Density-potential-functional theory (DPFT) is an alternative formulation of orbital-free density functional theory that may be suitable for modeling the electronic structure of large systems. To date, DPFT has been applied mainly to quantum gases in one- and two-dimensional settings. In this work, we study the performance of DPFT when applied to real-life systems: atoms, dimers, and nanoparticles. We build on systematic Suzuki-Trotter factorizations of the quantum-mechanical propagator and on the Wigner function formalism, respectively, to derive nonlocal as well as semilocal functional approximations in complete analogy to their well-established lower-dimensional versions, without resorting to system-specific approximations or ad hoc measures of any kind. The cost for computing the associated semiclassical ground-state single-particle density scales (quasi)linearly with particle number. We illustrate that the developed density formulas become relatively more accurate for larger particle numbers, can be improved systematically, are quite universally applicable, and, hence, may offer alternatives to existing orbital-free methods for mesoscopic quantum systems.

We investigate high-order above-threshold ionization (HATI) from intense quantum light-atom interaction, by numerically solving the fully quantum time-dependent Schrödinger equation based on quantum electrodynamics theory. Our simulations show that, in comparison with coherent-state (classical) light, the HATI spectra from intense quantum lights may have a higher cutoff energy and, moreover, for a given ponderomotive energy of the laser field, the cutoff energy is dependent on the laser wavelength and the ionization potential of the atom. The influence of quantum light on the HATI is revealed by examining the interplay between a quasiprobability distribution of the electric field of the quantum light and the Ammosov-Delone-Krainov ionization rate of the atom. Furthermore, analytical formulas of the cutoff energy of the HATI spectra for different quantum lights are derived.

Ultracold gases of highly magnetic lanthanide atoms have enabled the realization of dipolar quantum droplets and supersolids. However, future studies could be limited by the achievable atom numbers and hindered by high three-body loss rates. Here we study density-dependent atom loss in an ultracold gas of ${}^{166}\mathrm{Er}$ for magnetic fields below 4 G, identifying six previously unreported, strongly temperature-dependent features. We find that their positions and widths show a linear temperature dependence up to at least $15\mu \mathrm{K}$. In addition, we observe a weak, polarization-dependent shift of the loss features with the intensity of the light used to optically trap the atoms. This detailed knowledge of the loss landscape allows us to optimize the production of dipolar Bose-Einstein condensates with more than $2\times {10}^5$ atoms and points towards optimal strategies for the study of large-atom-number dipolar gases in the droplet and supersolid regimes.

We address the existence, stability, and propagation dynamics of multipole-mode solitons in cubic-quintic nonlinear media with an imprinted annular (ring-shaped) potential. The interplay of the competing nonlinearity with the potential enables the formation of a variety of solitons with complex structures, from dipole, quadrupole, and octupole solitons to necklace complexes. The system maintains two branches of soliton families with opposite slopes of the power-vs.-propagation-constant curves. While the solitons' stability domain slowly shrinks with the increase of even number $n$ of lobes in the multipole patterns, it remains conspicuous even for $n>16$. The application of a phase torque gives rise to stable rotation of the soliton complexes, as demonstrated by means of analytical and numerical methods.

We discuss the properties of electromagnetic transverse electric and transverse magnetic (TE and TM) pulses based on a family of oscillatory solutions of the wave equation. The energy and momentum densities of the TE and TM pulses are annular. These pulses are sufficiently localized to have finite norm, total energy, momentum, and angular momentum, which are evaluated analytically. All the solutions given are proved to be strictly forward propagating (unidirectional). The wave function is characterized by a length $a$ and a wave number $K$. The longitudinal extent of the pulses and the number of oscillations are of order $2a$ and $Ka$, respectively. Far from the focal region, the annular TE and TM pulses diverge from the axis of propagation on a cone, the angle of which is determined.

We investigate the properties of the cooperative decay modes of a cold atomic cloud, characterized by a Gaussian distribution in three dimensions, initially excited by a laser in the linear regime. We study the properties of the decay rate matrix $S$, whose dimension coincides with the number of atoms in the cloud, in order to get a deeper insight into properties of cooperative photon emission. Since the atomic positions are random, $S$ is a Euclidean random matrix whose entries are a function of the atom distances. We show that in the limit of a large number of atoms in the cloud, the eigenvalue distribution of $S$ depends on a single parameter $b_0$, called the cooperativeness parameter, which can be viewed as a quantifier of the number of atoms that are coherently involved in an emission process. For very small values of $b_0$, we find that the limit eigenvalue density is approximately triangular. We also study the nearest-neighbor spacing distribution and the eigenvector statistics, finding that although the decay rate matrices are Euclidean, the bulk of their spectra mostly behaves according to the expectations of classical random matrix theory. In particular, in the bulk, there is level repulsion and the eigenvectors are delocalized, therefore exhibiting the universal behavior of chaotic quantum systems.

