A rich variety of quantum features can be found in a collection of atoms driven only by an incoherent bath. To demonstrate this, we discuss a sample of three-level atoms in ladder configuration interacting via the surrounding bath, and show that the fluorescence light emitted by this system exhibits nonclassical properties. Realizations could be thermal baths for microwave transitions, or incoherent broadband fields for optical transitions. In a small sample of atoms, the emitted light can be switched from sub- to super-Poissonian and from antibunching to superbunching controlled by the mean number of atoms in the sample. Larger samples allow us to generate superbunched light over a wide range of bath parameters and thus fluorescence light intensities. We also identify parameter ranges where the fields emitted on the two transitions are strongly correlated or anticorrelated, such that the Cauchy-Schwarz inequality is violated. As in a moderately strong bath this violation occurs also for larger numbers of atoms, such samples exhibit macroscopic quantum effects.

A recent experiment by Antonini et al. [Phys. Rev. A 71, 050101 (2005)] set new limits on Lorentz violating parameters in the framework of the photon sector of the standard model extension, ${\tilde{\kappa }}_{e-}^{ZZ}$, and the Robertson–Mansouri–Sexl framework, $\beta -\delta -1∕2$. The experiment had significant systematic effects caused by the rotation of the apparatus which was only partly analyzed and taken into account. We show that this is insufficient to put a bound on ${\tilde{\kappa }}_{e-}^{ZZ}$, and that the bound on $\beta -\delta -1∕2$ represents a fivefold, not tenfold, improvement as claimed.

An improved analysis of the Michelson–Morley-type experiment by Antonini et al. [Phys. Rev. A. 71, 050101 (2005)] yields the Robertson–Mansouri–Sexl theory parameter combination $\beta -\delta -1∕2=(-0.6\pm 2.1\pm 1.2)\times {10}^{-10}$ and the standard model extension theory parameter ${\left({\tilde{\kappa }}_{e-}\right)}^{ZZ}=(-2.9\pm 2.2)\times {10}^{-14}$.

We construct the complete $U\text{−}\mu$ phase diagram for harmonically confined ultracold fermionic atoms with repulsive and attractive interactions($\mu$ is the chemical potential and $U$ the interaction strength). Our approach is based on density-functional theory, and employs analytical expressions for the kinetic and correlation energy functionals, permitting us to obtain closed expressions for all phase boundaries and characteristic lines of the phase diagram, both for repulsive and attractive interactions.

The exact solution of the one-dimensional interacting mixed boson-fermion gas is used to calculate ground-state properties both for finite systems and in the thermodynamic limit. The quasimomentum distribution, ground-state energy, and generalized velocities are obtained as functions of the interaction strength both for polarized and nonpolarized fermions. We do not observe any demixing instability of the system for repulsive interactions.

We calculate the asymptotic behavior of correlators as a function of the microscopic parameters for an integrable Bose-Fermi mixture with repulsive interaction in one dimension. For two cases, namely polarized and unpolarized fermions the singularities of the momentum distribution functions are characterized as a function of the coupling constant and the relative density of bosons.

We investigate scarred resonances of a stadium-shaped chaotic microcavity. It is shown that two components with different chirality of the scarring pattern are slightly rotated in opposite ways from the underlying unstable periodic orbit, when the incident angles of the scarring pattern are close to the critical angle for total internal reflection. In addition, the correspondence of emission pattern with the scarring pattern disappears when the incident angles are much larger than the critical angle. The steady probability distribution gives a consistent explanation about these interesting phenomena and makes it possible to expect the emission pattern in the latter case.

We investigate the concentration of multiparty entanglement by focusing on a simple family of three-partite pure states, superpositions of Greenberger-Horne-Zeilinger states and singlets. Despite the simplicity of the states, we show that they cannot be reversibly concentrated by the standard entanglement concentration procedure, to which they seem ideally suited. Our results cast doubt on the idea that for each $N$ there might be a finite set of $N$-party states into which any pure state can be reversibly transformed. We further relate our results to the concept of locking of entanglement of formation.

