It has recently been shown how to replace spin-wave functions with purely spatial wave functions in the formulation of spin-density functional theory (SDFT), without any change to the resultant density functional expressions [F. Zahariev and M. Levy, Phys. Rev. A 100, 062507 (2019)]. The purely spatial wave functions that are obtained are much more convenient to use and thus allow for a relatively easy decomposition and manipulation of terms in the constrained-search formulation of SDFT. All the essential ingredients of SDFT, including the kinetic, exchange, and correlation contributions, are explicitly defined in terms of the universal functional expressed in this spin-free manner. Constrained-search derivations of the Oliver-Perdew relations for the kinetic and exchange terms are presented. The up-down spin component of the correlation term is found to contain the up-down spin component of the Hartree term. A spin-dependent generalization of the adiabatic connection is put forth and a connection with spin-dependent coordinate scaling is established for the correlation energy.

Arising from modulation instability, Turing rolls in optical Kerr microresonators have been used in the generation of optical frequency combs and the synthesis of microwave and terahertz frequencies. In this work, by applying electro-optic modulation on terahertz-frequency Turing rolls, we implement electro-optic frequency division with a microcomb to synthesize variable low-noise microwave signals. We also actively stabilize the terahertz oscillations to a microwave reference via intracavity power modulation, obtaining fractional frequency instabilities that are better than those of the free-running situation by up to six orders of magnitude. This study not only highlights the extraordinary spectral purity of Turing-roll oscillations but also opens the way for bidirectional terahertz-to-microwave links with hybrid optical-frequency-comb techniques.

The term “layers of classicality” in the context of quantum measurements was introduced by T. Heinosaari [Phys. Rev. A 93, 042118 (2016)]. The strongest layer among these consists of the sets of observables that can be broadcast and the weakest layer consists of the sets of compatible observables. There are several other layers in between those two layers. In this work, we study their physical and geometric properties and show the differences and similarities among the layers in these properties. In particular we show that (i) none of the layers of classicality respect transitivity property, (ii) the concept like degree of broadcasting similar to degree of compatibility does not exist, (iii) there exist informationally incomplete positive operator-valued measures that are not individually broadcastable, (iv) a set of broadcasting channels can be obtained through concatenation of broadcasting and nondisturbing channels, (v) unlike compatibility, other layers of classicality are not convex, in general. Finally, we discuss the relations among these layers. More specifically, we show that a specific type of concatenation relations among broadcasting channels decides the layer in which a pair of observables resides.

I show that classical capacity per unit cost of noisy bosonic Gaussian channels can be attained by employing a generalized on-off keying modulation format and a projective measurement of individual output states. This means that neither complicated collective measurements nor phase-sensitive detection is required to communicate over optical channels at the ultimate limit imposed by laws of quantum mechanics in the limit of low average cost.

The cross sections and rate coefficients for the inelastic processes of mutual neutralization, ion-pair formation, and (de)-excitation in lithium-hydrogen collisions are calculated based on the ab initio electronic structure [J. Phys. B: At. Mol. Opt. Phys. 32, 81 (1999)] and the hopping probability current method [Phys. Rev. A 88, 052704 (2013)] for the nonadiabatic nuclear dynamics. For each given set of collisional parameters, calculations of the probability current evolution were repeated $2.62144\times {10}^9$ times, and this leads to a high accuracy of the inelastic state-to-state transition probabilities, the cross sections, and finally, the rate coefficients. The isotopic effects on the processes in collisions ${}^6\mathrm{Li}/{}^7\mathrm{Li}+\mathrm{H}/\mathrm{D}/\mathrm{T}$, where (H/D/T) represents hydrogen, deuterium, and tritium, for different isotopes are studied. In addition, the deexcitation resonance processes ${}^{6,7}\mathrm{Li}(2p\to 2s)+\mathrm{H}/\mathrm{D}/\mathrm{T}$ were treated by the branching probability current method. It is found that the isotopic effects are different for different collisional partners and for different electronic states involved into a process, varying from negligible effects for high-lying Li states to strong effects for low-lying Li states. The resonance transition process is one with the strongest isotopic effects: replacing H by T changes the rate coefficient by up to four orders of magnitude. Unfortunately, there is no experimental data for the processes with strong isotropic effects, only for the processes with moderate effects, up to 60%.

