In distributed quantum information processing, flying photons entangle matter qubits confined in cavities. However, when a matter qubit is homogeneously broadened, the strong-coupling regime of cavity QED is typically required, which is hard to realize in actual experimental setups. Here, we show that a high-fidelity entanglement operation is possible even in the weak-coupling regime in which dampings (dephasing, spontaneous emission, and cavity leakage) overwhelm the coherent coupling between a qubit and the cavity. Our proposal enables distributed quantum information processing to be performed using much less demanding technology than previously.

This paper studies a scheme of two spatially distant oscillator systems that are connected by Gaussian fields and examines distributed entanglement generation between two continuous-mode output Gaussian fields that are radiated by the oscillators. It is demonstrated that using measurement-feedback control while a nonlocal effective entangling operation is on can help to enhance the Einstein-Podolski-Rosen–like entanglement between the output fields. The effect of propagation delays and losses in the fields interconnecting the two oscillators and the effect of other losses in the system are also considered. In particular, for a range of time delays the measurement-feedback controller is able to maintain stability of the closed-loop system and the entanglement enhancement, but the achievable enhancement is only over a smaller bandwidth that is commensurate with the length of the time delays.

We propose a method to improve the secret key rates of four-state continuous-variable quantum key distribution by using an optical preamplifier. The modified protocol allows the distribution of higher secret key rates over long distances. Included in this paper is a detailed investigation of the effects of inserting an optical parametric amplifier into the output of the quantum channel in the four-state protocol, which will be instructive and meaningful about the usage of amplifiers in order to achieve the optimal performance of the protocol in a specific scenario.

The ground-state hyperfine splittings of laser-cooled ${}^{113}$Cd${}^{+}$ and ${}^{111}$Cd${}^{+}$ ions confined in a linear Paul trap have been measured to be $15199862854.96\left(0.12\right)$ Hz and $14530507349.9\left(1.1\right)$ Hz, respectively, using a microwave-optical double-resonance technique. The results for ${}^{113}$Cd${}^{+}$ agrees with previous measurements very well, and the precision of the ${}^{111}$Cd${}^{+}$ is improved by 7 orders of magnitude compared to the best results reported in the literature. The possibility of a microwave frequency standard based on laser-cooled cadmium ions is also discussed.

We analyze a model of electron lattice between two metallic or polar dielectric slabs. Typically, the electrons are confined on a thin helium layer above a metallic slab and form, under given circumstances, the Wigner lattice. We assume that another slab is placed above the electron layer at a distance away to have negligible influence on basic properties of the Wigner lattice. Here we want to analyze the total force acting on the lattice electrons as well as the total force acting between the metallic (or dielectric) slabs. From the classical electrodynamics one can determine the image potential and calculate corresponding forces. In the quantum electrodynamics, fluctuations of the electromagnetic field will add in attraction between the slabs due to the Casimir effect. We calculate and compare those two forces on the slabs within the same model, since they are an explicit manifestation of the same field. We found out that the resulting forces can be of comparable strengths in the appropriate setup.

A statistical theory of resonant multielectron recombination based on properties of chaotic eigenstates is developed. The level density of many-body states increases exponentially with the number of excited electrons. When the residual electron-electron interaction exceeds the interval between these levels, the eigenstates (called compound states or compound resonances if these states are in the continuum) become “chaotic” superpositions of large numbers of Hartree-Fock configurational basis states. This situation takes place in some rare-earth atoms and many open-shell multiply charged ions excited in the process of electron recombination. Our theory describes resonant multielectron recombination via dielectronic doorway states leading to such compound resonances. The result is a radiative capture cross section averaged over a small energy interval containing several compound resonances. In many cases individual resonances are not resolved experimentally (since the interval between them is small, e.g., $\le$1 meV, possibly even smaller than their radiative widths); therefore, our statistical theory should correctly describe the experimental data. We perform numerical calculations of the recombination cross sections for tungsten ions W${}^{q+}$, $q=18$–25. The recombination rate for W${}^{20+}$ measured recently [Schippers et al. Phys. Rev. A 83, 012711 (2011)] is ${10}^3$ greater than the direct radiative recombination rate at low energies, and our result for W${}^{20+}$ agrees with the measurements.

