidence suggesting it may require using two copies.

New Quantum de Finetti Theorems Improve Error Bounds in Channels

Researchers have developed new versions of quantum de Finetti theorems, applicable to both quantum states and channels, and valid for any distance measure. These theorems, which traditionally approximate the k-system marginals of permutation-invariant states of n-systems by mixtures of independent and identically distributed (iid) states, are now presented in a stronger form of operator inequalities. This implies that the error bounds are valid for every Schatten norm and for every quantum Rényi divergence satisfying data processing. In the standard case, for a fixed local dimension, the new k/n error bound in max-relative entropy significantly improves upon the previously known k²/n scaling, even in the classical setting. The operator-inequality approach is particularly suited to study channel de Finetti representations because operator order between Choi states is equivalent to completely positive (CP) order between the underlying channels. For permutation-covariant channels N⁽ⁿ⁾:Aⁿ→Bⁿ, with d_A=dim A, it is proven that the k-system reduced channel is CP-dominated by a mixture of tensor-power channels with an error of O(k/√n) and a polynomial dependence on the local dimensions. This addresses a question previously raised by Berta et al. Furthermore, under the no-signalling condition, an exponential channel de Finetti theorem is proven, where the approximating mixture consists of Choi-almost-iid channels, whose normalized Choi states are almost-iid in the sense of Mazzola-Sutter-Renner. In the case of r defects, the representation error is at most poly(n)(2d_A⁴k³/(nr²))⁽ʳ⁺¹⁾/² and decays exponentially with n for a suitable choice of parameters r and k.

Hybrid Nanophotonic Trap Boosts Atomic Storage and Coherence Times

Scientists have developed a hybrid nanophotonic trap that enables the storage of neutral rubidium (⁸⁷Rb) atoms with exceptionally long coherence times. This breakthrough is crucial for the advancement of quantum computing and precision metrology, as the ability to maintain the quantum coherence of atoms for extended periods is a fundamental requirement for these technologies. The trap combines the advantages of traditional optical traps with those of nanophotonic devices, achieving more efficient light-matter interaction and greater control over atomic states. The trap's design is based on a silicon nitride (Si₃N₄) nanophotonic waveguide that generates an evanescent field to trap atoms at a distance of approximately 100 nm from the surface. This proximity allows for strong light interaction, while the device's geometry minimizes heating and decoherence effects. Researchers achieved storage times of up to 10 seconds and coherence times of up to 20 milliseconds for the spin states of ⁸⁷Rb atoms, significantly surpassing results obtained with purely optical or nanophotonic traps. The key to success lies in the integration of the nanophotonic waveguide with a state-of-the-art atomic cooling and trapping system. This hybrid approach enables the manipulation and readout of atomic quantum states with high fidelity, opening new avenues for building scalable quantum processors and highly sensitive quantum sensors. The results demonstrate the potential of nanophotonic platforms to control and exploit the quantum properties of neutral atoms in the strong interaction regime. This advance represents a significant step towards the realization of neutral atom-based quantum computers, as well as the development of more precise atomic clocks and magnetometers. The ability to maintain quantum coherence in a nanophotonic environment paves the way for integrating these systems onto chips, which could lead to compact and robust quantum devices. Future research will focus on scaling the number of trapped atoms and exploring new architectures for quantum computing and quantum simulation.

New Quantum Algorithm Accelerates Equilibrium Correlation Function Calculation

Researchers have developed a novel quantum algorithm that significantly optimizes the calculation of equilibrium correlation functions, a fundamental quantity for describing the dynamic properties of physical systems. Unlike conventional quantum methods, which require a distinct quantum circuit for each time point, this new approach allows all time points to be computed simultaneously with a single circuit. This represents a substantial improvement in computational efficiency, drastically reducing the required quantum resources. Previous quantum algorithms for these functions, such as the Hadamard test or variational methods, scaled with a complexity of at least O(Nₜ) circuit shots and O(Nₜ²) quantum gates, where Nₜ is the number of time points. The new algorithm, however, uses O(log Nₜ) ancilla qubits to encode time as a quantum variable. This effective parallelization allows the circuit to be run only O(log Nₜ) times, resulting in a total complexity of O(Nₜ log Nₜ) quantum resources for large systems. The authors demonstrate that this scaling is optimal and validate their method by calculating the Green's function of a one-dimensional spinless Hubbard model. The ability to efficiently calculate equilibrium correlation functions is crucial for understanding and predicting the behavior of complex quantum materials and systems. This advance could accelerate research in fields such as materials science, quantum chemistry, and the design of new devices. The optimization in quantum resource usage brings closer the possibility of performing simulations of quantum systems that are currently intractable for classical computers, opening new avenues for scientific discovery.

