Domain walls in disordered ferromagnets—interfaces separating oppositely magnetized regions—are commonly modeled as elastic interfaces in random media. However, real domain walls possess internal degrees of freedom related to the local magnetization direction at the wall's midpoint, which are not captured by simple elastic interface models. Here, we investigate how these internal degrees of freedom influence Barkhausen noise arising from the jerky field-driven motion of linelike domain walls in disordered thin ferromagnetic films with strong perpendicular magnetic anisotropy. Using a recently developed reduced model that incorporates internal wall dynamics, we simulate systems two orders of magnitude larger than those accessible via full micromagnetic simulations. We focus on Bloch line dynamics and identify a disorder-driven mechanism where Bloch lines accumulate near the edges of moving wall segments during avalanches, thereby impeding further propagation. This mechanism leads to distinctive statistical features in Barkhausen avalanches, including pronounced bumps in the cutoffs of the avalanche distributions, skewness of the average avalanche shape evolving with the avalanche duration, and reduced domain wall roughness at large scales.

Simulating time evolution is one of the most natural applications of quantum computers and is thus one of the most promising prospects for achieving practical quantum advantage. Here, we develop quantum algorithms to extract thermodynamic properties by estimating the density of states (DOS), which is a central object in quantum statistical mechanics. We introduce several key innovations that significantly improve the practicality and extend the generality of previous techniques. First, our approach allows one to estimate the DOS only for a specific subspace of the full Hilbert space. This is crucial for fermionic systems, since both canonical and grand canonical ensemble thermal equilibrium properties depend on subspaces of fixed number. Second, in our approach, by time evolving very simple, random initial states, such as randomly chosen computational basis states, we can exactly recover the DOS on average. Third, due to circuit-depth limitations, we only reconstruct the DOS up to a convolution with a Gaussian window—thus all imperfections that shift the energy levels by less than the width of the convolution window will not significantly affect the estimated DOS. For these reasons, we find the approach is a promising candidate for early quantum advantage as even short-time, noisy dynamics can yield a semiquantitative reconstruction of the DOS (convolution with a broad Gaussian window), while early fault-tolerant devices will likely enable higher-resolution DOS reconstruction through longer time evolutions. We demonstrate the practicality of our approach in representative Fermi-Hubbard and spin models and indeed find that our approach is highly robust against algorithmic errors in the time evolution and against gate noise. We further demonstrate that our approach is compatible with noisy intermediate-scale quantum (NISQ) computing NISQ-friendly variational techniques, introducing and leveraging a technique for variational time evolution.

We use the transfer tensor method to analyze localization and transport in simple disordered systems, specifically the Anderson and Aubry-André-Harper models. Emphasis is placed on the memory effects that emerge when ensemble-averaging over disorder, even when individual trajectories are strictly Markovian. We find that transfer tensor memory effects arise to remove fictitious terms that would correspond to redrawing static disorder at each time step, which would create a temporally uncorrelated dynamic disorder. Our results show that while eternal memory is a necessary condition for localization, it is not sufficient. We determine that signatures of localization and transport can be found within the transfer tensors themselves by defining a metric called “outgoing pseudoflux.” This work establishes connections between theoretical research on dynamical maps and Markovianity and localization phenomena in physically realizable model systems.

By applying density functional theory calculations, we study the electronic and magnetic properties of ${\mathrm{KFe}}_2{\mathrm{As}}_2$, which exhibits two superconducting domes under pressure. We find there are competing magnetic instabilities from itinerant electrons in both superconducting phases. The lower $T_c$ of the first superconducting phase at ambient pressure can be attributed to the competing instabilities at $(\pi ,0,\pi )$, which favor the double stripe type or plaquette antiferromagnetic order. The second superconducting phase is further suppressed by the competing ferromagnetic and stripe antiferromagnetic instabilities at higher pressure. In addition, we find that applying pressure acts as self-doping, leading to variations in instabilities from the itinerant electrons. Our results imply that the instability of the Pauli susceptibility from itinerant electrons is a possible quantity that describes the various characters of ${\mathrm{KFe}}_2{\mathrm{As}}_2$ under pressure and have important implications for understanding the pressure manipulation of superconductivity in iron-based superconductors.

