We present electronic stopping power data for H and He ions in Zn and In obtained from backscattering experiments in the range of low velocities; for In data are obtained up to the stopping maximum. These results in combination with previous data are analyzed to unravel the influence of $d$ electrons in electronic stopping of slow light ions. We report on a systematic difference between the stopping behavior of H and He ions at velocities below 0.2 a.u. Furthermore, we find that the $d$-band offset is not correlated to the velocity at which deviations from velocity proportionality occur. This observation is in contrast to the state-of-the-art models of electronic stopping based on electron-hole pair excitation in a binary Coulomb collision. In this regard we discuss the possible relevance of electron promotion processes in atomic collisions.

We explore the three-body problem in two dimensions using the adiabatic hyperspherical representation. We develop the main equations in terms of democratic hyperangular coordinates and determine several symmetry properties and boundary conditions for both interacting and noninteracting solutions. From the analysis of the three-body effective potentials, we determine the threshold laws for low-energy three-body recombination, collision-induced dissociation, as well as inelastic atom-diatom collisions in two dimensions. Our results show that the hyperspherical representation can offer a simple and conceptually clear physical picture for three-body process in two dimensions which is also suitable for calculations using finite-range two-body interactions supporting a number of bound states.

Stationary afterglow measurements in conjunction with near-infrared absorption spectroscopy show that the recombination of the ${\text{H}}_3{}^{+}$ ion with electrons in ionized gas mixtures of He, Ar, and ${\text{H}}_2$ at 300 K is strongly enhanced by neutral helium and by molecular hydrogen. The ${\text{H}}_2$-assisted ternary recombination coefficient $K_{{\text{H}}_2}=(8.7\pm 1.5)\times {10}^{-23}{\text{cm}}^6{\text{s}}^{-1}$ substantially exceeds the value measured for ${\text{H}}_3{}^{+}$ in ambient helium $(K_{\mathrm{He}}\sim {10}^{-25}{\text{cm}}^6{\text{s}}^{-1})$ or predicted by the generally accepted classical theory of Bates and Khare $(\sim {10}^{-27}{\text{cm}}^6{\text{s}}^{-1})$ for atomic ions. Because of the extremely large value of $K_{{\text{H}}_2}$ in a hydrogen plasma the ternary recombination dominates over binary recombination already at pressures above 3 Pa. This can have consequences in plasma physics, astrophysics, recombination pumped lasers, plasma spectroscopy, plasmatic technologies, etc. The ternary processes provide a plausible explanation for the discrepancies between many earlier experimental results on ${\text{H}}_3{}^{+}$ recombination. The observation that the ternary process saturates at high He and ${\text{H}}_2$ densities suggests that recombination proceeds by a two-step process: formation of a long-lived complex [with a rate coefficient ${\alpha }_{\mathrm{F}}=(1.5\pm 0.1)\times {10}^{-7}{\text{cm}}^3{\text{s}}^{-1}]$ followed by collisional stabilization.

We predict the effect of the roton instability for a two-dimensional weakly interacting gas of tilted dipoles in a single homogeneous quantum layer. Being typical for strongly correlated systems, the roton phenomena appear to occur in a weakly interacting gas. It is important that in contrast to a system of normal to the layer dipoles, breaking of the rotational symmetry for a system of tilted dipoles leads to the convergence of the condensate depletion even up to the threshold of the roton instability, with mean-field approach being valid. Predicted effects can be observed in a wide class of dipolar systems. We suggest observing predicted phenomena for systems of ultracold atoms and polar molecules in optical lattices, and estimate optimal experimental parameters.

We present a microscopic model of cavity quantum electrodynamics based on a one-dimensional (1D) coupled cavity array (CCA), where a supercavity (SC) is composed by a segment of the 1D CCA with relatively smaller couplings with the outsides. The single-photon scattering problem for the SC empty or with a two level atom in it is investigated. We obtain the exact theoretical result on the transmission rate for our system, which predicts that the transmission peaks shall appear near the eigenenergies of the SC. Our numerical results further prove that the SC is a well-defined multimode cavity. When a two level atom resonant with the SC locates at the antinode of the resonant mode, the transmission spectrum shows a clear sign of vacuum Rabi splitting as expected. However, when the atom locates at the node of the resonant mode, we observe a deep valley in the transmission peak, which can be explained by the destructive interference of two transmission channels: one is the resonant mode, while the other is arising from the atom coupling with the nonresonance modes. The effect of nonresonance modes on vacuum Rabi splitting is also analyzed.