All Quantum Unitaries Can Have Constant Depth Circuits

A new study has revealed that any n-qubit unitary operation can be approximated with ε precision by a quantum circuit of depth polynomial in n and log(1/ε), using an exponential number of ancilla qubits. This finding is surprising, as it was traditionally assumed that implementing general unitaries required exponential depth. The ability to "parallelize" these operations to polynomial depth opens new avenues for quantum algorithm design. The advance is based on a novel connection between the unitary synthesis problem, previously explored by Aaronson and Kuperberg, and concepts from complexity theory and cryptography, such as locally-decodable codes and private information retrieval. This interconnection has allowed for the development of a construction that challenges previous assumptions about the complexity of quantum circuits. Furthermore, the researchers demonstrated that if unbounded fan-out gates are permitted (i.e., one output can be the input to multiple gates), these circuits can be further reduced to constant depth. This implies that, under certain hardware conditions, the execution of complex quantum operations could be performed in a computationally very short time, regardless of the number of input qubits, which has profound implications for large-scale quantum computing.

Multi-level time-bin encoded cluster states transmitted over 29 km of optical fiber

Scientists have successfully transmitted time-bin encoded quantum cluster states over 29 kilometers of partially deployed optical fiber. This achievement represents a significant step towards building robust and scalable quantum communication networks. Time-bin encoding, which uses the arrival of photons at different times to represent information, is less susceptible to decoherence and signal loss than other techniques, making it ideal for long-distance transmissions. Cluster states are a particular type of quantum entanglement considered a key resource for one-way quantum computing and quantum error correction. The ability to generate and distribute these states over significant distances is fundamental for the development of the quantum internet. The experiment used a configuration that allowed for the encoding of multiple levels of information (qudits) in each photon, increasing the transmitted information density and system efficiency. The optical fiber used in the experiment was not pristine laboratory fiber but rather existing, partially deployed infrastructure, which adds realism to the achievement and demonstrates the feasibility of integrating these technologies into current communication networks. This result opens new avenues for the practical implementation of quantum communication protocols and the interconnection of distributed quantum processors, laying the groundwork for future applications in quantum cryptography and quantum cloud computing.

New Local Decoding Method for Quantum LDPC Codes

Researchers have developed a local and distributed framework for decoding quantum low-density parity-check (LDPC) codes. This architecture, built upon belief propagation (BP), integrates strictly local processing of syndrome information, local feedback, autonomous operation, and highly parallelized decoding. This advancement is crucial for quantum computing, where LDPC codes hold promise for error correction, but efficient decoding remains a challenge. The study addresses two complementary levels. First, they show that any code exhibiting a threshold under iterative BP decoding can be promoted to an autonomous local measurement-and-feedback dynamics while preserving that threshold. This means the inherent robustness of certain codes against errors is maintained even with a more distributed decoding approach. The second level focuses on codes where naive BP itself does not exhibit threshold behavior. The key insight is that BP can nevertheless provide sufficiently accurate local marginal information. Active syndrome defects use these local beliefs to move, pair, and annihilate through asynchronous feedback. Numerically, this construction exhibits threshold behavior in the point-like sectors of the two- and three-dimensional toric codes, where conventional BP decoding fails. This represents a significant improvement in the ability to correct errors in quantum architectures. Exploiting the graph-generality of BP, the team also investigated the inference dynamics on the membrane-like sector of the three-dimensional toric code and in a family of bivariate-bicycle quantum LDPC codes, demonstrating the method's versatility. This approach could accelerate the development of fault-tolerant quantum computers by offering more scalable and efficient error decoding.

New Security Limit in Quantum Communication with Wiretap Channels

Researchers have determined the exact strong converse exponent for secret-key transmission and generation over general finite-dimensional quantum wiretap channels. This advance is crucial for understanding the fundamental limits of security in quantum communication, particularly in scenarios where an eavesdropper attempts to intercept information. Reliability and secrecy are jointly measured by squared fidelity to an ideal secret key, providing a robust metric for evaluating the performance of these systems. The work introduces a novel Rényi private information, whose regularization characterizes this exponent. An exact integral representation, expressed in terms of ordinary private information, establishes a uniform continuity bound. This demonstrates that the regularized quantity converges to private capacity as the Rényi order approaches one. This finding is significant because it establishes, for any wiretap channel, an exponential fidelity decay at every rate above private capacity, implying that attempting to transmit information above this threshold results in a rapid loss of security. This result has direct implications for the design and optimization of secure quantum communication protocols, such as quantum key distribution. By knowing the exact strong converse exponent, engineers can set precise limits on the maximum amount of secret information that can be reliably and securely transmitted, even in the presence of a quantum eavesdropper. Furthermore, understanding how fidelity decays exponentially above private capacity offers clear guidance for avoiding insecure transmission rates, thereby ensuring the robustness of quantum communication systems against interception attacks.

New Protocol Matches Efficiency of Parallel and Sequential Quantum Tomography

Researchers have developed a new protocol that enables quantum unitary channel tomography with parallel queries as efficiently as with sequential queries. Traditionally, sequential strategies were considered more powerful because they allow the learning strategy to be adapted as the channel estimate is refined. Parallel queries, on the other hand, do not allow such adjustments, which suggested an inherent limitation in their efficiency. The new protocol achieves learning an unknown d-dimensional unitary channel to within ε in diamond distance using O(d²/ε) parallel queries. This result matches the previously established theoretical lower bound, thus closing the efficiency gap that existed between parallel and sequential quantum tomography strategies. This advance is significant because it demonstrates that, contrary to previous intuition, parallel queries can be as efficient as sequential ones for this specific task. Applications of this protocol include optimizing query complexity for quantum channel tomography in the boundary regime and improving the query efficiency of the best-known protocol for fermionic linear optics tomography. Achieving optimal efficiency with parallel queries opens new avenues for the development of quantum technologies that require fast and precise characterization of quantum processes.

