Using perturbed relativistic coupled-cluster (PRCC) theory we compute the ground-state electric dipole polarizability, $\alpha$, of doubly ionized alkaline earth metal ions ${\mathrm{Mg}}^{2+}$, ${\mathrm{Ca}}^{2+}$, ${\mathrm{Sr}}^{2+}$, ${\mathrm{Ba}}^{2+}$, and ${\mathrm{Ra}}^{2+}$. In the present work we use the Dirac-Coulomb-Breit atomic Hamiltonian and we also include the Uehling potential, which is the leading-order term in the vacuum polarization corrections. We examine the correction to the orbital energies arising from the Uehling potential in the self-consistent field calculations as well as perturbatively. Our results for $\alpha$ are in very good agreement with the experimental data, and we observe a change in the nature of the orbital energy corrections arising from the vacuum polarization as we go from ${\mathrm{Mg}}^{2+}$ to Ra${}^{2+}$.

Cross sections for the multiple ionization of He, Ne, Ar, and Kr by H${}^0$ impact with and without the simultaneous ionization (electron loss) of the projectile are presented in the energy range 75–300 keV. The data were measured by coincident detection of the recoil target ions and the charge-state analyzed scattered projectiles. To obtain information about the role played by the electron of H${}^0$ in the collision, the measurements were repeated with protons under the same experimental conditions. The measured data are analyzed using the classical trajectory Monte Carlo (CTMC) method. CTMC describes well the experimental data for both projectiles for single vacancy creation; however, increasing deviation is observed between theory and experiment with increasing number of created vacancies and with decreasing target atomic number.

A theoretical model for subnanosecond evolution of a nonequilibrium, inhomogeneous free-electron gas in a laser filament or microfilament wake channel is presented. The spatial distributions of electron density and temperature calculated in axially symmetric geometry as a function of time reveal dynamics on the picosecond time scale that is principally driven by a combination of thermal conduction in the electron gas and impact ionization of residual neutral atoms. At high laser intensity, the electron density evolves into a widened distribution with a sharp edge while the temperature distribution evolves to a central peak surrounded by a wide plateau. At low laser intensity, little change is seen in the electron density while the temperature again evolves to a wide plateau. The calculations show that the homogeneous electron-density approximation becomes progressively invalid at higher laser intensity. Pump-probe experiments addressing Fraunhofer diffraction patterns, four-wave mixing, and dynamic Rabi sidebands are proposed for experimental verification of the results.

Following the bichromatic sub-Doppler cooling scheme on the $D_1$ line of ${}^{40}$K recently demonstrated in  Fernandes et al. [Europhys. Lett. 100, 63001 (2012)], we introduce a similar technique for ${}^7$Li atoms and obtain temperatures of $60\mu \mathrm{K}$ while capturing all of the $5\times {10}^8$ atoms present from the previous stage. We investigate the influence of the detuning between the the two cooling frequencies and observe a threefold decrease of the temperature when the Raman condition is fulfilled. We interpret this effect as arising from extra cooling due to long-lived coherences between hyperfine states. Solving the optical Bloch equations for a simplified $\Lambda$-type three-level system we identify the presence of an efficient cooling force near the Raman condition. After transfer into a quadrupole magnetic trap, we measure a phase space density of $\sim$${10}^{-5}$. This laser cooling offers a promising route for fast evaporation of lithium atoms to quantum degeneracy in optical or magnetic traps.

We study the equilibrium correlations of a Bose gas in an elongated three-dimensional harmonic trap using a grand-canonical classical-field method. We focus in particular on the progressive transformation of the gas from the normal phase, through a phase-fluctuating quasicondensate regime to the so-called true-condensate regime, with decreasing temperature. Choosing realistic experimental parameters, we quantify the density fluctuations and phase coherence of the atomic field as functions of the system temperature. We identify the onset of Bose condensation through analysis of both the generalized Binder cumulant appropriate to the inhomogeneous system, and the suppression of the effective many-body $T$ matrix that characterizes interactions between condensate atoms in the finite-temperature field. We find that the system undergoes a second-order transition to condensation near the critical temperature for an ideal Bose gas in the strongly anisotropic three-dimensional geometry but remains in a strongly phase-fluctuating quasicondensate regime until significantly lower temperatures. We characterize the crossover from a quasicondensate to a true condensate by a qualitative change in the form of the nonlocal first-order coherence function of the field and compare our results to those of previous works employing a density-phase Bogoliubov–de Gennes analysis.

