We have calculated ac polarizabilities of the $2{}^{3}S$ and $2{}^{1}S$ states of both ${}^4\mathrm{He}$ and ${}^3\mathrm{He}$ in the range 318 nm to 2.5 $\mu \mathrm{m}$ and determined the magic wavelengths at which these polarizabilities are equal for either isotope. The calculations, only based on available ab initio tables of level energies and Einstein A coefficients, do not require advanced theoretical techniques. The polarizability contribution of the continuum is calculated using a simple extrapolation beyond the ionization limit, yet the results agree to better than $1\%$ with such advanced techniques. Several promising magic wavelengths are identified around 320 nm with sufficient accuracy to design an appropriate laser system. The extension of the calculations to ${}^3\mathrm{He}$ is complicated due to the additional hyperfine structure, but we show that the magic wavelength candidates around 320 nm are predominantly shifted by the isotope shift.

A high-precision relativistic calculation of Cd-like $\mathrm{Nd}{}^{12+},\mathrm{Sm}{}^{14+}$ and Sn-like $\mathrm{Pr}{}^{9+},\mathrm{Nd}{}^{10+}$ atomic properties is carried out using an approach that combines configuration interaction and a linearized coupled-cluster method. These ions have long-lived metastable states with transitions accessible by laser excitations, relatively simple electronic structure, high sensitivity to $\alpha$ variation, and stable isotopes. Breit and QED corrections were included into the calculations. Energies, transition wavelengths, electric- and magnetic-multipole reduced matrix elements, lifetimes, and sensitivity coefficients $q$ and $K$ to the variation of the fine-structure constant $\alpha$ were obtained. A detailed study of uncertainties was performed. Energies for similar Cd-like $\mathrm{Ba}{}^{8+},\mathrm{La}{}^{9+},\mathrm{Ce}{}^{10+},\mathrm{Pr}{}^{11+}$ and Sn-like $\mathrm{Ba}{}^{6+}$ ions were calculated and compared with experiment for further tests of the accuracy.

We demonstrate control of the differential Zeeman shift between clock states of ultracold rubidium atoms by means of nonresonant microwave dressing. Using the dc field dependence of the microwave detuning, we suppress the first- and second-order differential Zeeman shift in magnetically trapped ${}^{87}\mathrm{Rb}$ atoms. By dressing the state pair $5S_{1/2}$ $F=1$, $m_F=-1$ and $F=2$, $m_F=1$, a residual frequency spread of $<0.1$ Hz in a range of 100 mG around a chosen magnetic offset field can be achieved. This is one order of magnitude smaller than the shift of the bare states at the magic field of the Breit-Rabi parabola. We further identify double magic points, around which the clock frequency is insensitive to fluctuations both in the magnetic field and in the dressing Rabi frequency. The technique is compatible with chip-based cold-atom systems and allows the creation of clock and qubit states with reduced sensitivity to magnetic-field noise.

Spatial light modulator (SLM) controlled transmission of light through opaque media is a relatively new experimental method with wide applications in various fields. While there has been a surge in research into the technique, there has been little work reported considering the effects of various experimental parameters on the efficiency of optimization. In this study, we explore the effects of various experimental conditions on optimization and find that the intensity enhancement depends on the number of modulated channels, number of phase steps, feedback integration radius, beam spot size, and active SLM area. We also develop a model, based on the propagation of a Gaussian beam with a random phase front, to account for most of the measured effects.

We investigate the possibility of using the independence of the transmitted speckle pattern on the illumination condition as a signature of Anderson localization in a single configuration of a two-dimensional and open disordered medium. The analysis is based on exact numerical simulations of multiple light scattering. We introduce a similarity function that we propose as a reliable observable to probe Anderson localization without requiring any statistical averaging over an ensemble.

The semiclassical perturbation method developed by Henkel et al. [J. Phys. II 4, 1955 (1994)] to model cold-atom diffraction by optical standing waves, is applied to the diffraction of fast atoms on crystal surfaces at grazing incidence (GIFAD or FAD). We first show that the interaction time and interaction length embedded in the obliquity factor is well suited to explain the transition from three-dimensional to two-dimensional (2D) diffraction. The situation of a slightly misaligned primary beam, corresponding to oblique incidence in the effective 2D system, is addressed pointing out discrepancies such as the absence of net deflection of the atomic beam. Guided by time-reversal considerations, we propose an arbitrarily symmetrized form significantly improving the agreement with experimental data recorded in oblique incidence.