Optimal Method Developed for Tolerant Quantum Dissipation Testing

Researchers have developed an algorithm and established a lower bound for the optimal quantification of dissipation in open quantum systems. This new method allows determining whether the dissipative strength of an unknown, but bounded-strength, Lindblad generator remains within a prescribed tolerance threshold. Dissipation is measured in the canonical dissipator's normalized Frobenius norm, and the algorithm can distinguish between a maximum dissipative strength of ε₁ and a minimum strength of ε₂, where 0 ≤ ε₁ < ε₂. The study addresses the problem of characterizing the dynamics of open systems and controlling quantum noise, crucial aspects for the development of quantum technologies. Unlike previous work, this approach eliminates the bounded-degree requirement for jump operators, significantly broadening its applicability to a wider range of experimentally relevant settings. The ability to tolerantly assess dissipation is fundamental in experimental environments where residual noise is unavoidable. The proposed algorithm solves the task with constant success probability in a total evolution time of O(ε₂/(ε₂-ε₁)²) for all nontrivial thresholds. Furthermore, the researchers have demonstrated a matching lower bound of Ω(ε₂/(ε₂-ε₁)²) for adaptive protocols, establishing the algorithm's optimality. This result is crucial as it provides an efficient and rigorous tool for dissipation characterization, an essential step for noise suppression and control in quantum systems.

Particle Physics: Why Are Neutrinos So Light?

An international team of scientists has proposed a new theory that could explain why neutrinos, some of the most abundant elementary particles in the universe, possess such a tiny mass compared to other leptons. The hypothesis suggests that neutrinos acquire their mass through a different mechanism than other particles, involving a type of interaction not yet directly observed. The Standard Model of particle physics predicts that neutrinos should be massless particles. However, experiments like Super-Kamiokande and SNO have shown that they oscillate between different types (flavors), which implies they must have a non-zero mass. This discrepancy has been one of the most persistent problems in particle physics for decades. The new proposal seeks to resolve this anomaly by introducing new particles and symmetries that interact weakly with neutrinos. The theory is based on the existence of an "invisible" dark matter sector that interacts with neutrinos via an as-yet unknown force mediator. This mechanism, dubbed the "inverse seesaw mechanism," would allow neutrinos to obtain their mass indirectly, without needing to interact directly with the Higgs boson in the same way as other particles. This would result in extremely small masses, consistent with current observations. While the theory is purely speculative for now, it offers a promising direction for future research at the Large Hadron Collider (LHC) and other neutrino physics experiments. The detection of mediating particles or new interactions could provide the necessary evidence to validate this hypothesis and shed light on one of the greatest mysteries of the subatomic universe.

Optimal Performance Achieved in Chaotic Floquet Quantum Battery

Researchers have explored a new strategy to optimize the performance of quantum batteries, devices that store energy in quantum systems. The study focuses on a battery composed of N qubits interacting uniformly. The charging process begins from the system's ground state, applying periodic kicks in a transverse direction and, crucially, introducing disorder in the interaction direction. This approach allows the system's dynamics, even though the Hamiltonian and initial state preserve permutation symmetry, to deviate from the symmetric subspace, resulting in a larger amount of stored energy compared to systems without disorder. A key finding is that, in the limit of large disorder, the possibility of a chaotic charging phase emerges. In this regime, the stored energy can be precisely calculated using random matrix theory. More importantly, energy fluctuations decrease and become negligible in this large disorder limit, which is precisely when the stored energy reaches its maximum. This condition is highly desirable for the optimal functioning of a quantum battery, as it ensures maximum and stable charge. The battery cycle is completed by considering the storage phase in the presence of dephasing noise. The results indicate that, for larger disorder strengths, the ergotropy—the maximum extractable energy—remains close to the stored energy. This suggests nearly complete energy extractability, with an efficiency approaching unity. This advance could lay the groundwork for the development of more efficient and reliable quantum batteries, leveraging quantum mechanics principles to overcome the limitations of conventional energy storage devices.

New Quantum Algorithm Optimizes Linear Systems Solving

Researchers have developed a new quantum algorithm that establishes an optimal bound for the complexity of the Quantum Linear Systems Problem (QLSP). This breakthrough significantly improves upon previously known query complexities, which were O(κd log(1/ε)) and κ√d(κd/ε)ᵒ⁽¹⁾, by demonstrating that the actual complexity is Θ(κ√d log(1/ε)). The QLSP is fundamental to various applications in quantum computing, where the goal is to output a quantum state proportional to the solution of a system of linear equations A⃗x=⃗b, given a sparse matrix A and a vector ⃗b. The algorithm addresses the resolution of large-scale linear systems, a bottleneck in many scientific simulations and calculations. The improvement in query complexity is crucial because it reduces the number of operations required to achieve a solution with a desired precision ε. This result not only optimizes the efficiency of existing quantum algorithms but also sets a new benchmark for future development in this field. In addition to optimizing the QLSP, the study resolves an open problem posed by Berry and Childs. They have shown that any N×N unitary matrix can be implemented with bounded error using only O(√N) queries to its matrix entries. This ability to efficiently implement unitaries is vital for the construction of complex quantum circuits and has direct implications for the synthesis of quantum algorithms and the simulation of physical systems.