Adiabatic transport provides a powerful way to manipulate quantum states. By preparing a system in a readily initialized state and then slowly changing its Hamiltonian, one may achieve quantum states that would otherwise be inaccessible. Moreover, a judicious choice of final Hamiltonian whose ground state encodes the solution to a problem allows adiabatic transport to be used for universal quantum computation. However, the dephasing effects of the environment limit the quantum correlations that an open system can support and degrade the power of such adiabatic computation. We quantify this effect by allowing the system to evolve over a restricted set of quantum states, providing a link between physically inspired classical optimization algorithms and quantum adiabatic optimization. This perspective allows us to develop benchmarks to bound the quantum correlations harnessed by an adiabatic computation. We apply these to the D-Wave Vesuvius machine with revealing—though inconclusive—results.

Motivated by the remarkable experimental control of synthetic gauge fields in ultracold atomic systems, we investigate the effect of an artificial Rashba spin-orbit coupling on the spin polarization of a two-dimensional repulsive Fermi gas. By using a variational many-body wave function, based on a suitable spinorial structure, we find that the polarization properties of the system are indeed controlled by the interplay between spin-orbit coupling and repulsive interaction. In particular, two main effects are found: (1) The Rashba coupling determines a gradual increase of the degree of polarization beyond the critical repulsive interaction strength, at variance with conventional two-dimensional Stoner instability. (2) The critical interaction strength, above which finite polarization is developed, shows a dependence on the Rashba coupling, i.e., it is enhanced in case the Rashba coupling exceeds a critical value. A simple analytic expression for the critical interaction strength is further derived in the context of our variational formulation, which allows for a straightforward and insightful analysis of the present problem.

Expressions for the speed and lifetimes of the sound modes in a dilute Bose-Einstein condensate are obtained using Bogoliubov mean field theory. The condensate has two pairs of sound modes which undergo an avoided crossing as the equilibrium temperature is varied. The two pairs of sound modes decay at very different rates, except in the neighborhood of the avoided crossing, where the identity of the longest-lived mode switches. The predicted speed and lifetime of the longest-lived sound mode are consistent with recent experimental observations on sound in an ${}^{87}\mathrm{Rb}$ Bose-Einstein condensate.

We study the far-from-equilibrium statistical mechanics of periodically driven fermionic atoms in a lossy optical resonator. We show that the interplay of the Fermi surface with cavity losses leads to subnatural cavity linewidth narrowing, squeezed light, and nonthermal quantum statistics of the atoms. Adapting the Keldysh approach, we set up and solve a quantum kinetic Boltzmann equation in a systematic $1/N$ expansion with $N$ the number of atoms. In the strict thermodynamic limit $N,V\to \infty ,N/V=\text{const.}$ we find that the atoms (fermions or bosons) remain immune against cavity-induced heating or cooling. At next-to-leading order in $1/N$, we find a “one-way thermalization” of the atoms determined by cavity decay. In absence of an equilibrium fluctuation-dissipation relation, the long-time limit $\Delta t\to \infty$ does not commute with the thermodynamic limit $N\to \infty$, such that for the physically relevant case of large but finite $N$, the dynamics ultimately becomes strongly coupled, especially close to the superradiance phase transition.

We study an atom-cavity system composed of an ensemble of three-level atoms interacting with a high-finesse single-mode optical resonator. Two examples of physical models, the laser-driven $\Lambda$-type atomic system and the three-level ladder-type atomic system, are investigated in detail. By altering the collective atom-cavity dynamics via an intracavity optical parametric amplification process, a spin-boson model is realized when the composite system operates in the far-detuning regime. When the number of atoms exceeds a critical value, the quantum system undergoes a superradiant phase transition whose dependence on different experimentally adjustable parameters is explored in detail. The effects of the spontaneous emission of atoms and the cavity loss on the phase transition are considered, and measuring the cavity output field itself gives an experimental probe of the superradiant phase transition.

