We show that the decay rate and the frequency shift of the radiation modes from a small sphere of two-level atoms with varying radial density can be accurately obtained using an electrostatic model.

To overcome the fundamental limitations of the $\pi$ optical pulse-induced population inversion and optical decay-caused short storage time in conventional photon echoes, a coherent control of collective atoms is studied for inversion-free, optical decay-halted photon echoes, where the constraint of photon storage time is now replaced by a spin population decay process. Using phase-controlled double rephasing, an inversion-free photon echo scheme is obtained, where no spontaneous or stimulated emission-driven quantum noise exists. Thus, the present method can be applied for ultralong quantum memories in quantum repeaters for long-distance quantum communications.

It has been recently shown that by varying the potential parameters in time so that a non-Hermitian degeneracy [exceptional point (EP)] is cycled, the nonadiabatic couplings have a large effect on the dynamics. This cardinal effect does not disappear even when the potential parameter are changed arbitrarily slow (“adiabatic process”). For a specific Hamiltonian this counterintuitive effect has been shown to be associated with the Stokes phenomenon. We study analytically the effects of EP cycling on the transmission and reflection of light in a waveguide with a small complex refractive index modulation. We find that in the adiabatic limit the oscillations in the transmission spectrum are sharply attenuated above a certain frequency where the EP is no longer cycled.

We propose a hybrid photonic-plasmonic resonant structure which consists of a metal nanoparticle (MNP) and a whispering-gallery-mode (WGM) microcavity. It is found that the hybrid mode enables a strong interaction between the light and matter, and the single-atom cooperativity is enhanced by more than two orders of magnitude compared to that in a bare WGM microcavity. This improvement originates from two aspects: the MNP offers a highly enhanced local field in the vicinity of an emitter, and, interestingly, the high-$Q$ property of WGMs can be maintained in the presence of the MNP. Thus the present system has advantages over a single microcavity or a single MNP, and holds potential in quantum optics, nonlinear optics, and highly sensitive biosensing.

We present a Hamiltonian quantum-computation scheme universal for quantum computation. Our Hamiltonian is a sum of a polynomial number (in the number of gates $L$ in the quantum circuit) of constant-norm, time-independent, two-body interaction terms. Furthermore, each qubit in the system interacts only with a constant number of other qubits in a three-layer, geometrically local layout. The computer runs in three steps—it starts in a simple initial product state, evolves according to a time-independent Hamiltonian for time of order $L^2$ (up to logarithmic factors), and finishes with a two-qubit measurement. Our model improves previous universal two-local-Hamiltonian constructions, as it avoids using perturbation gadgets and large energy-penalty terms in the Hamiltonian, which would result in a large required run time.

Several scenarios are proposed and analyzed for engineering of arbitrary preselected SU(3) transformations of laser-driven atomic qutrits. Two of the most natural implementations of qutrits are considered, in which the three qutrit states are coupled to each other via two-photon transitions through either (i) a common state in a tripod linkage pattern or (ii) two upper states in an M-shaped linkage chain. The SU(3) transformation for the tripod qutrit can be realized by 3 Givens SU(2) rotations, which require 9 consecutive interaction steps. Alternatively, because under certain conditions the propagator of the tripod system reduces to the Householder reflection operator, any SU(3) transformation can be constructed physically by 3 Householder reflections, each of which is implemented in a single interaction step. As an example, the discrete Fourier transform can be synthesized by 7 consecutive interaction steps with Givens rotations or, alternatively, by only a single Householder reflection and a phase gate. For the M-qutrit, the propagator is given by coupled Householder reflections and it cannot be reduced to Givens rotations or independent Householder reflections. By using these coupled Householder reflections it is shown that an arbitrary SU(3) transformation of the M qutrit can be realized with just two fields in at most 3 interaction steps; the discrete Fourier transform, in particular, requires only 2 interaction steps.

We have investigated the formation of coherent vibrational wave packets in the ground electronic state of HD${}^{+}$ on exposure to intense ultrashort laser pulses of wavelength 1060 nm. The effects of the duration and field strength of the pulse on the final composition of the residual bound nuclear wave packet generated by such impulsive excitations have been studied. The resulting wave packet is allowed to evolve freely on the potential surface for some time, after which a weak pulse of sufficiently large duration is used for probing its composition. This pulse can cause only single-photon dissociation. The simulations have been performed with different probe wavelengths for accessing information about different portions of the wave packet in the vibrational quantum number space. Our aim was to investigate the extent to which information obtained from the kinetic-energy spectra of the photofragments induced by the probe pulse can be correlated to the structure of the wave packet. Simple time-dependent perturbation calculations have also been performed for obtaining the relative strengths of photofragment signals arising from different vibrational levels due to wave-packet dissociation. A comparison with our numerical results indicates that though the general features of the photofragment kinetic-energy spectra from a wave packet are consistent with the perturbation theory results in the intensity regime studied, dynamical evolution during a long pulse can modify the relative heights of the kinetic-energy peaks through nonperturbative interactions in some cases.

