The recent direct experimental measurement of quantum entanglement paves the way towards a better understanding of many-body quantum systems and their correlations. Nevertheless, the experimental and theoretical advances had so far been predominantly limited to bosonic systems. Here, we study fermionic systems. Using experimental setups where multiple copies of the same state are prepared, arbitrary-order Rényi entanglement entropies and entanglement negativities can be extracted by utilizing spatially uniform beam splitters and on-site occupation measurement. As an example, we simulate the use of our protocols to measure the entanglement growth following a local quench. We also illustrate how our paradigm could be used for experimental quantum simulations of fermions on manifolds with nontrivial spin structures.

Coherence is a basic phenomenon in quantum mechanics and considered to be an essential resource in quantum information processing. Although the quantification of coherence has attracted a lot of interest, the lack of efficient methods to measure the coherence in experiments limits the applications. We address this problem by introducing an experiment-friendly method for coherence and spectrum estimation. This method is based on the theory of majorization and can not only be used to prove the presence of coherence, but also result in a rather precise lower bound of the amount of coherence. As an illustration, we show how to characterize the freezing phenomenon of coherence with only two local measurements for any $N$-qubit quantum systems. Our approach also has other applications in quantum information processing, such as the characterization of distillability and entanglement transformations.

The hyperfine splitting of the ground state of selected B-like ions within the range of nuclear charge numbers $Z=49$–83 is investigated in detail. The rigorous QED approach together with the large-scale configuration-interaction Dirac-Fock-Sturm method are employed for the evaluation of the interelectronic-interaction contributions of first and higher orders in $1/Z$. The screened QED corrections are evaluated to all orders in $\alpha Z$ by using an effective potential. The influence of nuclear magnetization distribution is taken into account within the single-particle nuclear model.

The Casimir force between two extended charge sources, embedded in a background of one-dimensional massive Dirac fermions, is explored by means of original contour integration techniques. For identical sources with the same (positive) charge, we find that in the nonperturbative region the Casimir interaction between them can reach sufficiently large negative values and simultaneously reveal the features of a long-range force in spite of nonzero fermion mass. For large distances $s$ between sources we recover that their mutual interaction is governed primarily by the structure of the discrete spectrum of a single source, through which it can be tuned to give an attractive, a repulsive, or an almost compensated Casimir force with various rates of the exponential decay, quite different from the standard $exp(-2ms)$ law. A quite different behavior of the Casimir force is found for the system of two extended sources with the opposite charge. In particular, in this case, there is no possibility for a long-range interaction between sources. The asymptotics of the Casimir force follows the standard $exp(-2ms)$ law. Moreover, for small separations between sources the Casimir force, being calculated completely nonperturbatively, for symmetric and antisymmetric cases turns out to be of different sign and also opposite to the classic electrostatic force for such Coulomb sources. By means of the same (dubbed $ln\text{[Wronskian]}$) techniques, the case of two pointlike charge sources is also considered in a self-consistent manner with similar results for the variability of the Casimir force.

We show that which-way interference within ringlike unit cells enhances the propagation of electronic excitations (excitons) along linear arrays made upon these basic units. After providing an analytic approximate solution of the eigenvalue problem for such aggregates, we show that the constructive interference of wave packets leads to an excitonic population transferred across the array which is not a monotonic function of the coupling between nearest-neighbor rings. The nonmonotonicity depends on an interesting trade-off between the exciton transfer speed and the amount of energy transferred, arising from the interplay between paths within the ringlike cells and the interring coupling strength across the array.

A classical description of electron-emission differential ionization cross sections for highly charged high-velocity ions ($\sim 10$ a.u.) impinging on water molecules is presented. We investigate the validity of the classical statistical mechanics description of ionization ($\hslash =0$ limit of quantum mechanics) in different ranges of electron-emission energy and solid angle, where mechanisms such as soft and binary collisions are expected to contribute. The classical-trajectory Monte Carlo method is employed to calculate doubly and singly differential cross sections for ${\mathrm{C}}^{6+}$, ${\mathrm{O}}^{8+}$, and ${\mathrm{Si}}^{13+}$ projectiles, and comparisons with continuum-distorted-wave eikonal-initial-state theoretical results and with experimental data are presented. We implement a time-dependent screening effect in our model, in the spirit of mean-field theory, to investigate its effect for highly charged projectiles. We also focus on the role of an accurate description of the molecular target by means of a three-center potential to show its effect on differential cross sections. Very good agreement with experiments is found at medium to high electron-emission energies.

We report on experimental elastic differential and integral cross sections for electron scattering from zinc. The energy range of these measurements is 10–100 eV, while the scattered electron angular range in the differential cross-section data is ${10}^{\circ }\text{–}{150}^{\circ }$. We also supplement our measured data with applications of our optical potential and relativistic optical potential approaches to this problem. Where possible, the present results are compared against those from earlier B-spline R-matrix [O. Zatsarinny and K. Bartschat, Phys. Rev. A 71, 022716 (2005)] and convergent close coupling [D. V. Fursa, I. Bray, R. Panajotović, D. Šević, V. Pejčev, D. M. Filipović, and B. P. Marinković, Phys. Rev. A 72, 012706 (2005)] computations. Good overall qualitative accord is typically observed.

