The two-body Dirac equation for positronium is studied with the Coulomb and Breit interactions. When adding an annihilation correction, its energies agree with experiment up to fourth order in powers of the fine structure constant if the Breit term is treated perturbatively. We analyze why this is not true if the Dirac equation is solved numerically. Symmetries of the 16-component Dirac spinors are first detailed. For each symmetry, coupled differential equations verified by their radial components are established in a self-adjoint form convenient for numerical calculations. Nondifferential relations between these components are derived. The coupled equations are first solved in the Coulomb case with the Lagrange-mesh method. With only four mesh points, accurate binding energies are obtained which match the best published results except for the ${}^1{S}_0$ and ${}^3{S}_1$ states. A detailed study of the radial spinor components of the ground states of para- and orthopositronium explains why the convergence of their energies stops at an accuracy of about ${10}^{-9}$ atomic units. A curious property of the Lagrange-mesh method is emphasized. Analytical approximations of these radial components are derived and compared with numerical solutions. The obtained Coulomb-Dirac spinors provide an original variant of first-order perturbation treatment of the Breit operator. The Coulomb-Breit-Dirac coupled equations are then solved under the numerical conditions tested with the Coulomb case. The differences between these numerical energies and the correct perturbation results at fourth order are analyzed. The unphysical numerical values of energies are shown to be due to couplings with the negative continua.

We theoretically modeled and experimentally demonstrated the collision-induced interaction of ${}^{131}\mathrm{Xe}$ nuclear spins, including the Fermi contact interaction and wall-dependent quadrupole relaxation. We utilized heater-integrated, wafer-level microfabricated ${}^{87}\mathrm{Rb}\text{−}{}^{131}\mathrm{Xe}$ atomic vapor cells with a single-chamber cylindrical structure and a single-beam absorption-based atomic magnetometer to probe hyperpolarized ${}^{131}\mathrm{Xe}$ nuclei precession in situ. We investigated the interaction of ${}^{131}\mathrm{Xe}$ nuclear spins with ${}^{87}\mathrm{Rb}$ alkali-metal vapor atoms and atomic vapor cell walls consistent with the theoretical model. Furthermore, we measured a Fermi contact factor of approximately $510\pm 40$, which reveals an enhancement in the spin-exchange collisions for ${}^{87}\mathrm{Rb}\text{−}{}^{131}\mathrm{Xe}$, and an activation energy of $0.18\pm 0.02$ eV, which characterizes the desorption and diffusion process of ${}^{131}\mathrm{Xe}$ induced by the wall collision process. These results can inspire the development of chip-scale atomic spin devices with enhanced stability and sensitivity.

We study the effect of spin flip on tunneling ionization in the attoclock settings. Our study is based on a fully relativistic approach in which we seek a numerical solution of the time-dependent Dirac equation. We find that spin flip during strong field ionization significantly increases the attoclock offset angle, indicating longer tunneling delay relative to electrons with no spin flip. To interpret our numerical findings, we conduct additional modeling within the perturbative Breit-Pauli approach, which allows us to elucidate the role of different relativistic mechanisms. We show that the spin-orbit interaction is mainly responsible for the spin flip, rather than interaction with the magnetic component of the laser field. We further discuss the dynamic mechanisms of the spin-flip effect, suggesting that it takes place while the electron is still bound. Our findings indicate that the attoclock can be used to investigate electron spin-flip dynamics during the tunnel ionization process.

Free expansion following the removal of axial confinement represents a fundamental nonequilibrium scenario in the study of many-body ultracold gases. Using the stationary phase approximation, we analytically demonstrate that for all one-dimensional spinor gases with repulsive contact interactions, whether bosonic or fermionic, the asymptotic density and momentum distribution can be directly determined from the quasimomentum distribution (Bethe rapidities) of the trapped gas. We efficiently obtain the quasimomentum distribution numerically by solving the integral equations that characterize the ground state of the integrable system within the local density approximation. Additionally, we derive analytical solutions for both weakly and strongly interacting regimes. Unlike in bosonic gases, where rapidity distributions and density profiles vary significantly across interaction regimes, fermionic gases maintain similar profiles in both weakly and strongly interacting limits. Notably, the gas expands self-similarly only when released from a harmonic trap. For other power-law trapping potentials, the asymptotic density profile is strongly influenced by the initial confinement geometry. Our results extend readily to Bose-Fermi mixtures and finite temperatures.

We demonstrate the formation of vector Kerr cavity solitons in a pulse-pumped normal dispersion fiber Kerr resonator. Such solitonic structures formed in normal-dispersion resonators typically manifest as flat-top solitonic pulses, known as platicons. The two orthogonally polarized modes inside the fiber resonator are pumped with the same conditions and the generation of vector platicons (VPs) is achieved via a spontaneous symmetry breaking (SSB) of the intracavity structures in a process when scanning the pump frequency from the blue-shift to the red-shift detuning side. The characteristics of the SSB are studied in detail. Also, the impact of the third-order dispersion (TOD) and the desynchronization on the VP dynamics are studied, revealing that the oscillating structure on the leading edge of VPs can be alleviated by introducing a desynchronization effect. What's interesting is that desynchronization can be used as an additional degree of freedom to control VP behaviors. For a small desynchronization, we observe the energy exchange between the two orthogonally polarized modes. For a large desynchronization, periodic and deterministic switching dynamics between the two orthogonally polarized modes can be obtained.