New Quantum Algorithm Optimizes Ground-State Preparation

Researchers have developed two quantum algorithms that establish the optimal query complexity for preparing ground states of a Hamiltonian. These algorithms are capable of preparing a quantum state within a trace distance ε of the ground state, assuming an energy threshold in the spectral gap is known. This gap, Δ, is defined as the minimum energy separating the ground state from excited states, and state preparation relies on an oracle that generates a state with an overlap γ with the ground state. The proposed algorithms use a number of calls to a Hamiltonian oracle U_H that scales as O((α/Δ)(γ⁻¹+log(1/ε))) in expectation and O((α/(γΔ))log(1/ε)) in the worst case, where α is a measure of the Hamiltonian encoding complexity. These bounds have been shown to be optimal, matching theoretical lower limits. For calls to the state preparation oracle U_I, the bounds are O(1/γ) in expectation and O(γ⁻¹log(1/ε)) in the worst case, also being optimal in (N+1)-dimensional systems under certain conditions. The core method of the algorithms involves using a constant-accuracy spectral filter to construct a purifier. This purifier is sequentially composed during an amplitude amplification process, allowing the preparation of a state with constant overlap with the ground state. The expected-query algorithm repeats the preparation followed by a high-accuracy spectral filter until success, while the worst-case algorithm employs filters of increasing accuracy, limiting the total number of queries.

New Si-on-SiC Platform for Vanadium Quantum Interfaces

Researchers have developed a novel Si-on-SiC photonic platform that enables efficient photon extraction from vanadium color centers in silicon carbide (SiC). These vanadium centers emit in the telecom O-band and exhibit sub-microsecond optical lifetimes and second-scale spin relaxation times, making them promising candidates for quantum network nodes. The main challenge until now has been the efficient extraction of photons from SiC, a material with a high refractive index. The new platform utilizes silicon nanocavities that evanescently couple to shallow vanadium dopants in commercial 4H-SiC, facilitating zero-phonon line (ZPL) collection under resonant excitation. In as-grown vanadium ensembles, Purcell-enhanced photoluminescence was observed, with transient spectral hole burning linewidths of 50 MHz on microsecond timescales. In dilute implanted samples, scientists successfully isolated individual vanadium centers, demonstrating high-purity single photon emission and a Purcell-enhanced lifetime of 109 ns. The entire optical interface operates through a single lensed fiber, which includes a 905 nm repump laser capable of recovering the vanadium charge state with 95% efficiency. These results represent a significant step towards the development of scalable, foundry-compatible spin-photon interfaces for telecom wavelengths. The Si-on-SiC platform offers a practical solution to overcome photon extraction limitations in high-refractive-index materials, opening new avenues for building robust and efficient quantum networks. The ability to integrate these components into existing manufacturing processes is crucial for future technological implementation.

Plasma Effects Suppress Collisional Freeze-In Dark Matter Production

A new study has demonstrated that plasma effects suppress dark matter production via the collisional "freeze-in" mechanism, a finding with significant implications for the viability of sterile neutrinos as dark matter candidates. While plasma effects were previously known to inhibit oscillatory "freeze-in," their role in collisional "freeze-in" was less clear, leading to conflicting results in the literature, particularly concerning sterile neutrino production. Researchers utilized first-principles nonequilibrium Kadanoff-Baym equations, formulated in a flavor-covariant manner, to establish a general result. They determined that plasma effects also reduce the efficiency of collisional "freeze-in." Specifically, the vacuum mixing angle, a key parameter in the collision rate, must be replaced by its plasma-corrected counterpart. This correction implies a suppression in the production of dark matter particles. Applying this result to sterile neutrinos, a popular dark matter candidate, leads to a strong conclusion: sterile neutrinos cannot be dark matter in the keV-MeV mass range if produced from the Standard Model plasma. This finding closes a potential avenue for dark matter production in this mass range and reinforces the need to explore other models or mechanisms to explain the nature of dark matter.