We report calculation of the energy spectrum and the spectroscopic properties of the superheavy element ion: ${\mathrm{Rf}}^{+}$. We use the four-component relativistic Dirac-Coulomb Hamiltonian and the multireference configuration interaction model to tackle the complex electronic structure problem that combines strong relativistic effects and electron correlation. We determine the energies of the ground and the low-lying excited states of ${\mathrm{Rf}}^{+}$, which originate from the $7s^{2}6d^1,7s^{1}6d^2,7s^{2}7p^1$, and $7s^{1}6d^{1}7p^1$ configurations. The results are discussed vis-à-vis the lighter homolog ${\mathrm{Hf}}^{+}$ ion. We also assess the uncertainties of the predicted energy levels. The main purpose of the presented calculations is to provide a reliable prediction of the energy levels and to identify suitable metastable excited states that are good candidates for the planned ion-mobility-assisted laser spectroscopy studies.

We report calculations of QED corrections to the $g$ factor of Li-like ions induced by the exchange of two virtual photons between the electrons. The calculations are performed within QED theory to all orders of the nuclear binding strength parameter $Z\alpha$, where $Z$ is the nuclear charge number and $\alpha$ is the fine-structure constant. In the region of low nuclear charges we compare results from three different methods: QED, relativistic many-body perturbation theory, and nonrelativistic QED. All three methods are shown to yield consistent results. With our calculations we improve the accuracy of theoretical predictions of the $g$ factor of the ground state of Li-like carbon and oxygen by about an order of magnitude. Our theoretical results agree with those from previous calculations but differ by three to four standard deviations from the experimental results available for silicon and calcium.

We investigate theoretically and numerically the light-matter interaction in a two-band system (TBS) as a model system for excitation in a solid-state band structure. We identify five clearly distinct excitation regimes, categorized with well-known adiabaticity parameters: (1) the perturbative multiphoton absorption regime for small driving field strengths, and four light-field-driven regimes, where intraband motion becomes important (2) the impulsive Landau-Zener (LZ) regime, (3) the nonimpulsive LZ regime, (4) the adiabatic regime, and (5) the adiabatic-impulsive regime for large electric field strengths. This categorization is tremendously helpful to understand the highly complex excitation dynamics in solids, in particular, when the driving field strength varies, and this categorization naturally connects Rabi physics with Landau-Zener physics. In addition, we find an insightful analytical expression for the photon orders connecting the perturbative multiphoton regime with the light-field-driven regimes. Moreover, in the adiabatic-impulsive regime, adiabatic motion and impulsive LZ transitions are equally important, leading to a broken symmetry of the TBS and a residual current when applying few-cycle laser pulses of broken temporal symmetry. This categorization allows a deep understanding of strong-field excitation in solids, including current and high-harmonic generation in a large variety of settings, and will help to find optimal driving parameters for a given purpose.

In this work we establish constraints on the temperature dependence of the shear viscosity $\eta$ in the superfluid phase of a dilute Fermi gas in the unitary limit. Our results are based on analyzing experiments that measure the aspect ratio of a deformed cloud after release from an optical trap. We discuss how to apply the two-fluid formalism to the unitary gas and provide a suitable parametrization of the equation of state. We show that in expansion experiments the difference between the normal and superfluid velocities remains small and can be treated as a perturbation. We find that expansion experiments favor a shear viscosity that decreases significantly in the superfluid regime. Using an exponential parametrization, we find $\eta (T_c/2T_F)\lesssim 0.37\left[\eta (T_c/T_F)\right]$, where $T_c$ is the critical temperature and $T_F$ is the local Fermi temperature of the gas.