This paper considers the probability density and current distributions generated by a pointlike, isotropic source of monoenergetic charges embedded in a uniform magnetic field environment. Electron sources of this kind have been realized in recent photodetachment microscopy experiments. Unlike the total photocurrent cross section, which is largely understood, the spatial profiles of charge and current emitted by the source display an unexpected hierarchy of complex patterns, even though the distributions, apart from scaling, depend only on a single physical parameter. We examine the electron dynamics both by solving the quantum problem, that is, finding the energy Green's function, and from a semiclassical perspective based on the simple cyclotron orbits followed by the electron. Simulations suggest that the semiclassical method, which involves here interference between an infinite set of paths, faithfully reproduces the features observed in the quantum solution, even in extreme circumstances, and lends itself to an interpretation of some (though not all) of the rich structure exhibited in this simple problem.

In order to study ultracold charge-transfer processes in hybrid atom-ion traps, we have mapped out the potential-energy curves and molecular parameters for several low-lying states of the Rb, Yb${}^{+}$ system. We employ both a multireference configuration interaction and a full configuration interaction (FCI) approach. Turning points, crossing points, potential minima, and spectroscopic molecular constants are obtained for the lowest five molecular states. Long-range parameters, including the dispersion coefficients, are estimated from our ab initio data. The separated-atom ionization potentials and atomic polarizability of the ytterbium atom (${\alpha }_d=128.4$ atomic units) are in good agreement with experiment and previous calculations. We present some dynamical calculations for (adiabatic) scattering lengths for the two lowest (Yb, Rb${}^{+}$) channels that were carried out in our work. However, we find that the pseudopotential approximation is rather limited in validity and only applies to nK temperatures. The adiabatic scattering lengths for both the triplet and singlet channels indicate that both are large and negative in the FCI approximation.

We discuss the mean-field theories obtained from the leading order in a large-$N$ approximation for one- and two-component dilute Bose gases. For a one-component Bose gas, this approximation has the following properties: the Bose-Einstein condensation (BEC) phase transition is second order but the critical temperature $T_c$ is not shifted from the noninteracting gas value $T_0$. The spectrum of excitations in the BEC phase resembles the Bogoliubov dispersion with the usual coupling constant replaced by the running coupling constant which depends on both temperature and momentum. We then study two-component Bose gases with both inter- and intraspecies interactions and focus on the stability of the mixture state above $T_c$. Our mean-field approximation predicts an instability from the mixture state to a phase-separated state when the ratio of the interspecies interaction strength to the intraspecies interaction strength (assuming equal strength for both species) exceeds a critical value. At high temperature this is a structural transition and the global translational symmetry is broken. Our work complements previous studies on the instability of the mixture phase in the presence of BEC.

For a gas trapped in a harmonic potential, the sloshing (or Kohn) mode is undamped and its frequency coincides with the trap frequency, independently of the statistics, interaction, and temperature of the gas. However, experimental trap potentials have usually Gaussian shape and anharmonicity effects appear as the temperature and, in the case of Fermions, the filling of the trap are increased. We study the sloshing mode of a degenerate Fermi gas in an anharmonic trap within the Boltzmann equation, including in-medium effects in both the transport and collision terms. The calculated frequency shifts and damping rates of the sloshing mode due to the trap anharmonicity are in satisfactory agreement with the available experimental data. We also discuss higher-order dipole, octupole, and bending modes and show that the damping of the sloshing mode is caused by its coupling to these modes.

The nonclassical spectral and temporal features of entangled photons offer new possibilities to investigate the interactions of excitons in photosynthetic complexes and to target the excitation of specific states. Simulations of fluorescence in the bacterial reaction center induced by entangled light demonstrate a degree of selectivity of double-exciton states which is not possible using classical stochastic light with the same power spectrum.

We propose to generate a counter-Einstein-Podolsky-Rosen (CEPR) state which possesses a positively-correlated momentum and anticorrelated position directly from spontaneous parametric down-conversion. The general CEPR state is given under a tightly focused Gaussian pump based on type II noncritical phase matching. We analytically and numerically study how a positively-correlated momentum and anticorrelated position of a biphoton can be achieved by choosing the proper focal parameter of the pump. To verify the degree of entanglement of the CEPR state, we calculate its entanglement entropy and it shows a unique dependence on the pump's focal parameter. Suggestions for practical engineering of the state are also given. Furthermore, based on an ideal CEPR state, we deduce a type of erect quantum ghost imaging which can be applied as the entanglement criterion. This work gives applicable references to the spatial control of an entangled photon source in experiments and may stimulate the development of new types of quantum technologies.