New Approach for Charmonium-Like Meson Spectrum and Decay Widths

Researchers have extended a previous diabatic framework for the charmonium-like meson spectrum below 4.3 GeV by incorporating the complex scaling method. This new approach explicitly integrates couplings to the meson-meson continuum, allowing for a more comprehensive description of its contributions to physical states. A key advantage of this method is that it treats bound and resonant states on an equal footing, enabling the direct extraction of decay widths from complex energy eigenvalues without introducing any additional free parameters. The study has allowed for the calculation of channel weights and complex root-mean-square radii (rRMS) of these mesons, shedding light on their internal structures. Specifically, it was found that the χc1(3872), ψ(4040), and ψ(4230) states exhibit significant molecular characteristics. Furthermore, the authors propose the X(3940) as a candidate for a state with quantum numbers JPC=1++ and discuss in detail the nature of the χc0(2P) resonance. This advance in understanding the charmonium-like meson spectrum is crucial for particle physics. The ability to determine decay widths directly from energy eigenvalues without free parameters represents a substantial methodological improvement. The results contribute to unraveling the complex structure of these exotic states, which often challenge simple quark model descriptions, and open new avenues for future experimental and theoretical investigations in the search for new hadronic states.

Puzzling isotonic odd-even staggering of charge radii in deformed rare earth nuclei

A new study has revealed an unexpected pattern in the nuclear charge radii of rare earth isotones, specifically in the region of nuclear deformation. Isotones are nuclei with the same number of neutrons (N) but different numbers of protons (Z). A pronounced staggering was observed between nuclei with an even number of protons and those with an odd number, a phenomenon known as odd-even staggering. This behavior is particularly puzzling because, while odd-even staggering in charge radii is known in spherical nuclei, its manifestation in deformed rare earth nuclei, where intrinsic deformation should mitigate such effects, raises new questions about the proton-neutron interaction and nuclear structure. Traditionally, nuclei with an even number of protons are expected to be more stable and compact than neighboring nuclei with an odd number, due to nucleon pairing. This effect manifests as an odd-even staggering in charge radii. However, in the rare earth region, many nuclei exhibit strong deformation, implying that the nuclear charge distribution is not spherical. The presence of this odd-even staggering in deformed nuclei suggests that the underlying mechanisms are more complex than previously thought, and that nuclear deformation does not completely eliminate the influence of pairing effects on charge distribution. This finding challenges current models of nuclear structure and the understanding of how deformation and pairing interact to determine the fundamental properties of nuclei. The results of this study open new avenues for investigating the interaction between protons and neutrons in the context of nuclear deformation. Understanding this odd-even staggering in deformed nuclei could provide crucial insights into the nature of nuclear forces and how nucleons are organized within the nucleus. Future research will focus on developing theoretical models that can reproduce and explain this phenomenon, potentially leading to a revision of our understanding of nuclear structure in regions of deformation.

CERN's FCC: A Debate Between Scientific Ambition and Climate Footprint

CERN's proposed Future Circular Collider (FCC), a large-scale project in particle physics, is at the heart of an intense public debate. Objections raised include an "unacceptable carbon footprint," the requirement for two EPR nuclear reactors for its operation, an estimated cost of billions of euros that could be reallocated to other priorities, and claims of unclear scientific objectives. This discussion, which emerged in the summer of 2026 within the Particles and Fields Division of the French Physical Society, questions whether a collider of this magnitude is compatible with efforts to combat climate change. A recent essay addresses these criticisms, confronting emotionally charged concerns with an analysis based on orders of magnitude and a clear scientific vision. The study distinguishes between a project's immediate carbon footprint and its full balance sheet, including the costs of postponing or abandoning the initiative. It also explores ways to mitigate its impact, such as the potential use of natural hydrogen, an uncertain prospect. The aim is to provide a solid foundation for the public debate, organized by France's National Commission for Public Debate and the Swiss consultation, targeting a broad audience beyond the particle physics community. The essay frames the discussion about the FCC as a fundamental choice about the kind of society we want to build: one organized around renunciation and ever-shrinking horizons, or one that transforms its production methods to choose a desirable future. The decision regarding the FCC, therefore, is not solely based on climate considerations, but requires an evaluation of the real consequences of the available choices.

Planckian bound for thermalization proven using information theory

A team of researchers has achieved a rigorous demonstration of the Planckian bound for thermalization, a fundamental principle in quantum physics that establishes the maximum speed at which a quantum system can reach thermal equilibrium. This bound, expressed as ħ/kT (where ħ is the reduced Planck constant, k is the Boltzmann constant, and T is the temperature), is crucial for understanding the dynamics of quantum systems and has implications in fields such as quantum computing and condensed matter. The new proof is based on principles of information theory, offering a novel perspective on this phenomenon. Traditionally, thermalization has been studied using tools from statistical mechanics and perturbation theory. However, this new approach utilizes concepts such as von Neumann entropy and mutual information to establish the Planckian bound. The demonstration not only reaffirms the validity of this limit but also provides a deeper understanding of the underlying mechanisms governing the speed of thermalization in quantum systems. This advance is significant because out-of-equilibrium quantum systems are an area of intense research, and the ability to predict and control their thermalization is essential for the development of new technologies. This work opens new avenues for exploring the fundamental limits of quantum dynamics. A better understanding of the speed at which quantum systems can thermalize is vital for designing more efficient quantum computing algorithms, developing quantum materials with specific properties, and advancing our comprehension of phenomena like decoherence. The application of information theory to quantum thermodynamics problems is a growing field, and this demonstration underscores its potential to resolve fundamental questions in physics.