In the present work, we explore analytically and numerically the coexistence and interactions of ring dark solitons (RDSs) with other RDSs, as well as with vortices. The azimuthal instabilities of the rings are explored via the so-called filament method. As a result of their nonlinear interaction, the vortices are found to play a stabilizing role on the rings, yet their effect is not sufficient to offer complete stabilization of RDSs. Nevertheless, complete stabilization of the relevant configuration can be achieved by the presence of external ring-shaped barrier potentials. Interactions of multiple rings are also explored, and their equilibrium positions (as a result of their own curvature, the external trap, and their tail-tail interactions) are identified. In this case too, stabilization is achieved via multiring external barrier potentials.

We present a scheme for ground-state cooling of a mechanical resonator by simultaneously coupling it to a superconducting qubit and a cavity field. The Hamiltonian describing the hybrid system dynamics is systematically derived. The cooling process is driven by a red-detuned ac drive on the qubit and a laser drive on the optomechanical cavity. We have investigated cooling in the weak and the strong coupling regimes for both the individual system, i.e., qubit assisted cooling and optomechanical cooling, and compared them with the effective hybrid cooling. We show that in the weak coupling and resolved sideband regime, cooling is more efficient in the hybrid case for a specific choice of parameters. On the other hand, in the strong coupling, the hybrid cooling is found to be more effective compared to the individual cooling mechanisms in the resolved regimes.

The relativistic theory of the time- and position-dependent energy and momentum densities of light in glasses and other low-loss dispersive media, where different wavelengths of light propagate at different phase velocities, has remained a largely unsolved challenge until now. This is astonishing in view of the excellent overall theoretical understanding of Maxwell's equations and the abundant experimental measurements of optical phenomena in dispersive media. The challenge is related to the complexity of the interference patterns of partial waves and to the coupling of the field and medium dynamics by the optical force on the medium atoms. In this work, we use the mass-polariton theory of light [Phys. Rev. A 96, 063834 (2017)] to derive the stress-energy-momentum (SEM) tensors of the field and the dispersive medium. Our starting point, the fundamental local conservation laws of energy and momentum densities in classical field theory, is close to that of a recent theoretical work on light in dispersive media by Philbin [Phys. Rev. A 83, 013823 (2011)], which, however, excludes the power-conversion and force density source terms describing the coupling between the field and the medium. In the general inertial frame, we present the SEM tensors in terms of Lorentz scalars, four-vectors, and field tensors that reflect in a transparent way the Lorentz covariance of the theory. The SEM tensors of the field and the medium are symmetric, form-invariant for all inertial observers, and in full accordance with the covariance principle of the special theory of relativity. When the power-conversion and force density source terms are accounted for, there is no need to introduce asymmetric SEM tensors or heuristic symmetrization procedures even for the field and medium subsystems. Therefore, asymmetric SEM tensors based on strictly classical energy and momentum densities, which have been studied in previous literature, do not account for all aspects in the field-medium coupling of electromagnetic waves. However, being based on classical field theory, the present work prompts for further groundwork on the classical limit of the quantum mechanical spin of light in a medium. The SEM tensor of the coupled field-medium state of light also has zero four-divergence. Therefore, light in a dispersive medium has a well-defined four-momentum and rest frame. The volume integrals of the total energy and momentum densities of light agree with the model of mass-polariton quasiparticles having a nonzero rest mass. The coupled field-medium state of light drives forward an atomic mass density wave, which makes the constant center-of-energy velocity law of an isolated system—a fundamental conservation law of nature—satisfied. This provides strong evidence for the consistency of the theory. The predictions of our work, such as the atomic mass density wave associated with light, are accessible to experiments.

Environment-assisted quantum transport (ENAQT) is a unique situation where environmental noise can, counterintuitively, enhance the transport of an open quantum system. In this paper, we investigate how the presence of a one-dimensional single-particle mobility edge can generate strong ENAQT. For this purpose, we study the energy current of a generalized Aubry-André-Harper tight binding model coupled at its edges to spin baths of differing temperature and dephasing noise along the system. We find that the ENAQT increases by orders of magnitude and depends on the number of localized eigenstates and disorder strength nonmonotonically. We show that this enhancement is the result of the cooperation between population uniformization and localization.