We investigate the solitary propagation effect of a well-defined chirped femtosecond laser pulse in a resonance-absorbing medium which is modeled by a two-level quantum dot ensemble. Employing full time-dependent and space-dependent Maxwell-Bloch equations, the evolution of the pulse and the population inversion as well as the spectrum are obtained. The results reveal that the special chirped pulse can retain its shape and resist breakup while propagating through the absorbing medium. Besides, the spectrum does not exhibit obvious broadening but the central and near-central frequency components tend to vanish. Moreover, the stable soliton formed by the special chirped-pulse design based on stimulated Raman adiabatic passage does not obey the area theorem.

A decade ago Kitaev's toric code model established the new paradigm of topological quantum computation. Due to remarkable theoretical and experimental progress, the quantum simulation of such complex many-body systems is now within the realms of possibility. Here we consider the question, to which extent the ground states of small toric code systems differ from local-unitary (LU)-equivalent graph states. We argue that simplistic (though experimentally attractive) setups obliterate the differences between the toric code and equivalent graph states; hence we search for the smallest setups on the square and triangular lattices, such that the quasilocality of the toric code Hamiltonian becomes a distinctive feature. To this end, a purely geometric procedure to transform a given toric code setup into a local-Clifford (LC)-equivalent graph state is derived. In combination with an algorithmic computation of LC-equivalent graph states, we find the smallest nontrivial setup on the square lattice to contain five plaquettes and 16 qubits; on the triangular lattice the number of plaquettes and qubits is reduced to four and nine, respectively.

It is shown that $e-\mathcal{F}$, where $\mathcal{F}$ is the Hartree-Fock (HF) operator, can be inverted, for molecular systems, in numerical Cartesian coordinates. The method was originally applied to finding corrections to approximate Hartree-Fock orbitals [J. D. Talman, Phys. Rev. A 82, 052518 (2010)]. The approach is applied to determine basis set independent dipole polarizabilities for the water molecule using the Sternheimer method within the uncoupled HF and coupled perturbed HF approximations.

We investigate the dependence of the Casimir force between a superconducting plate (niobium) and a magnetodielectric plate (yttrium iron garnet) on the temperature near the transition point within a two-fluid model. For large separations between the plates, the direction of the Casimir force changes from attractive to repulsive as the temperature decreases below the transition temperature, because of an increase in superconducting current density. We show that this increase in the repulsive contribution to the Casimir force, which depends on the magnetic permeability of the magnetodielectric plate, can be experimentally verified by measuring the force acting on the magnetodielectric plate inserted midway between a superconducting plate and a normal conducting plate.

We utilize accurate experimental data available in the literature to yield bounds on the polarizabilities of the ground and first-excited states of atomic Yb. For the $6s^2{}^1{S}_0^{}$ ground state, we find the polarizability $\alpha$ to be constrained to $134.4<\alpha \le 144.2$ in atomic units while for the $6s6p{}^3{P}_0^o$ excited state, we find $280.1<\alpha \le 289.9$. The uncertainty in each of these values is 1.0. These constraints provide a valuable check for ab initio and semiempirical methods used to compute polarizabilities and other related properties in Yb.

We develop a method based on the relativistic coupled-cluster theory to incorporate a perturbative interaction to the no-pair Dirac-Coulomb atomic Hamiltonian. The method is general and suitable to incorporate any perturbation Hamiltonian in a many-electron atom or ion. Using this perturbed relativistic coupled-cluster (PRCC) theory, we calculate the electric-dipole polarizability $\alpha$ of neon. The linearized PRCC results are in very good agreement with the experimental value. However, the results of the nonlinear PRCC theory show larger uncertainty, but they are consistent with the observations from earlier works.

A time-domain atom interferometer with a cold calcium atomic ensemble has been developed with a phase resolution ${\sigma }_{\phi }=140/\sqrt{\tau }$ mrad/s${}^{1/2}$ at the integration time $\tau$. Using this atom interferometer, the dispersion-shaped ac Stark phase shift with a sharp reversal on resonance was demonstrated under the perturbation light near the resonance frequency of the ${}^1{S}_0,m_J=0{-}^3{P}_1,m_J=-1$ transition. The phase shift was well described by a convolution of functions of the ac Stark potential and a Rabi excitation, and it was found that the lifetime of the ${}^3{P}_1$ state is 0.45 ± 0.03 ms.