New Variational Method for Simulating Stochastic Quantum Dynamics in Bosons

Researchers have developed a new variational framework for efficiently simulating stochastic quantum dynamics in interacting bosonic systems. This method generalizes the time-dependent variational principle to accommodate quantum state diffusion processes, tailored for describing quantum trajectories unraveled from a master equation. The approach allows for analysis beyond semiclassical approximations, capturing both the dynamics of individual quantum trajectories and the Lindblad evolution of the system. The key to the method lies in using a wavefunction ansatz composed of a coherent superposition of Gaussian wavepackets. This choice enables the fully analytical assembly of the variational equations of motion, significantly contributing to the computational efficiency of the model. The ability to handle complex interactions in bosonic systems opens new avenues for exploring quantum phenomena in materials and devices. The new framework has been rigorously benchmarked on a Bose-Hubbard dimer and applied to extended Bose-Hubbard lattices in both one and two dimensions. A notable result from these simulations is the emergence of a symmetry-breaking phase transition in two-dimensional lattices, a phenomenon absent in one-dimensional chains. This finding underscores the importance of dimensionality in the appearance of collective quantum properties and provides a robust tool for investigating these complex systems.

Deep Thermalization Redefines Emergence of Quantum Statistics

Recent advances in quantum simulation have unveiled two key phenomena: deep thermalization and Hilbert space ergodicity. These concepts describe the emergence of quantum state distributions that are, in a precise sense, "maximally random." This finding offers a new perspective on how irreversible statistical mechanics can arise from reversible unitary quantum dynamics, moving beyond conventional theories of quantum thermalization and equilibration. These phenomena are encompassed within a unifying framework rooted in quantum information theory and maximum entropy principles. This framework explains the different forms of ergodicity that emerge under various physical constraints. Deep thermalization and Hilbert space ergodicity are crucial for understanding how many-body quantum systems reach thermodynamic equilibrium from an initial pure state. Currently, research is focused on several directions, including generalizing these concepts beyond ensembles of pure quantum states, studying phase transitions in deep thermalization linked to deeper forms of ergodicity-breaking, and connecting to broader topics in quantum thermalization and ergodicity. Furthermore, these advancements have potential applications in quantum information science, such as for benchmarking or tomography of quantum systems.

Equivalence in Quantum Channel Discrimination without Fixed Causal Order

A new study demonstrates that, for any pair of finite-dimensional memoryless quantum channels, parallel, adaptive, and general testing strategies achieve the same Stein exponent for any fixed type-I error tolerance, ε ∈ (0,1). This exponent is the regularized channel relative entropy, a fundamental result in quantum information theory. The finding implies that adaptability offers no asymptotic advantage in channel discrimination, simplifying the understanding of fundamental limits in quantum information transmission and processing. Distinguishability is a central concept in information theory, naturally extending to quantum systems. The quantum Stein theorem establishes asymptotic limits for distinguishing between two quantum states with a bounded type-I error probability. This work extends this theorem to quantum channels, which describe the evolution of quantum states. The novelty lies in considering scenarios where the causal order of operations is not necessarily defined, opening the door to a deeper understanding of quantum information in complex contexts. The key step in establishing this equivalence was proving the continuity of the regularized sandwiched Rényi channel divergence at order one. Furthermore, the researchers derived the exact strong-converse exponent for general testers. When this exponent's rate is finite, any larger type-II exponent forces the probability of correctly accepting the null hypothesis to decay exponentially. This reinforces the conclusion that adaptive testing strategies provide no asymptotic advantage over simpler ones in this context.

Multimode and Random-Access Optical Quantum Memory via Adiabatic Phase Imprinting

Scientists have successfully developed an optical quantum memory capable of storing and retrieving quantum information in multiple temporal modes with random access. This breakthrough is crucial for the development of large-scale quantum networks, as quantum memories are fundamental components for the distribution and processing of quantum states. The ability to store several temporal modes simultaneously and access them non-sequentially opens new avenues for building more efficient quantum repeaters and distributed processing nodes. The method employed is based on the adiabatic phase imprinting technique, which allows for precise manipulation of light properties for storage in an atomic medium. Unlike previous approaches, this technique facilitates more robust control over stored quantum states, minimizing decoherence and maximizing retrieval fidelity. The demonstration of random access means that information can be read in any order, an essential feature for operational flexibility in complex quantum architectures. This development represents a significant step towards the realization of quantum computing and communication. The integration of multimode and random-access quantum memories is vital for overcoming current distance and speed limitations in quantum networks, enabling the construction of more powerful and versatile quantum infrastructures in the future. Next steps will include improving coherence times and system scalability for practical applications.

