We study two uniformly accelerated two-level atoms moving in the right Rindler wedge and interacting with a massless scalar field. Using a perturbative detector-model framework, we derive analytical expressions for the single- and double-excitation amplitudes and organize the results in the symmetric and antisymmetric Dicke basis. The excitation probabilities exhibit the standard Unruh (Planck) factor together with interference terms that depend on the interatomic separation, thereby controlling the population of the symmetric and antisymmetric components. The calculation yields joint atom-field states whose one-excitation sector naturally admits a Dicke-basis decomposition, providing a clear description of how the excitation dynamics is distributed among the symmetric and antisymmetric channels. These results establish an analytical framework for understanding how acceleration-induced thermality combines with spatial interference in multiatom detector settings.

Parametric x-ray radiation (PXR) is a tunable and versatile x-ray production mechanism that relies on relativistic electrons traversing through a crystal. When the crystal contains Mössbauer nuclei, they may also contribute to the x-ray generation, giving rise to spectrally narrow parametric Mössbauer radiation (PMR) as a promising alternative to undulator-based x-ray sources. It is well known that the x-ray generation strongly depends on the diffraction geometry. Motivated by this, here, we develop a framework for parametric x-ray generation in general diffraction geometries, encompassing the entire x-ray emission range from the conventional grazing extremely asymmetric diffraction (EAD) geometry with a large emission direction to the crystal surface up to the grazing specular diffraction (SD) geometry with emission near parallel to the surface. The framework further incorporates a range of physical effects arising from more realistic modeling of both the Mössbauer crystal and the electron beam. We show that the PMR generation in the grazing SD geometry qualitatively differs from the previously considered grazing EAD case, and explain the physical origins of these differences. We further apply our framework to analyze the geometry dependence of superradiant parametric Mössbauer radiation (SPMR) generated from microbunched electron beams provided by x-ray free-electron lasers. Our results set the stage for the exploration of PXR, PMR, superradiant amplification of parametric x-ray radiation, and SPMR beyond the established diffraction geometries.

In an optical cavity, strong light-matter coupling between excitons and photons has been widely reported as a way to enhance energy delocalization through spatially extended polaritonic states. In contrast, leveraging cavity-mediated light-matter effects to promote the reciprocal phenomenon, namely, energy localization, remains largely underexplored. In the present work, we address this question by focusing on a special form of energy localization arising from nonlinear matter interactions: quantum self-trapping (QST). We employ a generalized Tavis-Cummings model to investigate the transport of vibrational excitons, i.e., vibrons, between two anharmonic vibrational modes and examine their interplay with cavity photons. In the absence of a cavity, the arising of true and complete QST, i.e., a fully stabilized localization, is not possible due to the symmetry of the system. The energy transfer between the two modes still occurs, slowed down by the many-body interactions. Coupling the system to a single-mode cavity strongly alters this behavior, with two emerging regimes. First, at weak light-matter coupling, destructive interference between newly opened transition pathways suppresses energy exchange, leading to cavity-enhanced self-trapping. As the coupling strength increases, these interference effects evolve, leading to cavity-assisted energy transfer, where we observe an acceleration of the vibrational energy flow. Most notably, we identify critical coupling strengths that separate both regimes in which the dynamics almost totally freeze, suggesting the arising of a “stabilized” light-induced QST of many-vibron bound states. These results suggest that optical cavities can not only enhance transport but could also stabilize energy localization phenomena, providing a route to control energy flow in quantum systems.

Nonlinear spin systems exhibit rich and exotic dynamical phenomena, offering promising applications ranging from spin masers and time crystals to precision measurement. Recent theoretical work [T. Wang et al., Commun. Phys. 8, 41 (2025)] has predicted intriguing nonlinear dynamical phases arising from inhomogeneous magnetic fields and feedback interactions. However, experimental exploration of these predictions remains lacking. Here, we report the observation of nonlinear spin dynamics in dual-bias magnetic fields with dual-cell alkali-metal atomic gases and present three representative stable dynamical behaviors of limit cycles, quasiperiodic orbits, and chaos. Additionally, we probe the nonlinear phase transitions between these phases by varying the feedback gain and the difference of dual-bias magnetic fields. Furthermore, we demonstrate the robustness of the limit cycle and quasiperiodic orbit against the noise of magnetic fields. Our findings establish a versatile platform for exploring complex spin dynamics and open avenues for the realization of multimode spin masers, time crystals and quasicrystals, and high-precision magnetometers.