A careful inspection on the tin $L$ x-ray spectrum induced by electron impact was carried out, with the aim of obtaining fundamental atomic properties related to core ionizations and subsequent emissions. A bulk tin standard was irradiated, the resulting spectrum being recorded with a commercial wavelength dispersive spectrometer, and processed by means of a parameter optimization method previously developed. Characteristic energies, relative transition probabilities and natural linewidths were therefore experimentally determined for this element, and compared with data from the literature, when available. Satellite and radiative Auger effect structures were also investigated, for which the corresponding energy shifts and relative intensities were obtained. A number of these parameters were determined, even in overlapping peaks and weak transitions, which evidences the robustness of the spectral processing method used.

The magnetic dipole transitions in the homonuclear molecular ion ${\mathrm{H}}_2{}^{+}$ are obtained for a range of $v$ and $L$, vibrational and total orbital momentum quantum numbers, respectively. Calculations are performed in the nonrelativistic approximation. Spin consideration is also included.

We analyze the conditional states in which one part of a split spin-squeezed state is left, upon performing a collective spin measurement on the other part. For appropriate measurement directions and outcomes, we see the possibility of obtaining states with high quantum Fisher information, even reaching the Heisenberg limit. This allows us to propose a metrological protocol that can outperform standard approaches, for example, in a situation where the number of particles in the probe is bounded. The robustness of this protocol is investigated by considering realistic forms of noise present in cold-atom experiments, such as particle number fluctuations and imperfect detection. Ultimately, we show how this measurement-based state-preparation approach can allow for the conditional (i.e., heralded) preparation of spin Schrödinger's cat states even when the initial state before splitting is only mildly squeezed.

This paper explores the propagation of optical vortices of slow light inside a four-level tripod atomic light-matter coupling system. Initially, the system is prepared in a coherent superposition of two out of the three lower levels by, for example, coherent population trapping, while the third lower state remains unoccupied. The unoccupied state is coupled to a strong control laser field with a constant Rabi frequency, lacking orbital angular momentum (OAM). Simultaneously, one of the remaining lower states interacts with a weak vortex beam. The third lower state has no initial coupling to any field. This arrangement effectively closes the level transitions, resulting in a phase-dependent configuration. By solving the Maxwell-Schrödinger equations, we provide analytical evidence that the application of a strong control field can generate an additional optical vortex of slow light. This vortex possesses the same OAM as the incident vortex beam. Furthermore, we explore the matching of optical vortices at different propagation distances, contingent upon the intensity of the control field. Additionally, our analysis extends to a more complex five-level tripod and $\mathrm{\Lambda }$ scheme, where we introduce two additional strong control fields to prepare the atoms in a coherent superposition of two lower levels.

Fast quantum gates are of paramount importance for enabling efficient and error-resilient quantum computations. In the present work we analyze Landau-Zener-Stückelberg-Majorana (LZSM) strong driving protocols, tailored to implement fast gates with particular emphasis on small-gap qubits. We derive analytical equations to determine the specific set of driving parameters for the implementation of single-qubit and two-qubit gates employing single-period sinusoidal pulses. Our approach circumvents the need to scan experimentally a wide range of parameters and instead it allows to concentrate on fine-tuning the device near the analytically predicted values. We analyze the dependence of relaxation and decoherence on the amplitude and frequency of the pulses, obtaining the optimal regime of driving parameters to mitigate the effects of the environment. Our study focus on the single-qubit $X_{\frac{\pi }{2}}, Y_{\frac{\pi }{2}}$ and identity gates. Also, we propose the $\sqrt{\mathrm{bSWAP}}$ as the simplest two-qubit gate attainable through a robust LZSM driving protocol.

A construction of a quasirandom potential for cold atoms using dark states emerging in $\mathrm{\Lambda }$ level configuration is proposed. Speckle laser fields are used as a source of randomness. Anderson localisation in such potentials is studied and compared with the known results for the speckle potential itself. It is found out that the localization length is greatly decreased due to the nonlinear fashion in which dark-state potential is obtained. In effect, random dark-state potentials resemble those occurring in random Kronig-Penney-type Hamiltonians.

Hardy's argument constitutes an elegant proof of quantum nonlocality. In this work, we report an exotic application of Hardy's nonlocal correlations in two-party communication setup. We come up with a task, wherein a positive payoff can be through 1 bit of communication from the sender to the receiver if and only if the communication channel is assisted with a no-signaling correlation exhibiting Hardy's nonlocality. This further prompts us to establish a counterintuitive result in a correlation-assisted reverse zero-error channel coding scenario, where the aim is to simulate a higher input-output noisy classical channel by a lower input-output noiseless one in assistance with preshared correlations. We show that there exist such reverse zero-error channel simulation tasks where nonmaximally entangled states are preferable over the assistance with a maximally entangled state, even when the former states carry an arbitrarily small amount of entanglement. Our work thus establishes that within the operational paradigm of local operations and limited classical communication the structure of entangled resources is even more complex to characterize.

