We establish efficient algorithms for weakly interacting quantum spin systems at arbitrary temperature. In particular, we obtain a fully polynomial-time approximation scheme for the partition function and an efficient approximate sampling scheme for the thermal distribution over a classical spin space. Our approach is based on the cluster expansion method and a standard reduction from approximate sampling to approximate counting.

Using a generalized quantum gate teleportation protocol, copies of a gate encoding a known or unknown parameter can be sent to multiple receivers. We extend this quantum gate broadcast protocol to a quantum network on directed acyclic graphs in which agents can add phase gates to be distributed and pass unknown phase gates to subsequent receivers. The broadcast resource state is defined by a sequential local unitary circuit over the graph and requires a unitary circuit depth equal to the distance from the sink to the source of the directed acyclic graph, similarly to the Greenberger-Horne-Zeilinger state on a spin chain. We show that, using projective measurements and feedforward unitary operations, the broadcast resource state can be prepared efficiently in finite time.

The process of dissociative electron attachment to the ${\mathrm{HC}}_3\mathrm{N}$ molecule is considered using a theoretical approach. Two ${}^2\mathrm{\Pi }$ resonances located, respectively, at 1.089 and 6.373 eV play the major role in the process and are accounted for in the present approach. The cross section for dissociative electron attachment through the two resonances is computed using a model in which the molecular dissociation—occurring after the electron is captured into one of the two resonance states—proceeds along two vibrational degrees of freedom, responsible for dissociation towards the $\mathrm{H}+{\mathrm{C}}_3{\mathrm{N}}^{-}$ products.

The nonradiative electron-capture process was investigated for 66.3-MeV/u hydrogenlike cesium ions colliding with ${\mathrm{N}}_2$, Ar, Kr, and Xe gas targets at the internal gas-jet-target terminal of the Heavy Ion Research Facility at Lanzhou-Cooling Storage Ring. For each target, the cesium $K$-shell x-ray emissions were measured at ${35}^{\circ }$ relative to the projectile beam. These spectra contain contributions from He-like cesium formed through single-electron capture, as well as from H-like cesium produced via $1s$-electron excitation. From the measured x-ray spectra, the intensity ratio of x rays emitted from H-like cesium to those from He-like cesium was extracted and found to decrease rapidly with increasing target atomic number. Based on these ratios, the absolute electron-capture cross sections into $n>1$ were determined in combination with calculated electron excitation cross sections. The results were compared with predictions from the relativistic eikonal approximation and continuum distorted-wave theory. The experimental data exhibit reasonable agreement with the continuum distorted-wave calculations, whereas the relativistic eikonal approximation overestimates systematically the measured cross sections.

We examine the spin-resolved photoionization of randomly oriented chiral molecules via circularly polarized light, and revisit earlier predictions of Cherepkov [J. Phys. B: At. Mol. Phys. 16, 1543 (1983)]. We will show that the dynamical origin of spin- and enantio-sensitive observables arise from two intrinsic mechanisms that are quantified by two pseudovectors stemming from the geometric properties of the photoionization dipoles in spin space and in real space, and an extrinsic mechanism which is a directional bias introduced by the well-defined direction of light polarization. These mechanisms arise solely from electric-dipole interactions. Consequently, this means that the ten independent parameters that were earlier predicted by Cherepkov to fully describe the spin-resolved photoionization of chiral molecules can be reduced as moments of these three pseudovectors. We also find that the molecular pseudoscalars describing the spin- and enantio-sensitive components of the yield can be described by the flux of these pseudovectors through the energy shell, which changes sign upon switching enantiomers. Our results provide compact expressions for these observables which provide an intuitive picture on what determines the strength of these spin- and enantio-sensitive observables. The approach can be readily generalized to photoexcitation, multiphoton processes, and arbitrary field polarizations. Regardless of the specific driving conditions, the resulting spin- and enantio-sensitive observables are still controlled by the same three pseudovectors, underscoring their universal role as the primary generators of chirality-induced spin asymmetries, emphasizing their fundamental geometric origin and the universality of the mechanism identified here.