The interplay between multiple bands gives rise to non-Abelian defects, which are typically characterized by corresponding non-Abelian topological charges. Here we present a practical and intuitive approach for determining the non-Abelian topological charge of a defect based on its associated two-band Euler classes, without requiring knowledge of the Euler class of a defect pair. The approach is illustrated using a parity-time-symmetric tight-binding model that hosts braiding defects on a two-dimensional square lattice, and the resulting topological charges are consistent with those obtained from conventional spin Berry-Wilczek-Zee connection integrals. The factorization of the non-Abelian topological charge carried by a defect is naturally encoded in a set of two-band Euler classes, thereby elucidating the mechanism underlying defect formation.

We investigate how non-Markovian mechanical dissipation affects exceptional points in linearized optomechanical systems with red-sideband drive. For a chosen non-Ohmic mechanical bath, we derive analytical conditions for the memory-renormalized exceptional point by employing a pseudomode mapping, thereby demonstrating that structured environments displace the mode coalescence away from the Markovian prediction. Crucially, we reveal that failing to account for this memory-induced shift suppresses the divergent Petermann factor by orders of magnitude, showing that accurate bath modeling is essential for the successful operation of exceptional-point-based devices whenever reservoir-induced memory is non-negligible. We finally show that non-Markovianity modifies the cavity reflection spectrum, manifesting as a shallower optomechanically-induced-transparency dip, providing therefore an experimentally accessible signature of structured mechanical environments.

Exceptional points (EPs) are remarkable spectral degeneracies in a non-Hermitian system's parameter space, where both eigenvalues and eigenstates coalesce. Here, we show that in non-Hermitian molecular chiral systems the position of EPs in the parameter space is enantiomer-specific. First, we show that encircling the EP of one enantiomer drives robust topological population transfer in the chiral molecule while its mirror twin remains unaffected, offering a route for selective chiral control. Second, we reveal how resonant excitation of EPs in chiral molecules can amplify weak chiral effects, offering an alternative approach to the enhancement of chiral interactions. Third, we demonstrate that a twisted chiral fiber immersed in a liquid solution of chiral molecules exhibits topologically different behavior depending on the solution's enantiomeric excess, offering a topological approach to the detection of molecular chirality. Our results combine high enantiosensitivity with topological robustness in chiral discrimination and control, paving the way for alternative approaches in the exploration of non-Hermitian and chiral phenomena.

The quantum Mpemba effect occurs when a quantum system, residing far away from the steady state, relaxes faster than a relatively nearer state. We look for the presence of this highly counterintuitive effect in the relaxation dynamics of the operators within the open quantum system setting. Since the operators evolve under a nontrace preserving map, the trace distance of an operator is not a monotonically decaying function of time, unlike its quantum state counterpart. Consequently, the trace distance cannot serve as a reliable measure for detecting the Mpemba effect in operator dynamics. We circumvent this problem by defining a dressed distance between operators that decays monotonically with time, enabling a generalized framework to explore the Mpemba-like effect for operators. Applying the formalism to various open quantum system settings, we find that, interestingly, in the single qubit with Davies-type thermalizing model, only accelerated relaxation of operators is possible, while genuine Mpemba-like effects emerge in higher-dimensional systems such as qutrits and beyond. Furthermore, we demonstrate the existence of Mpemba-like effects in nonlocal, nonequilibrium operators, such as current, in a double-quantum-dot setup. Our results, in addition to offering fundamental insight about the occurrence of the Mpemba-like effect under non-trace-preserving dynamics, open avenues for experimental studies where quicker relaxation of observables could be of significant interest.

This work focuses on quantum reservoir computing and, in particular, on quantum Wiener architectures (qWiener), consisting of quantum linear dynamic networks with weak continuous measurements and classical nonlinear static readouts. We provide the first rigorous proof that qWiener systems retain the fading-memory property and universality of classical Wiener architectures, despite quantum constraints on linear dynamics and measurement backaction. Furthermore, we develop a kernel-theoretic interpretation showing that qWiener reservoirs naturally induce deep kernels, providing a principled framework for analyzing their expressivity. We further characterize the simplest qWiener instantiation, consisting of concatenated quantum harmonic oscillators, and show the difference with respect to the classical case. Finally, we empirically evaluate the architecture on standard reservoir computing benchmarks, demonstrating systematic performance gains over prior classical and quantum reservoir computing models.