Central spin models, where a single spinful particle interacts with a spin environment, find wide application in quantum information technology and can be used to describe, e.g., the decoherence of a qubit over time. We propose a method of realizing an ultracold quantum simulator of a central spin model with $XX$ (spin-exchanging) interactions. The proposed system consists of a single Rydberg atom (central spin) and surrounding polar molecules (bath spins), coupled to each other via dipole-dipole interactions. By mapping internal particle states to spin states, spin-exchanging interactions can be simulated. As an example system geometry, we consider a ring-shaped arrangement of bath spins and show how it allows us to exact precise control over the interaction strengths. We numerically analyze two example dynamical scenarios which can be simulated in this setup: a decay of central spin polarization, which can represent qubit decoherence in a disordered environment, and a transfer of an input spin state to a specific output spin, which can represent the transmission of a single bit across a quantum network. We demonstrate that this setup allows us to realize a central spin model with highly tunable parameters and geometry for applications in quantum science and technology.

The nuclear recoil effect, known also as the mass shift, is one of the theoretical contributions to the energy levels in muonic atoms. Accurate theoretical predictions are needed for extracting, e.g., the nuclear charge radii from experimental spectra. We report rigorous QED calculations of the nuclear recoil correction in muonic atoms, carried out to all orders in the nuclear binding strength parameter $Z\alpha$ (where $Z$ is the nuclear charge number and $\alpha$ is the fine-structure constant). The calculations show differences with the previous approximate treatment of this effect, most pronounced for the lowest-lying bound states. The calculated recoil correction was found to be sensitive to the nuclear charge radius, which needs to be accounted for when extracting nuclear parameters from measured spectra.

Divalent atoms and ions with a singlet $S$ ground state and triplet $P$ excited state form the basis of many high-precision optical atomic clocks. Along with the metastable ${}^{3}P_0$ clock state, these atomic systems also have a nearby metastable ${}^{3}P_2$ state. We investigate the properties of the electric quadrupole ${}^1{S}_0\leftrightarrow {}^{3}P_2$ transition with a focus on enhancing already existing optical atomic clocks. In particular we investigate the ${}^1\mathrm{S}_0\leftrightarrow {}^{3}P_2$ transition in ${}^{27}\mathrm{Al}^{+}$ and calculate the differential polarizability, hyperfine effects, and other relevant atomic properties. We also discuss potential applications of this transition, notably that it provides two transitions with different sensitivities to systematic effects in the same species. In addition, we describe how the ${}^1\mathrm{S}_0\leftrightarrow {}^{3}P_2$ transition can be used to search for physics beyond the Standard Model and motivate investigation of this transition in other existing optical atomic clocks.

We present measurements of three distinctive state-dependent wavelengths for the ${}^{1}S_0\text{−}{}^{3}P_0$ clock transition in ${}^{\text{174}}\text{Yb}$ atoms. Specifically, we determine two magic wavelengths at 652.281(21) and $542.50205\left(19\right)\text{THz}$, where the differential light shift on the ${}^{1}S_0\text{−}{}^{3}P_0$ clock transition vanishes, and one tune-out wavelength at $541.8325\left(5\right)\text{THz}$, where the polarizability of the ${}^{1}S_0$ ground state exhibits a zero crossing. The two magic wavelengths are identified by spectroscopically interrogating cold ${}^{\text{174}}\text{Yb}$ atoms on the clock transition in a one-dimensional optical lattice. The ground-state tune-out wavelength is determined via a parametric heating scheme. With a simple empirical model, we then extrapolate the ground- and excited-state polarizability over a broad range of wavelengths in the visible spectrum.

We derive tight bipartite Bell inequalities for various scenarios. A bipartite Bell scenario $(X,Y,A,B)$ is defined by the numbers of settings and outcomes per party, $X,A$, and $Y,B$ for Alice and Bob, respectively. We derive the complete set of facets of the local polytopes of (6,3,2,2), (3,3,3,2), (3,2,3,3), and (2,2,3,5). We provide extensive lists of facets for (2,2,4,4), (3,3,4,2), and (4,3,3,2). For each inequality we compute the maximum quantum violation, the resistance to noise, and the minimal symmetric detection efficiency required to close the detection loophole, for qubits, qutrits, and ququarts. Based on these results, we identify scenarios which perform better in terms of visibility, resistance to noise, or both, when compared to the Clauser-Horne-Shimony-Holt inequality. Such scenarios could find important applications in quantum communication.