We present a general model for quantum channels with memory and show that it is sufficiently general to encompass all causal automata: any quantum process in which outputs up to some time $t$ do not depend on inputs at times $t^{\prime }>t$ can be decomposed into a concatenated memory channel. We then examine and present different physical setups in which channels with memory may be operated for the transfer of (private) classical and quantum information. These include setups in which either the receiver or a malicious third party have control of the initializing memory. We introduce classical and quantum channel capacities for these settings and give several examples to show that they may or may not coincide. Entropic upper bounds on the various channel capacities are given. For forgetful quantum channels, in which the effect of the initializing memory dies out as time increases, coding theorems are presented to show that these bounds may be saturated. Forgetful quantum channels are shown to be open and dense in the set of quantum memory channels.

Electron-positron annihilation rates calculated directly from the electron and positron densities are known to underestimate the true annihilation rate. A correction factor, known as the enhancement factor, allows for the local increase of the electron density around the positron caused by the attractive electron-positron interaction. Enhancement factors are given for positrons annihilating with the $1s$ electron in H, ${\mathrm{He}}^{+}$, He, ${\mathrm{Li}}^{2+}$, and ${\mathrm{Li}}^{+}$. The enhancement factor for a free positron annihilating with ${\mathrm{He}}^{+}$ and He is found to be close to that of ortho-positronium (i.e., Ps in its triplet state) annihilating with these atoms. The enhancement factor for Ps-He scattering is used in conjunction with the known annihilation rate for pickoff annihilation to derive a scattering length of $1.47a_0$ for Ps-He scattering. Further, enhancement factors for $e^{+}$-Ne and $e^{+}$-Ar annihilation are used in conjunction with the pickoff annihilation rate to estimate scattering lengths of $1.46a_0$ for Ps-Ne scattering and $1.75a_0$ for Ps-Ar scattering.

We report fully differential cross sections (FDCS) for the single ionization of helium by a $75\mathrm{keV}$ incident energy proton. Previous three-body distorted wave (3DW) calculations for this collision system are in poor agreement with the absolute magnitude of the experimental measurements. The 3DW approximation treats the four-body problem as an effective three-body problem in which the passive electron does not participate in the collision. We have developed a full four-body approach in which the passive electron fully participates in the collision. It will be shown that the FDCS is very sensitive to the treatment of the passive electron-ejected electron interaction. Results of our full four-body approach will be compared with recent absolute experimental measurements.

We derive an effective Hamiltonian that describes a cross-Kerr-type interaction in a system involving a two-level trapped ion coupled to the quantized field inside a cavity. We assume a large detuning between the ion and field (dispersive limit) and this results in an interaction Hamiltonian involving the product of the (bosonic) ionic vibrational motion and field number operators. We also demonstrate the feasibility of operation of a phase gate based on our Hamiltonian. The gate is insensitive to spontaneous emission, an important feature for the practical implementation of quantum computing.

We propose a scheme for scalable and universal quantum computation using diatomic bits with conditional dipole-dipole interaction, trapped within an optical lattice. The qubit states are encoded by the scattering state and the bound heteronuclear molecular state of two ultracold atoms per site. The conditional dipole-dipole interaction appears between neighboring bits when they both occupy the molecular state. The realization of a universal set of quantum logic gates, which is composed of single-bit operations and a two-bit controlled-NOT gate, is presented. The readout method is also discussed.