Photodissociation of ozone following absorption of biologically harmful solar ultraviolet radiation is the key mechanism for the life protecting properties of the atmospheric ozone layer. Even though ozone photolysis is described successfully by post-Hartree-Fock theory, it has evaded direct experimental access so far, due to the unavailability of intense ultrashort deep ultraviolet radiation sources. The rapidity of ozone photolysis with predicted values of a few tens of femtoseconds renders both ultrashort pump and probe pulses indispensable to capture this manifestation of ultrafast chemistry. Here, we present the observation of femtosecond time-scale electronic and nuclear dynamics of ozone triggered by ∼10-fs, ∼2-µJ deep ultraviolet pulses and, in contrast to conventional attochemistry experiments, probed by extreme ultraviolet isolated pulses. An electronic wave packet is first created. We follow the splitting of the excited B-state related nuclear wave packet into a path leading to molecular fragmentation and an oscillating path, revolving around the Franck-Condon point with 22-fs wave-packet revival time. Full quantum-mechanical ab initio multiconfigurational time-dependent Hartree simulations support this interpretation.

Angular distribution of photoelectrons released by the ionization of randomly oriented molecules with two laser fields of carrier frequencies $\omega$ and $2\omega$, which are linearly polarized in two mutually orthogonal directions, is analyzed in the perturbation limit for the case of a one- versus two-photon ionization process. In particular, we focus on the recently predicted [Ph. V. Demekhin et al., Phys. Rev. Lett. 121, 253201 (2018)] forward-backward asymmetry in the photoelectron emission, which is induced by the interference of two fields and depends on the external relative phase between them. The present theoretical analysis proves a chiral origin of the effect and suggests that a molecule introduces an additional internal relative phase between two ionizing fields.

The objective of this paper is the theoretical description of the Mott insulator to superfluid quantum phase transition of a Bose gas in an optical lattice. In former works the Rayleigh-Schrödinger perturbation theory was used within a mean-field approach, which yields partially nonphysical results since the degeneracy between two adjacent Mott lobes is not taken into account. In order to correct such nonphysical results we apply the Brillouin-Wigner perturbation theory to the mean-field approximation of the Bose-Hubbard model. Detailed explanations of how to use the Brillouin-Wigner theory are presented, including a graphical approach that allows one to efficiently keep track of the respective analytic terms. To prove the validity of this computation, the results are compared with other works. Besides the analytic calculation of the phase boundary from Mott insulator to superfluid phase, the condensate density is also determined by simultaneously solving two algebraic equations. The analytical and numerical results turn out to be physically meaningful and can cover a region of system parameters inaccessible until now. Our results are of particular interest provided a harmonic trap is added to the former calculations in a homogeneous system, in view of describing an experiment within the local-density approximation. Thus, the paper represents an essential preparatory work for determining the experimentally observed wedding cake structure of particle-density profile at both finite temperature and finite hopping.

The long-time existence of topologically nontrivial configurations of quantum vortices in the form of torus knots and links in trapped Bose-Einstein condensates is demonstrated numerically within the three-dimensional Gross-Pitaevskii equation with an external anisotropic parabolic potential. We identify parametric domains of trap anisotropy, characterized by the axial over planar frequency ratio $\lambda \approx 1.5–1.6$, where the lifetime of such quasistationary rotating vortex structures is many hundreds of typical rotation times. This suggests the potential experimental observability of the structures. We quantify the relevant lifetimes as a function of the model parameters (e.g., $\lambda$) and initial condition parameters of the knot profile.

We report theoretical studies of adiabatic population transfer using dressed spin states. Quantum optimal control using the chopped random basis (CRAB) algorithm has been implemented in a negatively charged diamond nitrogen vacancy center that is coupled to a strong and resonant microwave field. We show that the dressed spin states are highly effective in suppressing effects of spin dephasing on adiabatic population transfer. The numerical simulation also demonstrates that CRAB-based quantum optimal control can enable an efficient and robust adiabatic population transfer.

The well-studied quantum optical Schrödinger cat state is a superposition of two distinguishable states, with quantum coherence between these macroscopically distinguishable states being of foundational and, in the context of quantum-information processing, practical use. We refer to these quantum-optical cat states as quantum dichotomous states, reflecting that the state is a superposition of two options, and we introduce the term quantum multichotomous state to refer to a superposition of multiple macroscopically distinguishable options. For a single degree of freedom, such as position, we construct the quantum multichotomous states as a superposition of Gaussian states on the position line in phase space. Using this nomenclature, a quantum tetrachotomous state (QTS) is a coherent superposition of four macroscopically distinguishable states. We define, analyze, and show how to create such states, and our focus on the QTSs is due to their exhibition of much richer phenomena than for the quantum dichotomous states. Our characterization of the QTS involves the Wigner function, its marginal distributions, and the photon-number distribution, and we discuss the QTS's approximate realization in a multiple-coupled-well system.

