The finite-basis-set method is commonly used to calculate atomic spectra, including quantum electrodynamics contributions such as bound-electron self-energy. Still, it remains problematic and underexplored for vacuum-polarization calculations. We fill this gap by trying this approach in its application to the calculation of the vacuum-polarization charge density and the Wichmann-Kroll correction to the electron binding energy in a hydrogen-like ion. We study the convergence of the method with different types and sizes of basis sets. We cross-check our results for the Wichmann-Kroll correction by direct integration of the Green's function. As a relevant example, we consider several heavy hydrogen-like ions and evaluate the vacuum polarization correction for $S$ and $P$ electron orbitals.

Solitonlike localized wave solutions in a two-dimensional Fermi superfluid are studied by solving the Bogoliubov–de Gennes equations in the BCS regime of weak pairing interactions. The dispersion relations of these solitons are found to exhibit a peculiar swallow-tail shape, with cusps and multiple branches. The effective mass of the solitons is found to diverge and change sign at the cusp. This behavior is in contrast to the smooth dispersion relations and negative effective masses of solitons in the three-dimensional Fermi superfluid. The swallow-tail dispersion relations are shown to be a consequence of counterflow of the superfluid and sign-changing contributions to the superfluid current from different transverse momenta in the Bogoliubov–de Gennes formalism. The results are relevant for the understanding of solitonic excitations in two-dimensional Fermi superfluids, such as ultracold atomic gases and high-temperature superconductors.

Single-shot coherent Rayleigh-Brillouin scattering (CRBS) is modeled using direct simulation Monte Carlo (DSMC). In CRBS, an optical lattice generated by the interference of two pump lasers traps the neutral particles via the dipole force. The gradient of refractive index due to the trapped particles of the gaseous media leads to coherent scattering of a probe beam, resulting in the CRBS signal. Additionally, using a chirped laser beam, CRBS signals can be obtained in a single laser shot, shortening the measurement time of the gas flow (on the order of 100 ns). The DSMC results of single-shot CRBS assuming Maxwellian velocity distribution functions (VDFs) show good agreement with experimental data, including argon, carbon dioxide, and xenon, for different gas pressures. One of the key observations obtained from the present simulations is that the CRBS signals become asymmetric when using a fast chirp rate due to finite time for collisionless trapping and collisional thermalization.

The accurate determination of the position-dependent energy fluence rate of scattered light (which is proportional to the energy density) is crucial to the understanding of transport in anisotropically scattering and absorbing samples, such as biological tissue, seawater, atmospheric turbulent layers, and light-emitting diodes. While Monte Carlo simulations are precise, their long computation time is not desirable. Common analytical approximations to the radiative transfer equation (RTE) fail to predict light transport and could even give unphysical results. Therefore, we experimentally probe the position-dependent energy fluence rate of light inside scattering samples where the widely used $P_1$ and $P_3$ approximations to the RTE fail. The samples are three-dimensional (3D) aqueous suspensions of anisotropically scattering and both absorbing and nonabsorbing spherical scatterers, namely, microspheres ($r=0.5\mu \mathrm{m}$) with and without absorbing dye. To probe the energy fluence rate, we detect the emission of quantum-dot reporter particles that are excited by the incident light and that are contained in a thin capillary. By scanning the capillary through the sample, we access the position dependence. We present a comprehensive discussion of experimental limitations and of both random and systematic errors. Our observations agree well with the Monte Carlo simulations and the $P_3$ approximation of the RTE with a correction for forward scattering. In contrast, the $P_1$ and the $P_3$ approximations deviate increasingly from our observations, ultimately even predicting unphysical negative energies.

Based on (i) the physical principle of local causality and (ii) a certain notion of elements of reality, Einstein, Podolsky, and Rosen put forward an argument showing that physical instances may exist in which two noncommuting observables can be joint elements of the physical reality. Here, we introduce a new criterion of joint reality. We demonstrate that according to this criterion, quantum mechanics generally prevents noncommuting observables from having joint elements of reality. In addition, we introduce a measure to quantify the extent to which the criterion is violated and explore the implications of such a measure in connection with incompatibility and correlations. Our findings suggest new manners of interpreting quantum phenomena.