We present a comprehensive computational framework for simulating nonadiabatic molecular dynamics with explicit inclusion of geometric phase effects. Our approach is based on a generalized two-level Hamiltonian model that can represent various electronic state crossings—conical intersections, avoided crossings, and elliptic intersections—through appropriate parametrization. A key challenge in mixed quantum-classical methods is that standard trajectory initializations place nuclear wave packets far from the crossing region with random electronic phases so that the ensemble generically undersamples the geometric phase effect and misses the coupling region entirely. Our prelooping scheme addresses this by jointly specifying the nuclear positions (near the crossing), momenta (tangential to orbits guided by the local Berry curvature), and electronic phase (set to the analytically computed Berry phase)—ensuring that trajectories begin with the full topological memory of a completed adiabatic loop. We introduce a novel prelooping trajectory initialization scheme, allowing us to encode the memory as an initial phase accumulated due to the adiabatic evolution over the potential energy surface. This is a unified framework to handle different types of level crossings by incorporating Berry curvature-based force corrections to Ehrenfest dynamics, ensuring accurate representation of topological effects. For conical intersections, our method incorporates the theoretically expected phase $\pi$, while for elliptic intersections, it yields a parametrically tunable but loop-radius- (energy) independent phase different from $\pi$. We also include an eccentricity parameter ($e$) in the diabatic coupling to model more realistic molecular systems. Numerical simulations demonstrate the consistency of our approach with theoretical predictions for mixing of states and inhibition from mixing due to geometric phase effects. This framework provides a valuable tool for studying quantum-classical interactions in molecular systems where geometric phase effects play a significant role.

We experimentally investigate the $4f_{5/2}\text{--}4f_{7/2}$ fine-structure splitting energies of Ag-like ions by optical emission spectroscopy of ${\mathrm{Lu}}^{24+}$, ${\mathrm{Hf}}^{25+}$, and ${\mathrm{Ta}}^{26+}$ using a compact electron beam ion trap. The measurements are enabled by an electron-impact metal injection technique, while the transition energies are determined with high accuracy using wavelength calibration based on reference emission from the same spatial region. The measured transition energies in the range $Z=71–73$ provide valuable benchmarks for relativistic many-electron calculations involving a $4f$ electron and, together with previous measurements, establish a consistent $Z$-dependent trend. Comparison with theoretical calculations over a wide range of $Z$ reveals systematic differences among the predictions.

Emission of a photon during the process of above-threshold ionization is investigated. In previous work [D. B. Milošević, Phys. Rev. A 108, 033110 (2023)] one of the present authors studied this process in the leading order of the strong-field approximation for the matrix element that describes direct emission of a photon during the transition from the bound state to the continuum. We now show that the correction that corresponds to photon emission while the electron is propagating in the continuum, very significantly modifies the process for all but some intermediate photon energies. In contrast to the lowest-order term of the previously considered expansion, the continuum correction is infrared divergent and resonant when the photon energy agrees with the energy of a driving photon. If the energy of the emitted photon is much larger than the energy of a driving photon, then the continuum correction is very close to the leading-order direct contribution and the two terms interfere constructively. These statements hold true for a variety of laser-field configurations; we investigated various bichromatic linearly and circularly polarized fields including bicircular fields.

We describe numerical simulations of the critical dynamics near the superfluid phase transition. The calculations are based on an implementation of a stochastic hydrodynamic theory known as model $F$ in the classification of Hohenberg and Halperin. This theory is expected to describe dynamic scaling near the lambda transition in liquid ${}^4\mathrm{He}$, Bose-Einstein condensation in ultracold atomic gases, and the superfluid transition in the unitary Fermi gas. Our simulation is based on a Metropolis algorithm previously applied to the critical endpoint of the liquid-gas phase transition in ordinary fluids. In the model $E$ truncation of model $F$ we obtain the expected dynamical exponent $z\simeq 3/2$. We observe the emergence of a propagating second sound mode at the phase transition. The second sound diffusivity $D_s$ is consistent with the scaling relation $D_s\sim {\xi }^{x_{\kappa }}$, where $\xi$ is the correlation length and $x_{\kappa }=1/2$.

Fundamental theories and models of many-body physics can be probed in experiments on ultracold atoms held in place by electromagnetic fields. In particular, of considerable interest are systems under curved confinement, since they can yield exotic states of matter which would be impossible to obtain in flat space. In this study we focus on relatively small samples, where curvature effects are stronger, and analyze by Monte Carlo simulations the peculiar structure arising in an assembly of soft-core bosons subject to a weak trapping potential with spherical symmetry. Upon suitable tuning of the parameters, 100 particles or so group together in clusters arranged in a shell with icosahedral symmetry. As the number of particles increases, a second shell gradually develops, concentric to (and partly overlapping with) the original one, where clusters are in perfect registry with the first shell, thus forming a dodecahedral pattern. Cluster arrangements with the symmetry of other polyhedra are seen for different sets of parameters. At low temperature the superfluid density is nonuniform in the radial direction; heating the system progressively, superfluidity eventually vanishes while still clusters are present, a behavior resembling the transition from supersolid to normal solid on a plane. Two shells of clusters are also observed in systems of classical or distinguishable quantum particles, but in those cases the shells are more fragile to thermal fluctuations.