The removal of photons from certain quantum light sources produces so-called photon-subtracted states with enhanced mean photon numbers and intricate quantum correlations. Here, we propose an integrated photon-subtraction scheme that, contrary to previous approaches, is not heralded by photon correlation (coincidence) measurements. In this way, our technique exploits the “Welcher Weg” (“which way”) information problem, as it does not provide information about the modes from which the subtracted photons emanated. We show that this lack of information allows the generation of multiphoton states endowed with rich quantum correlations that strongly depend on the parity, evenness or oddness, of the number of subtracted photons.

The Husimi phase distribution, an experimentally measurable quantity, is investigated for single-mode and two-mode squeezed vacuum states. The analysis highlights that non-Gaussian operations, i.e., photon subtraction (PS), photon addition (PA), and photon catalysis (PC), are effective tools for localizing phase distribution and enhancing phase robustness in the presence of noise, while PC results in comparatively slower delocalization accompanied by more intricate phase features. The work highlights the perspective that the combined effects of squeezing, beam-splitter transmittance, and environmental interactions must be carefully considered when quantum state engineering protocols are designed, and phase properties provide a valuable insight into this endeavor.

In this work we present a coupled-cluster theory for the propagation of multireference electronic systems initiating at general quantum mechanical states. Our formalism is based on the infinitesimal analysis of modified cluster operators, from which we extract a set of additional operators and their equations of motion. These cluster operators then can be used to compute electronic properties of interest in a size-extensive manner. The equations derived herein are also studied for the free propagation of linear superpositions. In this regime, we derive asymmetric matrix elements for observables, and a resymmetrization factor so they become consistent with their quantum mechanical counterparts. Our formalism is applied to a four-electron/four-level model with a Hubbard-like Hamiltonian, where we show that it reproduces the numerically exact results for observables, populations, and coherences.

In this article, we investigate the spectral broadening of cosh-Gaussian pulses which is induced by self-phase modulation (SPM) within the tunneling-induced transparency (TIT) window in an asymmetric multiple-quantum-well system. The study demonstrates how a single laser field, combined with a steady static electric field, enables dynamic control of absorption and dispersion profiles of the system, offering precise control of the group velocity of the applied field. The multiple-quantum-well system exhibits a large Kerr nonlinearity (approximately ${10}^6{\text{W}}^{-1}{\text{m}}^{-1}$), which enhances the SPM effect under the TIT window. Our exploration of SPM using cosh-Gaussian pulses demonstrates that higher probe laser powers result in broader spectral widths and increased internal oscillations, especially under strong tunneling strengths. These results suggest promising applications for fine-tuning light propagation in quantum-well systems and offer a more economical approach for optical light-matter interaction experiments, minimizing the need for extensive optics and optomechanical components.

Though known to be present, the accessibility of spacelike vacuum entanglement capable of being a fundamental resource for quantum information processing has remained in question at distances beyond the scale of vacuum fluctuations in massive fields. For a broad subclass of physical many-body mixed Gaussian states, including the free scalar field vacuum, the logarithmic negativity is here shown to be a necessary and sufficient measure of entanglement and to be entirely accessible by pairs of single-mode detectors in the continuum. By deriving exact and optimal detection profiles, entanglement resources in the massive field are demonstrated to be available at all distances.

Quantum annealing (QA) has attracted research interest as a sampler and combinatorial optimization problem (COP) solver. A recently proposed sampling-based solver for QA significantly reduces the required number of qubits, being capable of large COPs. In relation to this, a trainable sampling-based COP solver has been proposed that optimizes its internal parameters from a dataset by using a deep-learning technique called deep unfolding. Although learning the internal parameters accelerates the convergence speed, the sampler in the trainable solver is restricted to using a classical sampler owing to the training cost. In this study, to utilize QA in the trainable solver, we propose classical-quantum transfer learning, in which parameters are trained classically, and the learned parameters are used in the solver with QA. The results of numerical experiments demonstrate that the trainable quantum COP solver using classical-quantum transfer learning improves convergence speed and execution time compared with the original solver.