Electron-positron-pair creation in collisions of a modulated laser pulse with a high-energy photon (nonlinear Breit-Wheeler process) is studied by means of strong-field quantum electrodynamics. It is shown that the driving pulse modulations lead to the appearance of comb structures in the energy spectra of produced positrons (electrons). It is demonstrated that these combs result from a coherent enhancement of probability amplitudes of pair creation from different modulations of the laser pulse, thus resembling the Young-type double-slit experiment for antimatter (matter) waves.

In principle, quantum key distribution (QKD) offers unconditional security based on the laws of physics. Unfortunately, all previous QKD experiments assume perfect state preparation in their security analysis. Therefore, the generated key is not proven to be secure in the presence of unavoidable modulation errors. The key reason that modulation errors are not considered in previous QKD experiments lies in a crucial weakness of the standard Gottesman-Lo-Lütkenhaus-Preskill (GLLP) model, namely, it is not loss tolerant and Eve may in principle enhance imperfections through losses. Here, we propose a QKD protocol that is loss tolerant to state preparation flaws. Importantly, we show conclusively that the state preparation process in QKD can be much less precise than initially thought. Our method can also be applied to other quantum cryptographic protocols.

The permanent electric dipole moment of the $X^2{\Sigma }^{+}$ electronic ground state of the strontium monofluoride molecule is calculated using a relativistic coupled-cluster method. Our result is compared with those of other calculations and that of experiment. Individual contributions arising from different physical effects are presented. The result obtained suggests that the relativistic coupled-cluster method used in the present work is capable of yielding accurate results for the permanent electric dipole moments of molecules for which relativistic effects cannot be ignored.

We have measured fully differential cross sections for ionization of ${\mathrm{H}}_2$ by 75-keV proton impact. The coherence length of the projectile beam was varied by changing the distance between a collimating slit and the target. By comparing the cross sections measured for large and small coherence lengths pronounced interference effects could be identified in the data. A surprising result is that the phase angle in the interference term is primarily determined by the momentum transfer and only to a lesser extent by the recoil-ion momentum.

Bond oscillation, a phenomenon characteristic of the molecular electron-density distribution in momentum space, is demonstrated for the $1{\sigma }_g$ molecular orbital of ${\mathrm{H}}_2$ with an ($e$, $2e$) experiment at large momentum transfer. Analysis of the experimental data in terms of two-center interference effects has revealed that different oscillatory structures can be observed, depending on the model for describing ($e$, $2e$) ionization from the constituent H $1s$ atomic orbitals. It is shown that bond oscillation is highly sensitive to the spatial pattern and chemical bonding nature of the molecular orbital.

Several kinds of interference effects in laser-assisted photoionization of ${{\mathrm{H}}_2}^{+}$ molecular targets are theoretically investigated by means of a Coulomb-Volkov model. The photoelectron angular distributions in the single-photon ionization reaction are compared to more elaborated results, and the influence of usual approximations are studied. Several configurations of pulses and assistant lasers are considered. Moreover, we show that previous interference effects predicted for monochromatic pulses could be enhanced and/or diminished leading to a directional selectivity of photoelectron emission. Finally, we analyze the influence of the pulse duration on the photoelectron spectrum.

We have studied the influence of a linearly polarized laser field on the dynamics of low $(e,2e)$ collisions in atomic hydrogen. The influence of the laser on the target states is treated using a first-order perturbation approach. The continuum states of the scattered and ejected electrons are described respectively by Volkov and Coulomb-Volkov wave functions. The second Born approximation is used to calculate triple differential cross sections for laser-assisted ionization by low-energy electron impact. The required scattering amplitudes are evaluated by using the Sturmian basis expansion. The influence of the laser parameters (photon energy, intensity, and direction of polarization) on the triple differential cross sections is analyzed, and several illustrative examples are discussed. Our second Born approximation results agree very well with those obtained in the first-order Born approximation at larger incident energies. The margins between the second and first Born approximation results are large at low incident energies in the vicinity of the recoil peaks.

We suggest that nonequilibrium Bose-Einstein condensates may occur in time-periodically driven interacting Bose gases. Employing the model of a periodically forced bosonic Josephson junction, we demonstrate that resonance-induced ground-state-like many-particle Floquet states possess an almost perfect degree of coherence, as corresponding to a mesoscopically occupied, explicitly time-dependent single-particle orbital. In marked contrast to the customary time-independent Bose-Einstein condensates, the order parameter of such systems is destroyed by violent fluctuations when the particle number becomes too large, signaling the nonexistence of a proper mean-field limit.