Belief propagation (BP) can be a useful tool to approximately contract a tensor network, provided that the contributions from any closed loops in the network are sufficiently weak. In this article, we describe how a loop series expansion can be applied to systematically improve the accuracy of a BP approximation to a tensor network contraction, in principle converging arbitrarily close to the exact result. More generally, our result provides a framework for expanding a tensor network as a sum of component networks in a hierarchy of increasing complexity. We benchmark this proposal for the contraction of infinite projected entangled pair states, either representing the ground state of an Affleck-Kennedy-Lieb-Tasaki (AKLT) model or with randomly defined tensors, where it is shown to improve in accuracy over standard BP by several orders of magnitude while incurring only a minor increase in computational cost. These results indicate that the proposed series expansions could be a useful tool to accurately evaluate tensor networks in cases that otherwise exceed the limits of established contraction routines.

Entanglement underpins the power of quantum technologies, yet it is fragile and typically destroyed by dissipation. Paradoxically, the same dissipation, when carefully engineered, can drive a system toward robust entangled steady states. However, this engineering task is nontrivial, as dissipative many-body systems are complex, particularly when they support multiple steady states. Here, we derive analytic expressions that predict how the steady state of a system evolving under a Lindblad equation depends on the initial state, without requiring integration of the dynamics. These results extend Refs. [V. V. Albert and L. Jiang, Phys. Rev. A 89, 022118 (2014); V. V. Albert et al., Phys. Rev. X 6, 041031 (2016)], showing that while the steady-state manifold is determined by the Liouvillian kernel, the weights within it depend on both the Liouvillian and the initial state. We identify a special class of Liouvillians for which the steady state depends only on the initial overlap with the kernel. Our framework provides analytical insight and a computationally efficient tool for predicting steady states in open quantum systems. As an application, we propose schemes to generate metrologically useful entangled steady states in spin ensembles via balanced collective decay.

Magnetic domain walls (DWs) are topological defects that host robust low-energy modes, which can be harnessed for classical and neuromorphic computing. However, accessing their quantum dynamics has remained an outstanding challenge. Using concepts from cavity optomechanics, we show that a geometric, Coriolis-type interaction between localized DWs and extended magnon modes in short insulating ferromagnetic wires enables efficient cooling of DWs to their quantum ground state and the preparation of nonclassical states with negative Wigner functions, detectable in the power spectrum of emitted magnons. We further demonstrate that magnons can mediate long-range entangling interactions between qubits encoded in spatially separated DWs, providing a route toward universal quantum gate operations. Our proposal relies solely on intrinsic degrees of freedom of the ferromagnet and naturally extends to ferrimagnets and antiferromagnets, as well as to other confined magnetic textures such as vortices and skyrmions in insulating nanostructures.

Stability and efficiency are mutually exclusive in a thermodynamic process, e.g., in a thermal machine. Any effort to reduce the fluctuations of a certain output quantity is necessarily accompanied by an increase of entropy production, therefore lowering its efficiency. This interplay is beautifully captured by the so-called thermodynamic uncertainty relations (TURs), which set a lower bound on the relative uncertainty of a current for a given rate of entropy production. Their status in hybrid normal-superconducting (N-S) devices has remained unsettled. We show that, in the subgap regime, departures from the normal quantum TUR are governed by macroscopic superconducting coherence quantified by the pair amplitude, and that introducing a dephasing probe suppresses this coherence and restores the bound. We further derive a hybrid quantum TUR that is general for two-terminal N-S junctions in the Andreev regime: the inequality is never violated, is saturated only at vanishing current, and is related to the normal quantum bound under the replacement $e\to 2e$. For N-S quantum dot and Cooper-pair-splitter systems, we compute current and noise and show that deviations from the normal bound track the pair amplitude on the central region. The results establish a direct link between superconducting macroscopic coherence and nonequilibrium fluctuations and supply a general bound for the Andreev regime.

When an intense laser pulse strikes a flat solid target at normal incidence, only odd, equally spaced harmonics are typically generated. Using particle-in-cell simulations, we show that introducing transverse spatial modulation—either via two beams incident at small opposing angles or via a single beam reflecting from a corrugated surface—produces a two-dimensional harmonic spectrum. Selecting the emission through a narrow slit isolates an angle-dependent subset of harmonics, forming a tunable frequency comb controlled by both the viewing angle and the drive-laser frequency. Distinct combs arise for $s$- and $p$-polarized emission, while increasing the focal spot size reduces divergence and improves efficiency. Additional control is achieved by varying beam frequencies, target tilt, or beam number. Replacing the dual-beam configuration with a single beam and structured surface provides equivalent functionality with significantly reduced experimental complexity.