Atomic magnetometers based on Zeeman shift measurement have the potential for high sensitivity and long-term stability. Like other atomic sensors, including atomic clocks and atom interferometers, the atomic magnetometer could, in principle, be augmented with spin squeezing for further sensitivity enhancement. However, existing atomic magnetometers are not compatible with spin squeezing because the atoms can hardly be in a pure quantum state during operation. Moreover, a natural challenge faced by an atomic magnetometer is the arbitrary direction of the magnetic field. In this paper, we propose a cold-atom-based magnetometer with spin squeezing that can measure both the magnitude and the direction of an arbitrary magnetic field. For experimentally accessible parameters, we show that the technique described above could potentially achieve a sensitivity nearly two orders of magnitude higher than that of the best existing magnetometers.

The transmission of low-energy electrons through polyethylene terephthalate nanocapillaries is studied both by experiments and simulations. It is found that the transmission of electrons with incident energy occurs mostly within the geometrically allowed angle and they exit along the incident direction. The rate of the directly transmitted electrons decreases with time and their angular distributions broaden as the incident charge increases. A tentative simulation is performed to explain the experimental results by taking into account the deposited charges on the capillary walls, the image force, and electron scattering processes. The blocking of the transmission of the electrons with the incident energy is explained to be from the formation of the attractive Coulomb field, from the opposite (positive) charge to the incident electron inside the capillaries due to the impact of the projectiles and the production of secondary electrons. The broadening of the full angular distribution for the directly transmitted electrons is attributed to the charging of the bulk of the capillary membrane. This provides evidence for the controversial issue of the production of charge patches and their role in transmitting electrons through insulating nanocapillaries.

Detailed comparisons between theory and experiment for quantum electrodynamics (QED) effects in He-like ions have been performed in the literature to search for hints of new physics. Different frequentist statistical analyses of the existing atomic transition energy data have shown contradictory conclusions as to the presence of possible deviations from the theory predictions. We present here an approach using Bayesian statistics to assign quantitative probabilities to the different deviation models from theory for He-like ions for $Z=5–92$. Potential deviations beyond the standard model or higher-order QED effects are modeled with $f\left(Z\right)\propto Z^k$ functions. Considering the currently available data, no significant difference between theory and experiment is found, and we show that recent experiments have reduced the possible deviations previously observed in the literature. Using past measurements and a weighted average on the different deviation models, we estimate the accuracy required for future measurements to investigate possible anomalies.

The results of a study of the dynamics of atomic moments alignment in cesium under optical pumping by linearly polarized resonant light in ultraweak magnetic field are presented. It is shown that there are alignment components whose relaxation does not depend on spin-exchange broadening. The effect of suppression of spin-exchange relaxation in zero magnetic fields is detected, which is similar in its manifestations to the SERF (spin-exchange relaxation free) effect observed in orientation signals. This observation is interesting from the standpoint of general theory since the law of conservation of angular momentum responsible for maintaining orientation in the SERF mode should not guarantee the preservation of alignment under the same conditions. A comparison with theoretically calculated parameters of SERF resonances in orientation is given. A qualitative explanation of the observed effect is presented.

The preparation of highly entangled states involving multiparticle systems is of crucial importance in quantum physics, playing a fundamental role in exploring the nature of quantum mechanics and offering essential quantum resources for nascent quantum technologies that surpass classical limits. Here we present how to generate and manipulate a tripartite entangled state of photons, phonons, and magnons within a hybrid cavity magnomechanical system. It is shown that simultaneously applying two coherent driving fields to this system in opposite input directions enables a coherent and effective way to regulate the magnomechanical interaction by tuning the phase difference of the driving fields. Based on this feature, it is found that the tripartite entanglement also becomes phase dependent and can be enhanced for certain phase differences. More interestingly, it is shown that the robustness of tripartite entanglement against environmental thermal noises can also be improved by choosing a proper phase difference of the driving fields. Our findings offer a promising way to manipulate and protect fragile tripartite entanglement, which is applicable to a wide range of quantum protocols that require multipartite entangled resources such as quantum communication and quantum metrology.

Intermode breathers can be triggered by the avoided mode crossings in microresonators. Here, we numerically and experimentally explore the breather and single soliton regimes as the pump-power and pump-cavity detuning vary. Below a pump-power threshold, a single soliton regime is obtained within a detuning range. Above this threshold, an intermode breather section emerges, resulting in a soliton-breather-soliton pattern in which the system transits from a stable soliton to a breathing soliton and back to a stable soliton as the detuning increases.

We propose a scheme for the heralded generation of entangled states, such as Bell states, between two magnetic spheres individually coupled to a common waveguide. By deriving the quantum trajectory equation, we describe the system's dynamics under continuous monitoring and reveal how the waveguide output influences its internal evolution. Based on this formalism, we analyze the two- and three-magnon resonance conditions and demonstrate the two- and three-photon bundles reflected from the waveguide when it is driven from one side. Under these conditions, the detection outcomes allow us to infer the internal magnon states, namely the detection of the first photon in a bundle heralds the successful generation of an entangled state. We further examine abnormal photon emission events and potential experimental imperfections, highlighting the crucial role of detector efficiency in achieving high-fidelity entanglement. Our work provides a robust method for entanglement generation in waveguide magnonics systems.