We compare the performance of protective quantum measurements to that of standard projective measurements. Performance is quantified in terms of the uncertainty in the measured expectation value. We derive an expression for the relative performance of these two types of quantum measurements and show explicitly that protective measurements can provide a significant performance advantage over standard projective measurements.

Although genuine multipartite entanglement (GME), as one quantum resource, is indispensable in quantum information processing, most of the existing measures cannot detect GME faithfully. In this paper we present a GME measure, namely, the minimum pairwise concurrence (MPC), by introducing pairwise entanglement, which characterizes the entanglement between two single-qubit subsystems of a multipartite system without tracing out the remaining qubit. The pairwise entanglement can be obtained by combining the entanglement of the reduced subsystem and the three-tangle. Compared with existing measures, the MPC measure outperforms the previous ones in many aspects. Due to its fine properties, it thus is believed that the MPC could be a good candidate for achieving potential quantum tasks and also could facilitate the understanding of GME.

This paper presents strategies to improve the performance of digitized counterdiabatic quantum optimization algorithms by cooptimizing gate sequences, algorithm parameters, and qubit mapping. Demonstrations on real hardware validate the effectiveness of these strategies, leveraging both algorithmic and hardware advantages. Specifically, our approach achieves an average increase in approximation ratio of $4.49\times$ without error mitigation and 84.8% with error mitigation compared to Qiskit and Tket on IBM quantum processors. These findings provide valuable insights into the codesign of algorithm implementation, tailored to optimize qubit mapping and algorithm parameters, with broader implications for enhancing algorithm performance on near-term quantum devices.

We report the direct measurement of isotope shifts of the barium $6s^2$ ${}^{1}S_0\text{–}5d6p{}^{3}D_1^{\mathrm{o}}$ 413-nm electric quadrupole transition, which is utilized for efficient barium ion trapping via photoionization using a single coherent light source. The measured isotope shifts relative to ${}^{138}\mathrm{Ba}$ are $392.9\pm 0.9\mathrm{MHz}$, $178.1\pm 0.8\mathrm{MHz}$, $401.4\pm 1.2\mathrm{MHz}$, and $124.3\pm 1.3\mathrm{MHz}$ for isotopes with atomic numbers 137, 136, 135, and 134, respectively. We verify the measured isotopes with King plot analysis and compare the result with the formerly known shifts inferred from previous studies on neighboring transitions. The results can be used for efficient isotope selective loading of low-abundant barium ions, while careful suppression of line broadening is required for successful isotopic selectivity.

Cooperative effects such as super- and subradiance can be observed in the fluorescence emitted by a system of $N$ atoms in vacuum after interaction with a laser beam. In the vicinity of a dielectric or metallic surface, Casimir-Polder effects can modify collective atomic frequency shifts and decay rates. In this work, we study cooperative fluorescent emission next to resonant surfaces using the coupled-dipole model. We show that cooperative effects, expected in free space, are absent when the atoms are close to a surface whose polariton resonances coincide with the dominant atomic dipole coupling. In this case, cooperative effects are overshadowed by the very fast decay of the atomic fluorescence into surface modes. We illustrate our formalism and our results by considering a line of cesium $6D_{3/2}$ atoms in front of a sapphire surface. Finally, we propose the study of cesium $6P_{3/2}$ atoms in front of a resonant metasurface as the most promising scenario for experimentally demonstrating the results of our study.

Rydberg atoms in an optical tweezer array have been used as a quantum simulator of the spin-$1/2$ antiferromagnetic Ising model with longitudinal and transverse fields. We suggest how to implement the next-nearest-neighbor (NNN) interaction the sign of which is opposite to that of the nearest-neighbor one in the Rydberg atom systems. We show that this can be achieved by weakly coupling one Rydberg state with another Rydberg state. We further study the surface criticality associated with the first-order quantum phase transition between the antiferromagnetic and paramagnetic phases, which emerges due to the sign-inverted NNN interaction. From the microscopic model, we derive a Ginzburg-Landau (GL) equation, which describes static and dynamic properties of the antiferromagnetic order parameter near the transition. Using both analytical GL theory and numerical method based on a mean-field theory, we calculate the order parameter in the proximity of a boundary of the system in order to show that the healing length of the order parameter logarithmically diverges, signaling the surface criticality.