Resonances are ubiquitous in wave physics and shape the spectral response of every linear physical system. Yet their description remains fragmented, through partial and sometimes competing definitions. Here, we show that the resonance properties are fully encoded in the phase of the response functions of physical systems and can be decoded by deriving the spectral function of the phase. The frequency derivative of the phase is related to the Wigner-Smith time-delay operator, but is applied to individual elements of the scattering matrix. By deriving the meromorphic expansion of this scalar element-wise time delay in terms of poles and zeros in the complex frequency plane obtained from the knowledge of both phase and amplitude of the response function, we demonstrate that it yields a quality function whose peaks reveal both the strength and the position of each resonance. This expression proves that poles only are not sufficient to fully characterize resonances and that zeros bring the other part of the information. The numerical calculation of the quality function together with the analytical and balanced treatment of poles and zeros provide a rigorous framework for identifying and quantifying resonances without any prior knowledge of the physical system.

Superposed orders of quantum channels have already been proved—both theoretically and experimentally—to enable unparalleled opportunities in the quantum communication domain. Superposition of orders can be exploited within the quantum computing domain as well, by relaxing the (traditional) assumption underlying quantum computation about applying gates in a well-defined causal order. In this context, we address a fundamental question arising with quantum computing: Whether superposed orders of single-qubit gates can enable universal quantum computation. As shown in this paper, the answer to this key question is a definitive “yes.” Indeed, we prove that any two-qubit controlled quantum gate can be deterministically realized, including the so-called Barenco gate that alone enables universal quantum computation.

We present an experimental and theoretical study of the infrared-active phonon modes of heavily potassium (K) doped $p$-terphenyl, a material of great current interest due to reports of high-temperature superconductivity. First, we show that the phonon features in the fingerprint region of the midinfrared domain can be well reproduced through the density functional theory (DFT) quantum-chemical simulations. Compelling evidence for a distorted organic molecular framework is obtained here from the emergence of up to three absorption bands that are predicted to be infrared (IR)-inactive under idealized $D_{2h}$ planar symmetry and a systematic softening of specific vibrational modes. Computational models of twisted conformers exhibiting reduced $D_2$ or $C_{2h}$ symmetry are shown to successfully reproduce the activation of these silent modes and the redshifting shown by others. Second, we present an analysis of the lineshape of the experimental IR-phonon modes in a wide temperature range from 6 to 300 K. We show that several of the IR phonons have a strongly asymmetric shape that is indicative of a Fano-like interaction with the doped charge carriers. Notably, we find that the phonon asymmetries, and the corresponding Fano parameters, undergo anomalous changes around 100 K that are reminiscent of a charge localization transition around 90 K that has been previously reported from a corresponding study of the low-energy electronic response.

Encoding a qubit in the continuous degrees of freedom of a quantum system, such as bosonic modes, is a powerful alternative to modern quantum error correction (QEC). Among the most prominent bosonic QEC codes, binomial codes provide protection against loss and dephasing errors by encoding logical states in finite superpositions of Fock states with binomially weighted coefficients. While much attention has been given to their error-correcting capabilities and integration into fault-tolerant architectures, efficient methods for generating arbitrary binomial codewords remain scarce. In this work, we propose a scheme for generating these codewords by exploiting nonlinear multiphoton interactions between a continuous-variable bosonic mode (oscillator) and a two-level system (spin/qubit). Our proposed scheme assumes the ability to prepare the oscillator in an arbitrary Fock state and the qubit in an arbitrary superposition of its basis states and access to arbitrarily high multiphoton interactions. To enhance the experimental feasibility of our scheme, we further demonstrate how to reduce the required order parameter of multiphoton interactions by a factor of two for a special class of code states.

We present a broadband (THz to UV) ellipsometry study of the anisotropic dielectric response of the orthorhombic perovskite ${\mathrm{YAlO}}_3$. The ellipsometric measurements have been performed on ${\mathrm{YAlO}}_3$ crystals with three different surface cuts and for six high-symmetry configurations of the crystal axes with respect to the plane of incidence of the photons. The obtained data are presented in terms of the Mueller matrix elements $N, C$, and $S$, and their features are analyzed and discussed with respect to the anisotropy of the dielectric response tensor. In particular, in the infrared range we have identified all 25 infrared-active phonon modes that have been predicted from theoretical studies. We also discuss a negative refraction effect that naturally occurs in the vicinity of an anisotropic longitudinal-optical phonon. Moreover, we have determined the temperature dependence of the phonon parameters between 10 and 330 K. The dielectric response above the phonon range, from about 0.1 to 6.5 eV, is shown to be featureless and characteristic of an insulator with a large band gap above 6.5 eV and is well described by anisotropic Cauchy model.