In this article we present the three-body fragmentation dynamics of ${\mathrm{C}}_2{{\mathrm{H}}_4}^{3+}$ caused by $18\text{−}\mathrm{keV}/\mathrm{u}{\mathrm{Ne}}^{8+}$ ion impact. Utilizing the cold target recoil ion momentum spectroscopy, the complete kinematical information of three fragmentation channels, i.e., ${\mathrm{C}}_2{{\mathrm{H}}_4}^{3+}\to {\mathrm{H}}^{+}+{\mathrm{H}}^{+}+{\mathrm{C}}_2{{\mathrm{H}}_2}^{+}, {\mathrm{H}}^{+}+\mathrm{C}{\mathrm{H}}^{+}+\mathrm{C}{{\mathrm{H}}_2}^{+}$, and ${\mathrm{H}}^{+}+{{\mathrm{H}}_2}^{+}+{\mathrm{C}}_2{\mathrm{H}}^{+}$ is obtained. For each channel, both concerted and sequential fragmentation mechanisms are observed and differentiated through the momentum correlation of resultant fragments visualized by the Dalitz plot and the Newton diagram. In particular, for the channel ${\mathrm{C}}_2{{\mathrm{H}}_4}^{3+}\to {\mathrm{H}}^{+}+{\mathrm{H}}^{+}+{\mathrm{C}}_2{{\mathrm{H}}_2}^{+}$, it is found in addition to the sequential pathway that the two protons can be emitted concertedly in two ways, i.e., from the same carbon atom or from the two carbon atoms. The present results show a more diverse molecular fragmentation triggered by ion collisions in contrast to a previous study using intense laser field ionization.

Significant experimental progress has been made recently for observing long-sought supersolidlike states in Bose-Einstein condensates, where spatial translational symmetry is spontaneously broken by anisotropic interactions to form a stripe order. Meanwhile, the superfluid stripe ground state was also observed by applying a weak optical lattice that forces the symmetry breaking. Despite the similarity of the ground states, here we show that these two symmetry breaking mechanisms can be distinguished by their collective excitation spectra. In contrast to gapless Goldstone modes of the spontaneous stripe state, we propose that the excitation spectra of the forced stripe phase can provide direct experimental evidence for the gapped pseudo-Goldstone modes. We characterize the pseudo-Goldstone mode of such lattice-induced stripe phase through its excitation spectrum and static structure factor. Our work may pave the way for exploring spontaneous and forced or approximate symmetry breaking mechanisms in different physical systems.

In this work, we provide theoretical background for the analysis of second-order correlation functions in experiments performed at soft x-ray free-electron lasers (XFELs). Typically, soft x-ray beamlines at XFELs are equipped with variable line spacing (VLS) monochromators. We examine the beam propagation through such VLS monochromator especially taking into account the interplay between the finite monochromator resolution and the width of the exit slits. We then provide a general analysis of the second-order correlation intensities in spectral and spatial domains. Finally, we connect these functions with the statistical properties of the beam incoming to the monochromator unit in the limit of Gaussian Schell model pulses.

We investigate the topological phase transition and the edge states in a quasi-three-dimensional topological system mapped by a circuit quantum electrodynamic lattice via introducing two periodic spatial parameters. It is found that with the increasing of the periodically modulated on-site potential strength, the system undergoes a topological phase transition, corresponding to the change of the number of Weyl points under the periodic boundary condition. Under the open boundary condition, the phase transition is reflected by the energy band separation and the appearance of new edge states. Interestingly, the system holds two pairs of crossed edge states in the energy gaps when the periodic parameters take certain values. Furthermore, we show that, benefiting from the Bose statistical properties of the circuit quantum electrodynamic, the edge states of the system can be directly detected by measuring the average photon number of the cavity field in the steady state.