We study the states of one and two atoms in a rotating ring lattice in a Hubbard-type tight-binding model. The model is developed carefully from basic principles in order to properly identify the physical observables. The one-particle ground state may be degenerate and represent a finite flow velocity depending on the parity of the number of lattice sites, the sign of the tunneling matrix element, and the rotation speed of the lattice. Variation of the rotation speed may be used to control one-atom states and leads to peculiar behaviors such as wildly different phase and group velocities for an atom. Adiabatic variation of the rotation speed of the lattice may also be used to control the state of a two-atom lattice dimer. For instance, at a suitably chosen rotation speed both atoms are confined to the same lattice site.

We study bosonic atoms on the $p$ band of a two-dimensional optical square lattice in the presence of a confining trapping potential. Using a mean-field approach, we show how the anisotropic tunneling for $p$-band particles affects the cloud of condensed atoms by characterizing the ground-state density and the coherence properties of the atomic states both between sites and atomic flavors. In contrast to the usual results based on the local-density approximation, the atomic density can become anisotropic. This anisotropic effect is especially pronounced in the limit of weak atom-atom interactions and of weak lattice amplitudes, i.e., when the properties of the ground state are mainly driven by the kinetic energies. We also investigate how the trap influences known properties of the nontrapped case. In particular, we focus on the behavior of the antiferromagnetic vortex-antivortex order, which for the confined system is shown to disappear at the edges of the condensed cloud.

Microparticle self assembly under the influence of optical forces produced by higher-order optical beams or by projection of a hologram into the trapping volume is well known. In this paper, we report the spontaneous formation of a ring of identical microspheres (each with diameter 1.1 $\mu$m) in conventional single-beam optical tweezers with a usual TEM${}_{00}$ Gaussian beam coupled into a sample chamber having a standing wave geometry with a cover slip and glass slide. The effects of different experimental parameters on the ring formation are studied extensively. The experimental observations are backed by theoretical simulations based on a plane wave decomposition of the forward- and backward-propagating Gaussian beams. The ring patterns are shown to be caused due to geometrical aberrations produced by focusing the Gaussian beam using a high-numerical-aperture microscope objective into stratified media. It is found that the thickness of the stratified media and the standing wave geometry itself play a critical role in the formation of stable ring structures. These structures could be used in the study of optical binding, as well as of biological interactions between cells in an optical trap.

We develop a perturbative theory that describes a superluminal refractive perturbation propagating in a dispersive medium and the subsequent excitation of the quantum vacuum zero-point fluctuations. We find a pro-cess similar to the anomalous Doppler effect: Photons are emitted in correlated pairs and mainly within a Cerenkov-like cone, one on the forward and the other in backward directions. The number of photon pairs emitted from the perturbation increases strongly with the degree of superluminality and under realizable experimental conditions; it can reach up to $\sim$10${}^{-2}$ photons per pulse. Moreover, it is, in principle, possible to engineer the host medium so as to modify the effective group refractive index. In the presence of “fast-light” media, e.g., a with group index smaller than unity, a further $\sim$10$\times$ enhancement may be achieved and the photon emission spectrum is characterized by two sharp peaks that, in future experiments, would clearly identify the correlated emission of photon pairs.

Weak-interaction contributions to hyperfine splitting and Lamb shift in light electronic and muonic atoms are calculated. We notice that correction to hyperfine splitting turns into zero for deuterium. Weak correction to the Lamb shift in hydrogen is additionally suppressed in comparison with other cases by a small factor $(1-4{\sin }^2{\theta }_W)$.

Bistability, excitability, and self-pulsing regimes in an InP-based two-dimensional (2D) photonic crystal nanocavity with quantum wells as an active medium are investigated. A resonant cw beam is evanescently coupled into the cavity through a tapered microfiber. In such conditions, we show that the cavity exhibits class II excitability, which arises from the competition between a fast electronic nonlinear effect, given by carrier-induced refractive index change, and slow thermal dynamics. Multiple perturbation-pulse experiments allow us to measure the refractory time (“dead time” between two excitable pulses) of the excitable nanocavity system.