We show that the ultracold three-dimensional ${}^6$Li-${}^{40}$K mixture at unitarity can exhibit the highly exotic Larkin-Ovchinnikov superfluid phase. We determine the phase diagram for majorities of ${}^{40}$K atoms within mean-field theory taking the inhomogeneities of the fermion states into account exactly. We find two different inhomogeneous superfluid phases in mixtures with a majority of ${}^{40}$K atoms, namely the Larkin-Ovchinnikov (LO ) phase with one inhomogeneous direction and a cubic phase (LO${}^3$) where three spatial translational symmetries are broken. We determine the transition between these two phases by solving the Bogoliubov–de Gennes equations in the superfluid LO phase. Subsequently, we calculate the atomic density modulation of the atoms in the LO phase and show that it is sufficiently large to be visible in experiment.

The dynamics of an atom-molecule-conversion system subject to dephasing noises is studied in this paper. Using a dephasing master equation and mean-field theory, we derive a Bloch equation for the system, and this equation is compared with the Bloch equation derived by the Bogoliubov-Born-Green-Kirkwood-Yvon (BBGKY) hierarchy truncation approach. Fixed points of the system are calculated by solving both the Bloch equation and the master equation. Comparison between these two calculations suggests that while at short times the mean-field theory (MFT) is a good approximation for the atom-molecule-conversion system, a high-order hierarchy truncation approach is necessary for the system on a long time scale. Although the MFT cannot predict the fixed points correctly, its predictions for the stability of the fixed points are consistent with the BBGKY theory for a wide range of parameters.

We extend our time-transformation technique to include the delayed Raman response and use it to study propagation of ultrashort, few-cycle, optical pulses inside a dispersive nonlinear medium. Our technique deals directly with the electric field associated with an optical pulse and can be applied to pulses of arbitrary widths, as it does not make use of the slowly varying envelope approximation. We apply it to optical pulses containing several optical cycles and launched such that they form a third-order optical soliton. We vary the number of optical cycles within the pulse from 1 to 10 and study how the features such as soliton fission, intrapulse Raman scattering, and dispersive-wave generation depend on pulse width and soliton order. We find that for a fixed soliton order, the Raman-induced frequency shift becomes smaller, while the fraction of energy transferred to the dispersive wave increases, as pulse width is reduced. In the special case of a single-cycle pulse, the most dominant effect is self-steepening and it leads to dramatically different features in both the shape and spectrum of output pulses.

We implement a Maxwell-Bloch simulation for a two-level system within the finite-difference time-domain method to simulate the seeding of lasers by broadband pulse injection. The model does not make the slowly varying envelope approximation, and the full electromagnetic field is simulated so that we are able to obtain time-resolved seeding by few-cycle pulses. The model is compared to recent results on seeding of THz quantum cascade lasers to aid in the interpretation of their complex signals. The simulations are found to be in good agreement with the data when gain recovery times of 15 ps are used. Furthermore, we find that the emission from the laser depends only weakly on the seed used to initiate laser action. The model is readily applicable to any seeded laser system where few-cycle seed pulses are used.

We investigate the zero-temperature excitation spectrum of two-dimensional soft-core bosons for a wide range of parameters and across the phase transition from a superfluid to a supersolid state. Based on mean-field calculations and recent quantum Monte Carlo results, we demonstrate the applicability of the Bogoliubov-de Gennes equations, even at high interaction strengths where the system forms an insulating cluster crystal. Interestingly, our study reveals that the maximum energy of the longitudinal phonon band in the supersolid phase connects to the maxon energy of the superfluid at the phase transition.

There is a fundamental dimensional mismatch between the Hong-Ou-Mandel (HOM) interferometer and two-photon (2P) states: while the latter are represented using two temporal (or spectral) dimensions, the HOM interferometer allows access to only one temporal dimension. We introduce a linear 2P interferometer containing two independent delays spanning the 2P state. By “unlocking” the fixed phase relationship between the interfering 2P probability amplitudes in a HOM interferometer, one of these probability amplitudes now serves as a delay-free 2P reference against which the other beats, thereby resolving ambiguities in 2P state identification typical of HOM interferometry and extending its utility to a large family of 2P states.

A modified four-state CVQKD protocol is proposed to increase the maximum transmission distance and tolerable excess noise in the presence of Gaussian lossy and noisy channel by using a noiseless linear amplifier (NLA). An ideal NLA with gain $g$ can increase the maximum admission losses by $20{\log }_{10}g$ dB. Further, the imperfections in a practical NLA, such as the finite detection efficiency and imperfect single-photon source, are included in consideration.

Motivated by the possibility of universal quantum computation under noise perturbations, we compute the phase diagram of the two-dimensional (2D) cluster state Hamiltonian in the presence of Ising terms and magnetic fields. Unlike in previous analysis of perturbed 2D cluster states, we find strong evidence of a very well-defined cluster phase, separated from a polarized phase by a line of first- and second-order transitions compatible with the 3D Ising universality class and a tricritical end point. The phase boundary sets an upper bound for the amount of perturbation in the system so that its ground state is still useful for measurement-based quantum computation purposes. Moreover, we also compute the local fidelity with the unperturbed 2D cluster state. Besides a classical approximation, we determine the phase diagram by combining series expansion and variational infinite projected entangled-pair states methods. Our work constitutes an analysis of the nontrivial effect of few-body perturbations in the 2D cluster state, which is of relevance for experimental proposals.