Angular distributions of photoelectrons with a defined spin orientation, released by the absorption of a single circularly polarized photon from randomly oriented chiral molecules, are derived and analyzed within the electric-dipole approximation. The revealed symmetry properties of the distributions support previous findings that the total spin polarization of photoelectrons is independent of the enantiomer of a chiral molecule, whereas the angular distribution of the spin polarization is enantiosensitive. Numerical applications to the achiral HI and chiral 1-iodo-2-methylbutane molecules illustrate that these analytical results also hold for multiphoton ionization processes.

V-type three-level systems, where two excited states share a common ground state, serve as fundamental models for exploring coherent light-matter interactions in a range of quantum systems, from atomic gases to semiconductor nanostructures. In this work, we investigate the coherent evolution of such a system under strong femtosecond-pulse excitation by numerically solving the optical Bloch equations. Our analysis shows that the coherent evolution of a three-level system critically depends on the product of the excitation-pulse duration and energy separation between the excited states. Building on this understanding, we extend our analysis to simulate two-dimensional coherent spectra in a high-intensity regime. We demonstrate a control over the coherent pathway contributions to the nonlinear optical response of a V-type system by varying the intensity of the excitation pulses. This control is manifested through the ability to selectively turn individual spectral features on or off in the two-dimensional spectra, each corresponding to distinct quantum pathways. Furthermore, the pulse intensities are varied to precisely adjust the phase of these peaks. Our approach provides a simple and robust framework for achieving control of coherent response of multilevel systems.

A limitation of analog quantum simulators based on cold atoms in optical lattices is that readout is typically limited to observables diagonal in the charge basis, i.e., densities and density correlation functions. To overcome this limitation, we propose experiment-friendly schemes to measure charge-off-diagonal correlations (such as currents). Our protocols use noninteracting dynamics for random times followed by standard quantum gas microscope measurements to effectively measure in random bases. The main requirement of our scheme is the ability to turn off interactions, which can be done in many atomic species using Feshbach resonances. Importantly, our scheme requires no local control and otherwise also exhibits modest requirements in terms of total evolution time and number of repetitions. We numerically demonstrate efficient estimation of bilinear correlation functions, requiring less than 4000 samples to measure local currents to 5% error (system-size independent) and $\sim {10}^4$ samples to simultaneously measure all nonlocal correlations in 70-site systems. Due to its simplicity, our protocol is implementable in existing platforms, and thus paves the way to precision measurements beyond particle number measurements.

Memory devices for single photons are notable components for quantum information processing and quantum communications. The present study investigates the possibility of achieving the storage of light at the level of single photons inside nanofibers by exploiting stimulated Brillouin scattering. We present first the standard approach using a coherent buffer in a nanoscale waveguide by transferring the optical signal coherently to an acoustic wave that can be extracted by the reverse process. The life time of the acoustic wave put a limitation on the applicability of such an approach for single-photon signals. We introduce a configuration for achieving a slow signal at the level of single photons without gain or loss. The process utilizes photon-phonon Brillouin interactions involving two counterpropagating pump fields. The photon storage is achieved through the time delay of a significantly slow signal inside nanowires. We address the condition for obtaining negligible influence due to the scattering off thermal phonons.

We develop a systematic temporal coupled-mode theory tailored for a single Floquet-sheet resonator governed by time-modulated conductivities. By invoking photon-number conservation during frequency conversion, we derive characteristic radiative decay rates and coupling coefficients that explicitly account for the frequency ratio between different channels. We establish a direct analytical connection to the Floquet transfer-matrix method, providing closed-form analytical expressions that map scattering parameters to the Drude-type physics of the sheet. Our model captures the resonant coupling between the zeroth-order propagating channel and the negative first-order surface-mode channel. Validated by full-wave numerical simulations, the proposed theory remains accurate even with the inclusion of intrinsic material loss. This approach provides an intuitive pole-expansion representation for designing time-varying photonic interfaces and understanding the underlying physics in two-dimensional temporal systems.