Exceptional points (EPs) in open optical systems are rigorously studied using the resonant-state expansion (RSE). A spherical resonator, specifically a homogeneous dielectric sphere in a vacuum, perturbed by two pointlike defects which break the spherical symmetry and bring the optical modes to EPs, is used as a worked example. The RSE is a nonperturbative approach encoding the information about an open optical system in matrix form in a rigorous way, and thus offering a suitable tool for studying its EPs. These are simultaneous degeneracies of the eigenvalues and corresponding eigenfunctions of the system, which are rigorously described by the RSE and illustrated for perturbed whispering-gallery modes (WGMs). An exceptional arc, which is a line of adjacent EPs, is obtained analytically for perturbed dipolar WGMs. Perturbation of high-quality WGMs with large angular momentum and their EPs are found by reducing the RSE equation to a two-state problem by means of an orthogonal transformation of a large RSE matrix. WGM pairs have opposite chirality in spherically symmetric systems and equal chirality at EPs. This chirality at EPs can be observed in circular dichroism measurements, as it manifested itself in a squared-Lorentzian part of the optical spectra, which we demonstrate here analytically and numerically in the Purcell enhancement factor for the perturbed dipolar WGMs.

In the present work, the role of temperature in the occurrence of optical phases in a system of a large number of atoms interacting with photons is reported. To this end, it is assumed that the cavity inside which the constituents interact is in equilibrium with a heat reservoir, held at a fixed temperature. To achieve this goal, we start with the conventional density operator to calculate the mean values of electromagnetic quadratures. To make the realization of three distinct optical phases, trivial, electric, and magnetic superradiance, possible, the medium inside the cavity is assumed to be a second-order nonlinear one. The nonlinearity of the medium is further assumed to be activated by an externally applied classical pump field. As a consequence, now there are two external controlling agents, the temperature and nonlinearity strength. We then proceed to calculate the quadrature mean values in the thermodynamical limit and for a large number of atoms as a function of temperature. An analysis of the calculated mean values, along with relevant figures, reveals temperature-dependent conditions for realizing trivial, electric, and magnetic optical phases. Moreover, we shall demonstrate that the atom-field coupling strength gives rise to two distinct regimes where the optical phases behave quite differently. In one of the two regimes it is possible for the phases to coexist, while in the other it is not. As a profound result, it is demonstrated that for such a system there can exist a unique temperature (triple point) at which the three optical phases coexist.

We theoretically identify the noise-induced coherent contribution to the ergotropy of a four-level quantum heat engine coupled to a unimodal quantum cavity. We utilize a protocol where the passive state's quasiprobabilities can be analytically identified from the population-coherence coupled reduced density matrix. The reduced density matrix elements are evaluated using a microscopic quantum master equation formalism. Multiple ergotropies within the same coherence interval, each characterized by a positive and pronounced coherent contribution, are observed. These ergotropies are a result of population inversion as well as quasiprobability-population inversion, controllable through the coherence measure parameters. The optimal flux and power of the engine are found to be at moderate values of ergotropy with increasing values of noise-induced coherence. The optimal power at different coherences is found to possess a constant ergotropy.

The quantum dense-coding (DC) protocol, which has no security feature, deals with the transmission of classical information encoded in a quantum state by using shared entanglement between a single sender and a single receiver. Its appropriate variant has been established as a quantum key distribution (QKD) scheme for shared two-qubit maximally entangled states, with the security proof utilizing the uncertainty relation of complementary observables and the Shor-Preskill entanglement purification scheme. We present the DC-based QKD protocol for higher-dimensional systems and report the lower bounds on the secret key rate, when the shared state is a two-qudit maximally entangled state, and mixtures of maximally entangled states with different ranks. The analysis also includes the impact of noisy channels on the secure key rates, before and after encoding. In both the noiseless and the noisy scenarios, we demonstrate that the key rate as well as the robustness of the protocol against noise increases with the dimension. Further, we prove that the set of useless states in the DC-based QKD protocol is convex and compact.