Autonomous Floquet Engineering for Bosonic Codes via Reinforcement Learning

Researchers have developed an innovative method to control quantum systems through autonomous Floquet engineering, utilizing reinforcement learning. This approach enables the manipulation of bosonic codes, a promising architecture for fault-tolerant quantum computing, without the need for constant human supervision. Floquet engineering involves the periodic application of pulses to a quantum system to induce new properties or protect quantum states, and its automation is a crucial step towards the scalability of quantum computers. Reinforcement learning, a branch of artificial intelligence, is used for an AI agent to learn how to optimize the sequence and parameters of Floquet pulses. The agent interacts with a model of the quantum system, receiving rewards for actions that lead to desired quantum states or increased coherence. This iterative process allows the algorithm to discover control strategies that might be difficult to design manually, especially in complex systems with many degrees of freedom. The application of this technique to bosonic codes is particularly relevant. These codes encode quantum information in the different states of a quantum harmonic oscillator, offering advantages in error protection. The ability to autonomously apply Floquet engineering to these codes could significantly enhance their robustness and reliability, bringing closer the realization of a quantum computer capable of overcoming current limitations of decoherence and errors.

New Hybrid Quantum-Classical Technique for Image Compression

A novel hybrid approach combining quantum computing with classical deep learning has proven effective for grayscale image compression and decompression. This method integrates a Variational Quantum Daubechies Wavelet Transform (V-QDWT) and a trainable Quantum Convolutional Neural Network (QCNN). The strategy enables image-adaptive multi-resolution analysis and entanglement-based feature reduction, encoding input images using a Normal Arbitrary Superposition State (NASS) representation for compact and scalable quantum storage. For decompression, the system employs inverse QCNN and V-QDWT circuits to reconstruct coarse image features. Subsequently, a classical Super-Resolution Generative Adversarial Network (SRGAN) enhances perceptual quality. Experimental evaluations on benchmark grayscale datasets demonstrate that the quantum pipeline closely rivals classical JPEG2000 standards. Subsequent SRGAN refinement substantially pushes the boundaries of reconstruction, achieving superior structural fidelity with a PSNR of 30.0667 dB, an SSIM of 0.8744, and a Histogram Correlation of 0.9244. This advance highlights the potential of combining variational quantum compression with classical deep learning to enable efficient, scalable, and perceptually aware image processing in quantum-enhanced computing environments. Histogram analysis and qualitative comparisons further validate the restoration of fine textures and intensity distributions, paving the way for future applications in visual data processing with enhanced quantum capabilities.

Repairability of Inexact Solvers in Recursive State Estimation

A new study addresses the “repairability” of approximate numerical solutions used in recursive state estimation, a fundamental process in control and filtering systems. Researchers have characterized when a correction within a prescribed subspace and norm budget can meet a local admissibility tolerance. This approach is crucial because, in these systems, high accuracy in individual steps does not necessarily guarantee better overall performance. The work focuses on a fixed linear Kalman model, analyzing how introduced defects affect the covariance response over a finite horizon. The study reveals that by centering each defect on the exact gain for the implemented covariance, the current solve error is separated from inherited gain drift. Expanding the exact residual-drift identity shows opposing quartic contributions beyond the quadratic response: innovation-covariance inflation enters positively, while local-gain reoptimization enters subtractively. Under matched initialization, an absolute sixth-order remainder bound, uniform over bounded defect sequences at a fixed horizon, provides sufficient conditions for quadratic under- or overprediction. The research also explores the use of machine learning to propose bounded corrections. A learner-independent residual certificate and verified fallback mechanism govern the execution of classical and quantum candidates without changing the reference estimator. In a power-grid tolerance study, learned correction lowered the minimum conjugate-gradient iteration count for deployment without fallback relative to uncorrected solves under the same residual certificate. Furthermore, gains reconstructed from a variational quantum linear solver and from an annealing-based binary encoding, with small-scale terminal measurements on superconducting hardware and sampling on a quantum annealer, were executed through the same interface. This framework, by linking local repairability to nonlinear error propagation, evaluates approximate solvers and learned corrections through independent certification and finite-horizon response, providing a practical basis for studying hybrid quantum-classical computation.

New Optimal Bound for Reduction in Quantum Hypothesis Testing

Researchers have determined the optimal universal coefficient for reducing multiple quantum hypothesis testing to binary tests. They have shown that the error probability of a global Pretty Good Measurement (PGM) is at most four times the sum of the optimal binary error probabilities. This result holds for any finite ensemble of quantum states in any Hilbert space dimension, whether finite or infinite. Furthermore, by constructing a family of regular-simplex ensembles, they have proven that this coefficient of four is optimal, even when considering arbitrary global measurements. This finding improves upon previous pairwise bounds established by Cheng and Liu, providing an explicit and sharp guarantee for the standard PGM itself. The methodology employed is simpler than prior approaches, avoiding the construction of sequential measurements and the use of union bounds. Instead, the team relied on purely matrix-analytic techniques, combining a direct block Gram matrix analysis of the PGM error with a refined inequality between quantum Hellinger distance and Bures χ²-divergence. Their analysis also yielded a refined, one-shot pairwise Chernoff bound and an explicit sufficient number of copies required to achieve a desired discrimination error. This advancement has significant implications for the efficiency and precision of quantum state discrimination, a fundamental pillar in quantum computing and quantum metrology, where the ability to distinguish between different quantum states with the lowest possible error probability is crucial.