We address two distinct nonlinear propagation problems in nonlinear optical waveguide arrays (WGAs) with nonreciprocal (non-Hermitian) couplings. First, we investigate the light propagation launched by initial excitations of two different types. The single-channel excitation creates stable solitons supported by the interplay of the Kerr nonlinearity and non-Hermitian skin effect (NHSE). In this case, we derive, by means of the symbolic-regression method, an analytical formula defining the soliton existence boundary. For the broad-pulse excitation, we produce perturbed soliton solutions analytically in the continuum approximation, which is accurately corroborated by numerical results. We thus conclude that NHSE accelerates the propagation of the broad soliton toward the boundary, ultimately causing tight localization at the edge, which is a hallmark of the NHSE in the continuum limit. Second, we identify stationary solitons in the system—specifically, nonlinear bulk modes in the Hermitian regime and near-edge skin solitons in the non-Hermitian one. The nonlinear bulk modes are compressed toward the edge of the WGA under the action of the nonreciprocality, which is the nonlinear extension of NHSE.

Birefringence is commonly associated with anisotropic or chiral media, but optical nonlinearity can also induce effective symmetry breaking, enabling circular birefringence even in isotropic materials through helicity-dependent Kerr phase shifts. Extending this concept to the orbital angular momentum (OAM) degree of freedom, optical vortices provide a natural platform to explore handedness-dependent nonlinear effects. In homogeneous isotropic media, however, Kerr-induced coupling to higher-order vortex states leads to modal instability, rendering orbital-handedness-dependent phase shifts ill defined. Here, we show that this limitation is naturally lifted in optical fibers, where guided vortex modes with opposite orbital handedness are mutually orthogonal eigenmodes of the waveguide. Using a full-vectorial formulation, we demonstrate that modal orthogonality suppresses Kerr-induced OAM cascading, resulting in a well-defined and stable nonlinear phase splitting between vortex modes of opposite topological charge. Our results establish optical fibers as a robust platform for Kerr-induced orbital handedness birefringence and provide a unified framework for understanding nonlinear birefringence phenomena in structured light beyond polarization.

We consider the motion of a quantum particle whose position is measured in random places at random moments in time. We contrast this motion with the motion of a quantum particle in a potential which varies randomly in space and in time, which could also be thought of as (possibly thermal) noise. We calculate expectations of observables both linear and nonlinear in the density matrix. We demonstrate explicitly that while linear observables cannot distinguish between random measurements and random noise, measurable distinctions can be seen in nonlinear observables.

Accurate estimation of observables in quantum systems is a central challenge in quantum information science, yet practical implementations are fundamentally constrained by the limited number of measurement shots. In this work, we explore a variation of the classical shadows protocol in which the measurements are kept local while allowing the resulting classical shadows themselves to be correlated. By constructing locally optimal shadows, we obtain unbiased estimators that are competitive with state-of-the-art methods in terms of measurement overhead, while requiring only single-qubit measurements and allowing for the estimation of any observable in pure postprocessing, reducing estimation errors by orders of magnitude over standard classical shadows. We validate our approach through numerical experiments on molecular Hamiltonians with up to 40 qubits, consistently observing significant reductions in estimation errors, including for estimations of multiple chemically relevant observables simultaneously.

Many current quantum error-correcting codes that achieve full fault tolerance suffer from having low ratios of logical to physical qubits and significant overhead. This makes them difficult to implement on current noisy intermediate-scale quantum computers and results in the inability to perform quantum algorithms at useful scales with near-term quantum processors. As a result, calculations are generally done without encoding. We propose a middle ground between these two approaches: constructions in the $\left[\right[n,n-2,2\left]\right]$ quantum error-detecting code that can detect any error from a single faulty gate by measuring the stabilizer generators of the code and additional ancillas at the end of the computation. This achieves weak fault tolerance. As we show, this yields a significant improvement over no error correction for small computations with low enough physical error probabilities and requires much less overhead than codes that achieve full fault tolerance. We give constructions for a set of gates that achieve universal quantum computation in this error-detecting code, while satisfying weak fault tolerance up to analog imprecision on the physical rotation gate.