We study the dynamics of a driven optomechanical cavity coupled to a charged nanomechanical resonator via Coulomb interaction, in which the tunable double optomechanically induced transparency (OMIT) can be observed from the output field at the probe frequency by controlling the strength of the Coulomb interaction. We calculate the splitting of the two transparency windows, which varies near linearly with the Coulomb coupling strength in a robust way against the cavity decay. Our double-OMIT is much different from the previously mentioned double-EIT or double-OMIT, and might be applied to measure the Coulomb coupling strength.

We capitalize on a multipolar expansion of the polarization density matrix, in which multipoles appear as successive moments of the Stokes variables. When all the multipoles up to a given order $K$ vanish, we can properly say that the state is $K\mathrm{th}$-order unpolarized, as it lacks of polarization information to that order. First-order unpolarized states coincide with the corresponding classical ones, whereas unpolarized to any order tally with the quantum notion of fully invariant states. In between these two extreme cases, there is a rich variety of situations that are explored here. The existence of hidden polarization emerges in a natural way in this context.

The Talbot effect, i.e., the self-imaging property of a periodic wave in near-field diffraction, is a remarkable interference phenomenon in paraxial systems with continuous translational invariance. In crystals, i.e., systems with discrete translational invariance, self-imaging has been regarded so far as a rare effect, restricted to special sets of initial field distributions. Here it is shown that in a class of gapless $\mathcal{PT}$-symmetric complex crystals at the symmetry-breaking threshold Talbot revivals can arise for almost any initial periodic wave distribution which is commensurate with the lattice period. A possible experimental realization of commensurate Talbot self-imaging for light pulses in complex “temporal” crystals, realized in an optical dispersive fiber loop with amplitude and phase modulators, is briefly discussed.

The performance of a quantum information processor depends on the precise control of phases introduced into the system during quantum gate operations. As the number of operations increases with the complexity of a computation, the phases of the gates at different locations and different times must be controlled, which can be challenging for optically driven operations. We circumvent this issue by demonstrating an entangling gate between two trapped atomic ions that is insensitive to the optical phases of the driving fields while using a common master reference clock for all coherent qubit operations. Such techniques may be crucial for scaling to large quantum information processors in many physical platforms.

We study a wide range of neutral atoms and ions suitable for ultraprecise atomic optical clocks with naturally suppressed blackbody-radiation shift of clock transition frequency. Calculations show that scalar polarizabilities of clock states cancel each other for at least one order of magnitude for the considered systems. Results for calculations of frequencies, quadrupole moments of the states, clock transition amplitudes, and natural widths of upper clock states are presented.

High-order-harmonic generation is investigated for ${\mathrm{H}}_2{}^{+}$ and ${\mathrm{D}}_2{}^{+}$ with and without a Born-Oppenheimer approximation by a numerical solution of a full dimensional electronic time-dependent Schrödinger equation under four-cycle intense laser pulses of 800 nm wavelength and $I=4$, 5, 7, and $10\times {10}^{14}$ $\mathrm{W}/{\mathrm{cm}}^2$ intensities. For most harmonic orders, the intensity obtained for ${\mathrm{D}}_2{}^{+}$ is higher than that for ${\mathrm{H}}_2{}^{+}$, and the yield difference increases as the harmonic order increases. Only at some low harmonic orders, ${\mathrm{H}}_2{}^{+}$ generates more intense harmonics compared to ${\mathrm{D}}_2{}^{+}$. The results show that nuclear motion, ionization probability, and system dimensionality must be simultaneously taken into account to properly explain the isotopic effects on high-order-harmonic generation and to justify experimental observations.

A simple model of parametric coupling between two fermionic oscillators is considered. Statistical properties, in particular the mean and variance of quanta for a single mode, are described by means of a time-dependent reduced density operator for the system and the associated $P$ function. The density operator for fermionic fields as introduced by Cahill and Glauber [K. E. Cahill and R. J. Glauber, Phys. Rev. A 59, 1538 (1999)] thus can be shown to provide a quantum mechanical description of the fields closely resembling their bosonic counterpart. In doing so, special emphasis is given to population trapping, and quantum control over the states of the system.