The field of optomechanics provides us with several examples of quantum photon-phonon interface. In this paper, we theoretically investigate the generation and manipulation of quantum correlations in a microfabricated optomechanical array. We consider a system consisting of localized photonic and phononic modes interacting locally via radiation pressure at each lattice site with the possibility of hopping of photons and phonons between neighboring sites. We show that such an interaction can correlate various modes of a driven coupled optomechanical array with well-chosen system parameters. Moreover, in the linearized regime of Gaussian fluctuations, the quantum correlations not only survive in the presence of thermal noise, but may also be generated thermally. We find that these optomechanical arrays provide a suitable platform for quantum simulation of various many-body systems.

We provide a proposal to achieve single-photon router in the microwave domain based on a waveguide quantum electrodynamics structure, which consists of a double-resonator side coupled to a superconducting transmission line. A superconducting quantum interference device is used to realize tunable and switchable coupling between the resonators; thereby the transport of a single photon in the transmission line can be effectively controlled. Our calculations show that the device has high on-off ratio and is experimentally feasible in currently available superconducting quantum circuits.

In this work we compute bound solutions for particles and antiparticles of the Dirac equation for a pure tensor radial Coulomb potential plus a constant. We find that the binding depends on the sign of the tensor constant potential, and allows only bound solutions for a certain sign and magnitude of the $\kappa$ quantum number, which is related to the spin-orbit coupling in the Dirac equation. This relation is reversed for antiparticle solutions. On the other hand, the Coulomb tensor potential, although not binding, changes the range of $\kappa$ values for which there are bound solutions.

Properly designed control has been shown to be particularly advantageous for improving adiabatic quantum computation (AQC) accuracy and time complexity scaling. Here, an in situ quantum control optimization protocol is developed to indirectly optimize state fidelity without knowledge of the instantaneous spectral gap or the computational solution. The protocol is shown to converge to analytically derived time-optimal controls for Grover's search algorithm. Furthermore, the protocol is utilized to explore optimized control trajectories for the maximum two-bit satisfiability problem, where appreciable improvement in fidelity and the minimum spectral gap over a linear schedule is observed. The approach is also shown to be robust against system model uncertainties (unitary control errors). This method is designed to enable robust control optimization on existing quantum annealing hardware and future AQC processors.

We study the process of laser-assisted spontaneous electron bremsstrahlung by running classical trajectories in a combined Coulomb and laser (ac) fields. Due to chaotic scattering in the combined Coulomb and ac fields, the radiation probability as a function of the impact parameter and the constant phase of the laser field exhibits fractal structures. However, these structures are smeared out when the cross section is integrated over the impact parameter and averaged over the phase. We analyze the role of different types of orbits, including the trapped orbits, and the dependence of the radiation probability on the impact parameter and the initial phase of the ac field. We show that, at low incident electron kinetic energy, the Coulomb focusing leads to a substantial extension of the range of impact parameters contributing to the bremsstrahlung cross section and results in a substantial increase (by one to two orders of magnitude) of the cross section as compared with the pure Coulomb case. As examples, we discuss the case of relatively high ponderomotive energy $E_p$ when we obtain an efficient production of photons with frequencies up to $2E_p$, and the case of low $E_p$ when only infrared photons are produced. Overall accuracy of the classical approach is estimated to be very good, although it does not describe resonant processes studied previously by quantum-mechanical methods.

We study the dynamics of vortices formed in a superfluid film adsorbed on the curved two-dimensional surface of a cone. To this aim, we observe that a cone can be unrolled to a sector on a plane with periodic boundary conditions on the straight sides. The sector can then be mapped conformally to the whole plane, leading to the relevant stream function. In this way, we show that a superfluid vortex on the cone precesses uniformly at fixed distance from the apex. The stream function also yields directly the interaction energy of two vortices on the cone. We then study the vortex dynamics on unbounded and bounded cones. In suitable limits, we recover the known results for dynamics on cylinders and planar annuli.

A theoretical scheme is proposed to enhance the signal-to-noise ratio in ultrasensitive detection by quantum correlation in a composite optomechanical system. Introducing an auxiliary oscillator and treating it as an added probe for weak-field detection, the additional noise can be greatly suppressed, which may even break the standard quantum limit. A magnetic field is employed as an example to exhibit the detection capability of the scheme. The result shows that, compared with the traditional detection protocol, the scheme has a higher signal-to-noise ratio and a better detection accuracy. Furthermore, the signal intensity detection curve shows a good linearity. The results provide a promising platform for reducing the additional noise by utilizing quantum correlation in ultrasensitive detection.