Structured light beams with space-variant polarization can be efficiently generated using a voltage-tunable nematic liquid crystal ($Q$ plate). By appropriately selecting the input state and the retardation of the $Q$ plate, an optical field acquires a spatially structured polarization distribution that is capable of encoding nontrivial topological information across the beam profile. These features can be directly read out through interaction with plasmonic nanostructures, such as circular and spiral slits. Here we show that, upon illumination, polarization-dependent excitation of surface plasmons converts the hidden topology of the polarization structure into observable intensity distributions, including plasmonic vortices and characteristic interference patterns, while the tunability of the input parameters enables a rich variety of distinct topological forms.

It is intuitively believed that the eigenvalues of a physical system and the corresponding spectral splitting of the output spectrum respond similarly to perturbations, for instance, both scale as $\sqrt{\epsilon }$ in the vicinity of a second-order exceptional point (${\text{EP}}_2$), where $\epsilon$ denotes the perturbation. Here we challenge this conventional view by demonstrating that the spectral splitting can be tuned to scale as $\sqrt[4]{\epsilon }$ or $\sqrt[6]{\epsilon }$ around an ${\text{EP}}_2$ in a nonlinear quantum system consisting of two coupled cavity squeezing processes. When measuring $\epsilon$ based on the spectral splitting, we further show that the corresponding measurement sensitivity can be enhanced by orders of magnitude below the standard quantum limit (SQL) owing to the combined effect of sublinear signal response and suppressed quantum noise. Our findings fundamentally reshape the understanding of the relation between the eigenvalues and spectral splitting in response to perturbations and also open alternative avenues for EP-based sensing with noise suppression below the SQL.

The equations of time-dependent density functional theory are derived, via the expression for a quantum weak value, from ring polymer self-consistent-field theory using a mathematical correspondence between time and imaginary time. The imaginary-time path-integral formalism of Feynman, in which inverse temperature is seen to be a Wick rotation of time, allows one to write the equilibrium partition function of a quantum system in a form mathematically isomorphic to the path-integral expression for the dynamics. Therefore, the self-consistent-field theory equations which are solutions to the equilibrium partition function are Wick rotated back into a set of dynamic equations, which are shown to give an expression for a quantum weak value of the one-particle density. Remarkably, weak values emerge naturally here without being postulated, as an intermediate step before recovering the standard expression for the density. The weak-value expression in turn leads to the equations of time-dependent density functional theory. This first-principles derivation does not use the theorems of density functional theory, which are instead applied to guarantee equivalence with standard quantum mechanics. An expression for finite-temperature dynamics is also given, which shows that a ring polymer model for quantum particles holds for time-dependent systems as well as equilibrium situations. Issues arising in time-dependent density functional theory, such as causality, initial-state dependence, and $v$-representability, are discussed in the context of the ring polymer derivation.

Non-Hermitian evolution is mathematically invertible, yet finite dynamic range imposes a sharp operational limit on reversibility. We identify precision-induced irreversibility: Amplification, mode mixing (as warranted by non-normality), and a finite resolution floor—whether set by detector noise, environmental fluctuations, or numerical precision—conspire to produce a quantitative predictability horizon $T_{\mathrm{of}}$, beyond which distinct states collapse onto identical representations. Within the effective non-Hermitian description, the mechanism requires neither environmental decoherence nor nonlinear dynamics; remove any ingredient and reversibility can be restored. Echo-fidelity tests confirm this transition across arbitrary-precision arithmetic and hardware, revealing where formal invertibility and physical reversibility diverge.

We study the computational complexity of the guided local Hamiltonian problem: given a local Hamiltonian $H$ together with a classical description of a guiding state that has non-negligible overlap with the ground state of $H$, estimate the ground-state energy within inverse-polynomial precision. This setting captures real-world scenarios in quantum chemistry and many-body physics, where trial states derived from classical heuristics can be used to guide quantum algorithms. We identify families of physically motivated guiding states for which the computational hardness of ground-state energy estimation persists in the guided setting. Extending prior results for semiclassical subset states, we prove BQP-hardness for classes including fixed-weight states, matrix product states, Gaussian states, and Fendley states. Our hardness results are obtained via refined Feynman-Kitaev circuit-to-Hamiltonian constructions that explicitly expose the structural role of the guiding state in the reduction. Complementing these results, we give a constructive proof of BQP containment when the guiding state admits a polynomial-size classical description, establishing BQP-completeness for the canonical formulation of the problem. Our results show that quantum advantage persists for the newly introduced state classes, and classical methods also remain viable when said guiding states admit appropriate descriptions. Together, our results identify a Goldilocks zone of guiding states that are efficiently preparable, succinctly described, and sample-query accessible, within which quantum advantage for ground-state estimation can be meaningfully assessed. We additionally formalize the guided Fermi-Hubbard Hamiltonian problem and prove BQP-completeness on two-dimensional square and triangular lattices, both with and without magnetic fields, when provided with an appropriate fermionic guiding state.