Quantum entanglement, one of the most fundamental and nonclassical features of quantum systems, stands in sharp contrast to classical correlations. Meanwhile, exceptional points—spectral degeneracies unique to non-Hermitian systems—possess distinct topological structures that give rise to a range of intriguing phenomena. For example, dynamically encircling an exceptional point (EP) along a closed parameter loop leads to chiral mode transitions that depend critically on the encirclement direction. In this work, we propose a scheme to deterministically generate quantum entangled states by exploiting the chirality associated with different encircling directions in a non-Hermitian superconducting circuit. We reveal that the degree of entanglement is significantly enhanced—reaching its theoretical maximum—only when the encircling trajectory topologically encloses the EP. In realistic superconducting circuits, however, realizing such topological evolution poses a fundamental trade-off between the adiabaticity required for state following and the inevitable signal loss induced by environmental dissipation. To quantitatively resolve this trade-off, we employ dynamic magnetic flux modulation to precisely tune the superconducting parameters, pushing the driving period to its optimal value. These results establish a robust connection between non-Hermitian topology and quantum-state synthesis, suggesting a promising pathway toward novel applications in quantum information processing.

Temporal boundary effects provide a potential route to treat time as an additional degree of freedom in the design of optical devices, while a key challenge in this context is the controllable manipulation of temporal reflection and refraction components. In this work, we demonstrate that the abrupt phase modulation of couplings in lattice models enables tunable redistribution of the reflected and refracted intensities, and even allows control over their propagation angles. Our results demonstrate that phase engineering provides an effective strategy for controlling temporal boundary dynamics in lattice systems.

In this paper, we suggest a chiral interaction scheme to enhance magnon bundle emission by placing a qubit and a magnon into a cascaded-cavity setup, respectively. It is found that the unidirectional interaction prolongs the lifetime of the target excited state, thereby suppressing the magnon reexcitation and promoting both the average purity and number of two-magnon bundles. Consequently, the chiral interaction not only offers directional control but also improves the quality of the multimagnon source, which may find potential applications in quantum information processing.

We model and investigate the collective nonlinear optical response of an ensemble of two-level emitters that are weakly coupled to a waveguide featuring a single spatial mode. Our approach generalizes the insight that photon-photon correlations in the light scattered by a single two-level emitter result from two-photon interference to the case of many emitters. Using our model, developed for coherent driving in the weak-saturation regime, we study different configurations for probing the nonlinear response of the ensemble, e.g., through the waveguide or via external illumination, and derive analytical expressions for the second-order quantum coherence function, $g^{\left(2\right)}\left(\tau \right)$, as well as for the squeezing spectrum of the output light in the waveguide, $S_{\theta }\left(\omega \right)$. For the transmission of resonant guided light, we recover the same predictions as previously made with more involved theoretical models when analyzing experimental results regarding $g^{\left(2\right)}\left(\tau \right)$ [Prasad et al., Nat. Photon. 14, 719 (2020)] and $S_{\theta }\left(\omega \right)$ [Hinney et al., Phys. Rev. Lett. 127, 123602 (2021)]. We also study the transmission of light that is detuned from the transition of the two-level emitter, a situation that we recently studied experimentally [Cordier et al., Phys. Rev. Lett. 131, 183601 (2023)]. Our model predictions show how the collectively enhanced nonlinear response of weakly coupled emitters can be harnessed to generate nonclassical states of light using ensembles ranging from a few to many emitters.

This work extends quantum optical models of high harmonic generation towards open systems, by considering a quantum stochastic analysis of the field modes coupled to an environment. We study the open system dynamics by solving the quantum Langevin equation for a nonlinear driven cavity coupled to an environment. For an unstructured environment without memory, we show that the emission characteristics of the intense driven cavity is isomorphic to the process of high harmonic generation and a nonlinear antenna. This is achieved by using the quantum regression theorem for the Markovian dynamics, and further allows to obtain the upper bound of the radiated power due to the emitter fluctuations. This opens the path to connect current quantum optical approaches of high harmonic generation with open system descriptions and studies driven-dissipative systems in the nonlinear regime.