Radiative lifetimes are measured for metastable levels in the iron charge states ${\mathrm{Fe}}^{9+},{\mathrm{Fe}}^{10+}$, and ${\mathrm{Fe}}^{13+}$. The ions are generated in a 14 GHz electron cyclotron resonance ion source and trapped in a Kingdon ion trap. The Fe levels and their measured lifetimes are (a) $73.0\pm 0.8\mathrm{ms}$ for the $3s^{2}3p^4\left({}^{3}P\right)3d$ ${}^{4}F_{7∕2}$ level in ${\mathrm{Fe}}^{9+}$, (b) $9.91\pm 0.5\mathrm{ms}$ for the $3s^{2}3p^4$ ${}^{1}D_2$ level in ${\mathrm{Fe}}^{10+}$, and (c) $17.0\pm 0.2\mathrm{ms}$ for the $3s^{2}3p$ ${}^{2}P^o{}_{3∕2}$ level in ${\mathrm{Fe}}^{13+}$. Comparisons are made to other measured results using a Kingdon trap, an ion storage, and an electron-beam ion trap (EBIT).

A sum rule approach is used to calculate the zero temperature oscillation frequencies of a two component trapped atomic Fermi gas in the Bardeen, Cooper, and Schrieffer–Bose Einstein condensation crossover region. These sum rules are evaluated using a local density approximation which explicitly includes Feshbach molecules. Breathing modes show nonmonotonic behavior as a function of the interaction strength, while quadrupole modes are insensitive to interactions for both spherically symmetric and axially symmetric traps. Quantitative agreement is found with experiments on atomic ${}^6\mathrm{Li}$ systems and with other theoretical approaches.

We study the evolution of the excitation spectrum of a trapped rotating Bose-Einstein condensate as a function of increasing rotational frequency and discuss chiral edge states supported on the condensate surface. We describe these modes in the context of a mean-field Bogoliubov–de Gennes treatment. We find that they dominate the spectrum at fast rotation, and the dispersion relation for these excitations becomes linear in the limit of high angular momentum as in the quantum Hall effect. We also identify the edge states as primary agents in the nucleation and decay of the lattice.

A quantum system weakly interacting with a fast environment usually undergoes a relaxation with complex frequencies whose imaginary parts are damping rates quadratic in the coupling to the environment in accord with Fermi’s “golden rule.” We show for various models (spin damped by harmonic-oscillator or random-matrix baths, quantum diffusion, and quantum Brownian motion) that upon increasing the coupling up to a critical value still small enough to allow for weak-coupling Markovian master equations, a different relaxation regime can occur. In that regime, complex frequencies lose their real parts such that the process becomes overdamped. Our results call into question the standard belief that overdamping is exclusively a strong coupling feature.

The four-level atomic system in an inverted-Y configuration is investigated for large Kerr nonlinearities. The cross-Kerr nonlinearity generated in such a system can produce a phase shift of order $\pi$ and can be used for realizing polarization quantum phase gates.

In this paper, we present relaxation optimized methods for the transfer of bilinear spin correlations along Ising spin chains. These relaxation optimized methods can be used as a building block for the transfer of polarization between distant spins on a spin chain, a problem that is ubiquitous in multidimensional nuclear magnetic resonance spectroscopy of proteins. Compared to standard techniques, significant reduction in relaxation losses is achieved by these optimized methods when transverse relaxation rates are much larger than the longitudinal relaxation rates and comparable to couplings between spins. We derive an upper bound on the efficiency of the transfer of the spin order along a chain of spins in the presence of relaxation and show that this bound can be approached by the relaxation optimized pulse sequences presented in the paper.

The magnetic-dipole transition probabilities between the fine-structure levels $\left(1s^{2}2s^{2}2p\right)$ ${}^{2}P_{1∕2}–{}^{2}P_{3∕2}$ for B-like ions and $\left(1s^{2}2s2p\right)$ ${}^{3}P_1–{}^{3}P_2$ for Be-like ions are calculated. The configuration-interaction method in the Dirac-Fock-Sturm basis is employed for the evaluation of the interelectronic-interaction correction with negative-continuum spectrum being taken into account. The $1∕Z$ interelectronic-interaction contribution is derived within a rigorous QED approach employing the two-time Green function method. The one-electron QED correction is evaluated within framework of the anomalous magnetic-moment approximation. A comparison with the theoretical results of other authors and with available experimental data is presented.