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.

We report a nondissipative optomechanical approach to separate the spin and orbital components of transverse energy flows of light beams. Focusing on uniformly polarized paraxial fields, the method relies on detecting the radiation-pressure-induced reorientation of the optical axis in anisotropic media—specifically, liquid crystals. Within a perturbative analytical approach, we demonstrate the selective encoding of the transverse spin and orbital components of the Poynting vector into orthogonal orientational elastic modes, whose relative weights are mediated by the Gouy phase. The spatial distributions of spin and orbital momentum can therefore be retrieved without resorting to point-by-point measurements using absorbing or scattering probe particles. Analytical results are supported by a fully self-consistent numerical solution coupling Maxwell’s equations and Landau-de Gennes nematodynamics in three dimensions. This exact framework captures higher-order and nonparaxial effects beyond the analytical perturbative regime and allows us to validate the predicted Gouy-phase-mediated optomechanical response. These results establish a practical route toward optomechanical imaging of the internal momentum structure of light, fostering momentum-resolved optical metrology and light-matter interaction studies with structured beams.

Fluctuating hydrodynamics provides a framework connecting mesoscopic fluctuations with macroscopic transport behavior. To bridge mesoscopic and macroscopic transport from microscopic dynamics, we introduce a wave-number-dependent viscosity, defined via the equilibrium correlation of time-averaged Fourier components of the fine-grained shear stress field. Two-dimensional molecular dynamics simulations reveal its small-wave-number divergence characteristic of the renormalized viscosity, while its large-wave-number behavior determines the bare viscosity, thereby establishing a link between mesoscopic and macroscopic transport based on microscopic dynamics.

We classify gapped phases and characteristic nodal points of non-Hermitian band structures on two-dimensional nonorientable parameter spaces. Such spaces arise in a wide range of physical systems in the presence of nonsymmorphic parameter space symmetries. For gapped phases, we find that nonorientable spaces provide a natural setting for exploring fundamental structural problems in braid group theory, such as torsion and conjugacy. Gapless systems, which host exceptional points (EPs), explicitly violate fermion doubling, even in two-band models. We demonstrate that EPs traversing the nonorientable parameter space exhibit non-Abelian charge inversion. These braided phases and their transitions leave distinct signatures in the form of bulk Fermi arc degeneracies, offering a concrete route toward experimental realization and verification.

We employ the operational quasiprobability (OQ) as a work distribution, which reproduces the Jarzynski equality and yields the average work consistent with the classical definition. The OQ distribution can be implemented through two simple experimental settings: the end-point measurement and the two-point measurement. Using this framework, we demonstrate the explicit contribution of coherence to the work, which can be formulated by the second-law-type relation. In a two-level system, we show that nonjoint measurability, a generalized notion of measurement incompatibility, can increase the amount of extractable work beyond the classical bound imposed by jointly measurable measurements. We further prove that the real part of Kirkwood-Dirac quasiprobability (KDQ) and the OQ are equivalent in two-level systems, and they are non-negative for binary unbiased measurements if and only if the measurements are jointly measurable. In a three-level nitrogen-vacancy center system, the OQ and the KDQ exhibit different amounts of negativities while enabling the same work extraction, implying that the magnitude of negativity is not a faithful indicator of nonclassical work. These results highlight that coherence and nonjoint measurability play fundamental roles in the enhancement of work.

Complex fluids such as colloids and emulsions are ubiquitous in biopharmaceuticals, manufacturing, and the oil industry, but controlling their viscosity can pose a significant challenge. Low viscosities are ideal for mixing, high viscosities are helpful for pharmaceuticals, and switchable viscosities are useful for smart materials. One strategy to make multipurpose fluids is to make nonequilibrium modifications to the microstructure of colloids and emulsions using external fields, which can result in emergent changes in their rheological characteristics. We introduce an experimental technique, electrorheoimaging (ERI), which directly visualizes microstructure during rheological measurements while simultaneously applying an electric field. We also introduce a continuous phase that lowers electrohydrodynamic thresholds and enables experiments at modest field strengths, revealing behaviors previously inaccessible in laboratory settings. Using this combination of technique and material design, we observe shear-induced banding that only appears in the presence of electric depending on the frequency of the electric field, at either high or low shear rates. These microstructural transitions produce dramatic, reversible viscosity modulation—not only increases, but also decreases, including negative intrinsic viscosities. Frequency control enables instantaneous switching among multiple effective viscosities spanning a 30-fold range, establishing ERI as a powerful tool for probing and controlling rheology in field-responsive soft materials.