We theoretically propose and experimentally demonstrate the use of a configurable four-qubit photonic system to generate publicly verifiable quantum random numbers, to perform entanglement verification, and to generate a secure public and private key. Quantum circuits, to generate the desired four-qubit states and its experimental realization in the photonic architecture, are carried out using photon pairs entangled in polarization and path degrees of freedom. By performing measurements on the four-qubit system and accessing partial information of the four-qubit state for public verification, we generate publicly verified and purely secured random bits at the rate of 185 kbps from collective data of 370 kbps. When the system is used for generating public and private keys, an equal number of public and private keys are generated simultaneously. We also record about 97.9% of sampled bits from four-qubit states passing entanglement verification and demonstrate the use of public and private key generated for image encryption-decryption. The theoretical model of noise on the four-qubit state and its effect on the generation rate of verified and secured bits are in perfect agreement with the experimental results. This demonstrates the practical use of the small-scale multiqubit photonic system for quantum-safe applications by providing the option for real-time verification of the security feature of the quantum system.

This work shows that feed-forward neural networks can predict the final rovibrational state distributions of inelastic and reactive processes of the reaction of $\mathrm{Ca}+{\mathrm{H}}_2\to \mathrm{CaH}+\mathrm{H}$ in the hyperthermal regime, relevant for buffer gas chemistry. Furthermore, these models can be extended to the isotopologues of the reaction involving deuterium and tritium. In addition, we develop a neural network model that can learn across the chemical space based on the isotopologues of hydrogen. The model can predict the outcome of a reaction whose reactants have never been seen. This is done by training on the $\mathrm{Ca}+{\mathrm{H}}_2$ and $\mathrm{Ca}+{\mathrm{T}}_2$ reactions and subsequently predicting the $\mathrm{Ca}+{\mathrm{D}}_2$ reaction.

In this paper we derive local-field corrections for interactions within chiral media using the Onsager real cavity model. The problem is not fully solvable analytically, so we introduce a matrix formalism to determine the correct local-field corrections. We find that left- and right-handed circularly polarized excitations yield different correction factors inside a chiral medium, analogous to their different propagation velocities. These chiral correction factors can significantly alter theoretical predictions for chiral effects within the medium. We validate our solution in various limits and special cases, particularly when the source and absorption points coincide, enabling analytical comparison for nonabsorbing media. For spontaneous emission in a chiral medium, we extend the local-field correction to absorbing media, revealing fundamentally different corrections analogous to the nonchiral case.

The interaction between solid materials and an optical vortex has garnered significant attention in recent years. Here, we study theoretically the high-order harmonic generation (HHG) of crystalline solids under the spatiotemporal optical-vortex light. We develop a quantum-orbit model to describe the intricate electron motion driven by a strong optical vortex, demonstrating the conservation law of spin and transverse orbital angular momenta in the HHG process. Harnessing the bicircular spatiotemporal vortex light field, we generate high-order harmonics with the hybrid spin and transverse orbital angular momentum. The proportions of different orbital angular-momentum components of harmonics in solids can also be tuned by adjusting the intensity ratio of the two light beams.

We theoretically investigate the topological edge-state impact on high-order harmonic generation (HHG) in solids, with a focus on the modification of the topological edge-state dynamics by spatially structured (SS) light. Our findings reveal an underlying mechanism coupling the spatial inhomogeneity of light with the topological states, which leads to a notable but counterintuitive enhancement of HHG yields by SS light, even with a lower average field-intensity compared to the conventional spatially homogeneous light. We attribute this giant coupling effect to the dynamical modulation of the energy bands by the SS light in the HHG process, building an “electron lift” scenario of the topological edge states. Our study not only highlights the intricate interplay between spatial inhomogeneity of laser fields and topological condensed matter but also paves pathways for manipulating electron dynamics in solids using the unique properties of SS light.