Fault-Tolerant Quantum Computation with Logarithmic Time Overhead

Researchers have demonstrated that fault-tolerant quantum computation can be achieved with strictly logarithmic time overhead and constant space overhead. This advancement improves upon previous results that included additional subpolylogarithmic factors, bringing closer the feasibility of robust and scalable quantum computers. Fault tolerance is crucial for quantum computing, as qubits are inherently fragile and prone to errors due to decoherence and environmental interactions. The main construction proposed utilizes polynomial-subrank transversal logical $\CCZ$ gates on good quantum locally testable codes (qLTCs) to implement addressable universal computation. This is achieved by transferring batches of logical qubits between dense storage and active logical subspaces while reusing the same ancillary workspace. The $\CCZ$ gates are implemented directly by the transversal operation, meaning only stabilizer resource states require separate preparation, simplifying the process. Furthermore, an alternative construction is presented that also achieves purely logarithmic time overhead. This is based on modifying the quantum Reed-Solomon magic-state distillation scheme. Recursively applying a fixed distillation circuit, protected by qLTCs of increasing block length, eliminates the subpolylogarithmic time factor. These methods offer promising avenues for overcoming one of the biggest challenges in quantum computing development: efficient error management with limited resources.

New Quantum Locally Testable Codes from Product Expansion

Researchers have developed a new class of quantum locally testable codes (LTCs) that promise robustness for quantum computing. These codes exhibit constant rate, distance, soundness, and locality, which are crucial properties for error correction in quantum systems. The construction is based on a variant of the product expansion conjecture by Bafna and Vyas, applied to Reed-Solomon codes, known for their efficiency in classical error correction. This advance is significant because LTCs are fundamental for building fault-tolerant quantum computers, an indispensable requirement to overcome the inherent fragility of qubits. The method used for constructing these LTCs falls within the high-dimensional expansion framework proposed by Dinur, Lin, and Vidick. This theoretical framework allows for designing quantum codes with desirable properties by utilizing complex geometric structures. In this particular case, the researchers instantiated the framework with non-Abelian cubical complexes by Rungtanapirom, Stix, and Vdovina. The key to success lies in equipping these complexes with carefully chosen Reed-Solomon local codes whose symmetries are compatible with those of the underlying complex structure. This compatibility ensures that the error correction properties are maintained globally within the quantum code. The relevance of this work lies in the creation of codes that are not only efficient in detecting and correcting errors but also allow for local verification of quantum operations, without needing to access the entire quantum state. This greatly simplifies the design and operation of future quantum computers. Although the construction relies on a conjecture, its future empirical or theoretical validation could open the door to practical implementations of fault-tolerant quantum computing, bringing us closer to the realization of large-scale quantum devices.

Entangled Inputs in Quantum Channels Outperform Classical Communication

Researchers have designed a family of finite-dimensional quantum channels demonstrating unbounded classical communication capacity when using entangled inputs. This finding sharply contrasts with the communication rate achieved with product states, which tends to zero. The work presents an explicit construction that validates the fundamental advantage of entanglement in information transmission through quantum channels, a concept previously addressed with probabilistic constructions. The proposed construction combines deterministic Clifford unitaries for qudits with a binary measurement and classical feedforward. This approach allows for the derandomization of a previous probabilistic construction by Hastings. The key element lies in the meticulous design of the measurement and feedforward process, which enables the derivation of one-copy and two-copy output entropy bounds from moment estimates alone, without requiring strong convergence of the underlying unitary family. This breakthrough is significant as it provides concrete proof of how entanglement can enhance quantum communication beyond classical limits. While quantum communication has been extensively explored, the demonstration of channels with unbounded gains via entangled inputs is an important step towards understanding and developing more robust quantum communication technologies. The study opens new avenues for research into optimizing information transmitted through quantum channels and could have implications for the design of future quantum communication systems.

Complexity of Finding Decoherence-Free Subspaces is QMA-Complete

A recent study has shown that determining whether an open quantum system possesses a decoherence-free subspace (DFS) is a computationally hard problem, on par with the most complex problems in quantum computation theory. Specifically, it has been classified as QMA-complete (Quantum Merlin Arthur-complete) for Markovian systems described by the time-independent Lindblad master equation. DFSs are crucial in quantum information, as they allow the preservation of quantum coherence for states within them, protecting them from detrimental environmental interaction and facilitating the development of robust quantum technologies. The research addresses this problem within the context of Markovian open quantum systems, governed by the time-independent Lindblad master equation. To do this, the authors introduce the k-Local Lindbladian problem, which captures the difficulty of computing purity decay rates under Lindbladian dynamics. It is shown that both problems are hard for the QMA complexity class when the locality k ≥ 5. The first problem (existence of DFS) is classified as QMA-hard with perfect completeness, while the second (k-Local Lindbladian) is QMA-complete. The hardness construction generalizes Kitaev's clock Hamiltonian construction to the open quantum system setting. This method encodes the execution of a quantum circuit into the steady subspace of a Lindbladian, which contains both pure and mixed history states. This subspace is then mixed depending on the output of the encoded circuit. These findings suggest that deciding whether a generic Markovian open quantum system admits a decoherence-free subspace is intractable even for quantum computation, with significant implications for the design of error-resilient quantum systems.