Entanglement-induced nonadditivity of classical communication capacity in networks consisting of quantum channels is considered. Communication lattices consisting of butterfly-type entanglement-breaking channels augmented, with some probability, by identity channels are analyzed. The capacity superadditivity in the network is manifested in directed correlated bond percolation which we consider in two flavors: simply directed and randomly oriented. The obtained percolation properties show that high-capacity information transfer sets in much faster in the regime of superadditive communication capacity than otherwise possible. As a by-product, this sheds light on a type of entanglement-based quantum capacity percolation phenomenon.

We propose a scalable scheme for a unitary multimode operator using an optical cavity with an atomic ensemble. We exemplify three-mode and four-mode cases and engineer the squeeze operators that are decoupled from the atomic degrees of freedom. The squeeze parameters can be large since they are proportional to the number of atoms. Using the input-output theory we show that ideal squeezed states and perfect squeezing could be approached at the output. At the same time, we show that it is possible to obtain tripartite and quadripartite Greenberger-Horne-Zeilinger entangled states for continuous variables. The responsible mechanism for both the multimode squeezing and the genuine multipartite entanglement is based on the atomic coherence controlled parametric interactions. The scalability of the scheme is simply obtained by including more transitions in the atomic system.

Fast highly charged C and O ion-induced total ionization of an RNA base molecule, uracil (C${}_4$H${}_4$N${}_2$O${}_2$, $m=112$ amu), has been investigated in a wide energy range of keV to MeV. A combined study of the collision products using a time-of-flight mass spectrometer and an electron spectrometer allows one to determine absolute total ionization cross sections (TCSs). Experimental measurements of TCSs are compared to theoretical predictions performed in the classical trajectory Monte Carlo and classical over-barrier (CTMC-COB) and quantum mechanical (Continuum Distorted Wave with Eikonal Initial State and first-order Born with correct boundary condition) frameworks. The overall energy dependence of the TCSs is approximately reproduced by the models, especially well in the high energy range. The CTMC-COB model provides an excellent agreement for the high-energy data. The projectile charge-state $q$ dependence of TCSs deviates from the well-known quadratic behavior in ion-atom collisions.

The Fredholm equations for one-dimensional two-component fermions with repulsive and with attractive $\delta$-function interactions are solved by an asymptotic expansion for strong repulsion, weak repulsion, weak attraction, and strong attraction. Consequently, we obtain the first few terms of the expansion of the ground-state energy for the Fermi gas with polarization for these regimes. We also prove that the two sets of Fredhom equations for weakly repulsive and attractive interactions are identical as long as the integration boundaries match each other between the two types. Thus the asymptotic expansions of the energies of repulsive and attractive fermions are identical to all orders in this region. The identity of the asymptotic expansions may not mean that the energy connects analytically.

We derive the first few terms of the asymptotic expansion of the Fredholm equations for one-dimensional $\kappa$-component fermions with repulsive and attractive $\delta$-function interaction in strong- and weak-coupling regimes. We thus obtain a highly accurate result for the ground-state energy of a multicomponent Fermi gas with polarization for these regimes. This result provides a unified description of the ground-state properties of the Fermi gas with higher spin symmetries. However, in contrast to the two-component Fermi gas, there does not exist a mapping that can unify the two sets of Fredholm equations as the interacting strength vanishes. Moreover, we find that the local pair-correlation functions shed light on the quantum statistic effects of the $\kappa$-component interacting fermions. For the balanced spin case with repulsive interaction, the ground-state energy obtained confirms Yang and You's result [Chin. Phys. Lett. 28, 020503 (2011)] that the energy per particle as $\kappa \to \infty$ is the same as for spinless Bosons.

Equal-mass two-component Fermi gases under spherically symmetric external harmonic confinement with large $s$-wave scattering length are considered. Using the stochastic variational approach, we determine the lowest 286 and 164 relative eigenenergies of the $(2,2)$ and $(3,1)$ systems at unitarity as a function of the range $r_0$ of the underlying two-body potential and extrapolate to the $r_0\to 0$ limit. Our calculations include all states with vanishing and finite angular momentum $L$ (and natural and unnatural parity $\Pi$) with relative energy up to 10.5 $\hslash \Omega$, where $\Omega$ denotes the angular trapping frequency of the external confinement. Our extrapolated zero-range energies are estimated to have uncertainties of 0.1$\%$ or smaller. The $(2,2)$ and $(3,1)$ energies are used to determine the fourth-order virial coefficient of the trapped unitary two-component Fermi gas in the low-temperature regime. Our results are compared with recent predictions for the fourth-order virial coefficient of the homogeneous system. We also calculate small portions of the energy spectra of the $(3,2)$ and $(4,1)$ systems at unitarity.