Recent experimental realizations of artificial gauge fields for cold atoms are promising for generating steady states carrying a mass current in strongly correlated systems, such as the Bose-Hubbard model. Moreover, a homogeneous condensate confined by hard-wall potentials from laser sheets has been demonstrated, which provides opportunities for probing the intrinsic transport properties of isolated quantum systems. Using the time-dependent density matrix renormalization group, we analyze the effect of the lattice and interaction strength on the current generated by a quench in the artificial vector potential when the density varies from low values (continuum limit) up to integer filling in the Mott-insulator regime. There is no observable mass current deep in the Mott-insulator state as one may expect. Other observable quantities used to characterize the quasisteady state in the bulk of the system are the Drude weight and entanglement entropy production rate. The latter in particular provides a striking signature of the superfluid–Mott-insulator transition. Furthermore, an interesting property of the superfluid state is the formation of shock and rarefaction waves at the boundaries due to the hard-wall confining potentials. We provide results for the height and the speed of the shock front that propagates from the boundary toward the center of the lattice. Our results should be verifiable with current experimental capabilities.

We develop theory for fluctuations in atom number and spin within finite-sized cells of a spinor Bose-Einstein condensate. This theory provides a model of measurements that can be performed in current experiments using finite resolution in situ imaging. We develop analytic results for quantum and thermodynamic limits of the fluctuations and apply our theory to the four equilibrium phases of a spin-1 condensate. We then validate these limits and examine the behavior over a wide parameter regime using numerical calculations specialized to the case of a spinor condensate confined to be quasi-two-dimensional.

We employ coupled wave theory to enumerate the lasing modes of structurally chiral lasers. The elliptical modes are shown to be fundamentally distinct from those of a scalar distributed feedback laser. High threshold modes are shown to lase with the opposite chirality as the active medium, in contrast to their low-threshold counterparts that lase with the same chirality as the active medium. The lasing mode structure suggests the intriguing possibility of dynamically changing the polarization handedness of a chiral laser, as well as the possibility of lasing within the forbidden band-gap region. These observations arise from the fundamental interplay between the distributed chirality-preserving reflections within the active medium and the localized chirality-reversing reflections at the medium's boundaries.

Single photons exhibit nonclassical, counterintuitive behavior that can be exploited in the developing field of quantum technology. They are needed for various applications such as quantum key distribution, optical quantum information processing, quantum computing, intensity measurement standards, and others yet to be discovered in this developing field. This drives the current intensive research into the realization of true deterministic sources of single photons on demand. Lacking such a source, many researchers default to the well-established workhorse: spontaneous parametric down-conversion that generates entangled signal-idler pairs. Since this source is thermal statistical in nature, it is common to use a detected idler photon to herald the production of a signal photon. The need exists to determine the quality of the single photons generated in the heralded signal beam. Quite often, the literature reports a heralded second-order coherence function of the signal photons conditioned on the idler photons using readily available single-photon detectors. In this work we examine the applicability of this technique to single-photon characterization and the consequences of the fact that the most commonly used single-photon detectors are not photon-number resolving. Our results show that this method using non-photon-resolving detectors can only be used to characterize the signal-idler correlations rather than the nature of the signal-photon state alone.

We study the influence of a detector on the decay law of a quantum state whose undisturbed survival probability is purely exponential. In particular, we consider a detector with a finite energy band of detection, i.e., it interacts only with decay products having an energy within a certain range of values. In one case, we assume that the detector performs many repeated measurements at short time intervals in all of which a collapse of the wave function occurs (bang-bang or pulsed-type measurements). In the second case, we assume a continuous measurement, which preserves unitarity. We confirm the slowing down of the decay in presence of a measuring apparatus, the quantum Zeno effect, but the outcomes of the detector are in general qualitatively and quantitatively different in the two cases. In turn, this implies that the so-called Schulman relation (the equivalence of pulsed and continuous measurements) does not hold in this case and that it is in principle possible to experimentally access how a certain detector performs a measurement.

We demonstrate, in the regime of ultrastrong matter-field coupling, the strong connection between the dynamical Casimir effect (DCE) and the performance of quantum-information protocols. Our results are illustrated by means of a realistic quantum communication channel and show that the DCE is a fundamental limit for quantum computation and communication and that novel schemes are required to implement ultrafast and reliable quantum gates. Strategies to partially counteract the DCE are also discussed.