Establishing an information-theoretic secret key between two parties using a quantum key distribution (QKD) system is only possible when an accurate characterization of the quantum channel and proper device calibration routines are combined. Indeed, security loopholes due to inappropriate calibration routines have been shown for discrete-variable QKD. Here, we propose and provide experimental evidence of an attack targeting the local oscillator calibration routine of a continuous-variable QKD system. The attack consists in manipulating the classical local oscillator pulses during the QKD run in order to modify the clock pulses used at the detection stage. This allows the eavesdropper to bias the shot-noise estimation usually performed using a calibrated relationship. This loophole can be used to perform successfully an intercept-resend attack. We characterize the loophole and suggest possible countermeasures.

We investigate the stability of supercurrents in a Bose-Einstein condensate with one-dimensional spin-orbit and Raman couplings. The consequence of the lack of Galilean invariance is explicitly discussed. We show that in the plane-wave phase, characterized by a uniform density, the supercurrent state can become dynamically unstable, the instability being associated with the occurrence of a complex sound velocity, in a region where the effective mass is negative. We also discuss the emergence of energetic instability in these supercurrent states. We argue that both the dynamical and the energetic instabilities in these systems can be generated experimentally through excitation of the collective dipole oscillation.

In this work, an analytical and numerical study of the relativistic interaction of a linearly polarized laser field of ${\omega }_1$ and ${\omega }_2$ frequencies with highly overdense plasma at normal and oblique incidence is presented. Combining harmonics of ${\omega }_1$ and ${\omega }_2$ frequencies causes the amplitude oscillation of relativistic mirrors to be increased. Therefore, the intensity of generated harmonics will be increased considerably. Also, in agreement with the fully relativistic one-dimensional particle-in-cell (PIC) simulation, the relativistic mirror model explains the enhancement of the harmonic intensity of the upper spectra in the interaction of the two-color mixing laser pulse with overdense plasma. Our results show that increasing the intensity of high-order harmonics by using the two-color pulse will enhance sharply the intensity of ultrashort attosecond pulses.

We have studied the phase diagram of a quasi-two-dimensional interacting Bose gas at zero temperature in the presence of random potential created by laser speckles. The superfluid fraction and the fraction of particles with zero momentum are obtained within the mean-field Gross-Pitaevskii theory and in diffusion Monte Carlo simulations. We find a transition from the superfluid to the insulating state when the strength of the disorder grows. Estimations of the critical parameters are compared with the predictions of the percolation theory in the Thomas-Fermi approximation. Analytical expressions for the zero-momentum fraction and the superfluid fraction are derived in the limit of weak disorder and weak interactions within the framework of the Bogoliubov theory. Limits of validity of various approximations are discussed.

We consider the dispersion energy of a pair of dipoles embedded in a metallic waveguide with transverse dimension $a$ smaller than the characteristic dipolar wavelength. We find that $a$ sets the scale that separates retarded, Casimir-Polder-like from quasistatic, van der Waals-like interactions. Whereas in the retarded regime the energy decays exponentially with interdipolar distance, typical of evanescent waves, in the van der Waals regime, the known free-space result is obtained. This short-range scaling implies that the additivity of the dispersion interactions inside a waveguide extends to denser media, along with modifications to related Casimir effects in such structures.

Two different models are presented that allow for efficiently performing routing of a quantum state. Both cases involve an $XX$ spin chain working as a data bus and additional spins that play the role of sender and receivers, one of which is selected to be the target of the quantum state transmission protocol via a coherent quantum coupling mechanism making use of local and/or global magnetic fields. Quantum routing is achieved in the first of the models considered by weakly coupling the sender and the receiver to the data bus. On the other hand, in the second model, local magnetic fields acting on additional spins located between the sender and receiver and the data bus allow us to perform high-fidelity routing.

Quantum communication theory sets the maximum rates at which information can be encoded and decoded reliably given the physical properties of the information carriers. Here we consider the problem of readout of a digital optical memory, where information is stored by means of the optical properties of the memory cells that are in turn probed by shining a laser beam on them. Interesting features arise in the regime in which the probing light has to be treated quantum mechanically. The maximum rate of reliable readout defines the quantum reading capacity, which is proven to overcome the classical reading capacity—obtained by probing with classical light—in several relevant settings. We consider a model of optical memory in which information is encoded in the (complex-valued) attenuation factor and study the effects on the reading rates of thermal and correlated noise. The latter type of noise arises when the effects of wave diffraction on the probing light beam are taken into account. We discuss the advantages of quantum reading over the classical one and show that the former is substantially more robust than the latter under thermal noise in the regime of low power per pulse.