We discuss the application of the static path plus random-phase approximation $(\mathrm{SPA}+\mathrm{RPA})$ and the ensuing mean $\text{field}+\mathrm{RPA}$ treatment to the evaluation of entanglement in composite quantum systems at finite temperature. These methods involve just local diagonalizations and the determination of the generalized collective vibrational frequencies. As an illustration, we evaluate the pairwise entanglement in a fully connected $XXZ$ chain of $n$ spins at finite temperature in a transverse magnetic field $b$. It is shown that already the mean $\text{field}+\mathrm{RPA}$ provides an accurate analytic description of the concurrence below the mean field critical region $(\mid b\mid <b_c)$, exact for large $n$, whereas the full $\mathrm{SPA}+\mathrm{RPA}$ is able to improve results for finite systems in the critical region. It is proven as well, that for $T>0$ weak entanglement also arises when the ground state is separable $(\mid b\mid >b_c)$, with the limit temperature for pairwise entanglement exhibiting quite distinct regimes for $\mid b\mid <b_c$ and $\mid b\mid >b_c$.

The anticommuting properties of fermionic operators, together with the presence of parity conservation, affect the concept of entanglement in a composite fermionic system. Hence different points of view can give rise to different reasonable definitions of separable and entangled states. Here we analyze these possibilities and the relationship between the different classes of separable states. The behavior of the various classes when taking multiple copies of a state is also studied, showing that some of the differences vanish in the asymptotic regime. In particular, in the case of only two fermionic modes all the classes become equivalent in this limit. We illustrate the differences and relations by providing a complete characterization of all the sets defined for systems of two fermionic modes. The results are applied to Gibbs states of infinite chains of fermions whose interaction corresponds to a $XY$ Hamiltonian with transverse magnetic field.

We consider a dilute gas of dipole moments in an arbitrary harmonic trap and treat both the short-range, isotropic $\delta$ interaction and the long-range, anisotropic dipole-dipole interaction perturbatively. In this way, we calculate the leading shift of the critical temperature with respect to that of an ideal gas as a function of the relative orientation of the dipole moments with respect to the harmonic trap axes. In particular, we determine those magic angles, where the dipolar shift of the Bose-Einstein condensation temperature vanishes. Furthermore, we show for the parameters of the ongoing ${}^{52}\mathrm{Cr}$ BEC experiment of Pfau and co-workers that this dipolar shift can be enhanced by increasing the number of particles, the geometrical mean trap frequency, and the anisotropy of the trap.

We propose a technique to measure multispin correlation functions of arbitrary range as determined by the ground states of spinful cold atoms in optical lattices. We show that an observation of the atomic version of the Stokes parameters, using focused lasers and microwave pulsing, can be related to $n$-spin correlators. We discuss the possibility of detecting not only ground state static spin correlations, but also time-dependent spin-wave dynamics as a demonstrative example using our proposed technique.

We report an experimental and theoretical study of a rotating diode-pumped Nd-YAG ring laser with active beat-note stabilization. Our experimental setup is described in the usual Maxwell-Bloch formalism. We analytically derive a stability condition and some frequency response characteristics for the solid-state ring-laser gyroscope, illustrating the important role of mode coupling effects on the dynamics of such a device. Experimental data are presented and compared with the theory on the basis of realistic laser parameters, showing very good agreement. Our results illustrate the duality between the very rich nonlinear dynamics of the diode-pumped solid-state ring laser (including chaotic behavior) and the possibility to obtain a very stable beat note, resulting in a potentially new kind of rotation sensor.

We consider a nondegenerate three-level cascade laser with a subthreshold nondegenerate parametric oscillator coupled to a vacuum reservoir. Applying the pertinent master equation, we analyze the squeezing and entanglement properties of the two-mode light produced by this quantum optical system inside and outside the cavity. We also determine the normalized second-order correlation function for the two-mode light as well as for individual mode. We find that the light generated by this system is in a two-mode squeezed state and the state of the system is strongly entangled at steady state. Moreover, the presence of the parametric oscillator leads to an increase in the degree of squeezing and entanglement. We also find that the intermode correlation decreases as the injected atomic coherence decreases in the system.

Triple-differential cross sections (TDCSs) for electron-impact ionization of $\mathrm{He}{\left(1s^2\right)}^{1}S$ leading to ${\mathrm{He}}^{+}\left(1s\right)$ are calculated using the highly accurate convergent close-coupling (CCC) method for incident projectile energies of 268.6 and $112.6\mathrm{eV}$, with at least one of the outgoing electrons having an energy of $44\mathrm{eV}$. These results are used to obtain absolute TDCSs from the recent experimental data [Bellm et al., Phys. Rev. A 75, 042704 (2007)] for TDCS ratios of ionization with no excitation to ionization with excitation resulting in ${\mathrm{He}}^{+}(n=2,3,4)$. The TDCSs can be used as comparison benchmarks in future studies, and already allow us to test the accuracy of the TDCSs obtained from the hybrid distorted-$\text{wave}+R$-matrix (close-coupling) model, which was fairly successful in predicting the ratios, using CCC for $n=1$ and experimental results for $n=2,3,4$.

We prove that previous various forms of the inseparability of a Gaussian state are equivalent to the negativity of a partial transposed density matrix in the one-photon Fock subspace. We give an infinite series of inequalities; the fulfillment of each inequality shows the presence of entanglement in bipartite systems. For many non-Gaussian states, we show that Fock-space inseparability criteria are theoretically more efficient and simpler than Shchukin-Vogel criteria, which are based on the moments of the states.

The analytic continuation methods of complex scaling (CS), smooth exterior scaling (SES), and complex absorbing potential (CAP) are investigated for the supercritical quasimolecular ground state in the ${\mathrm{U}}^{92+}\text{−}{\mathrm{Cf}}^{98+}$ system at an internuclear separation of $R=20\mathrm{fm}$. Padé approximants to the complex-energy trajectories are used to perform an extrapolation of the resonance energies, which, thus, become independent of the respective stabilization parameter. Within the monopole approximation to the two-center potential is demonstrated that the extrapolated results from SES and CAP are consistent to a high degree of accuracy. Extrapolated CAP calculations are extended to include dipole and quadrupole terms of the potential for a large range of internuclear separations $R$. These terms cause a broadening of the widths at the $‰$ level when the nuclei are almost in contact, and at the % level for $R$ values where the $1S\sigma$ state enters the negative continuum.

Hydrogen atoms in supersonic expansions with velocities in the range from $700\text{to}800\mathrm{m}∕\mathrm{s}$ have been excited to Rydberg-Stark states with principal quantum number $n$ between 20 and 40, decelerated to zero velocity in the lab frame using time-dependent inhomogeneous electric fields, and trapped in a two-dimensional electrostatic trap with an initial density of $\approx 5\times {10}^6{\mathrm{cm}}^{-3}$. The motion of the atomic cloud in the trap was observed by measuring the times of flight and images of the ${\mathrm{H}}^{+}$ ions produced by pulsed field ionization. The velocity distribution of the trapped atoms can be described by an effective temperature of $350\mathrm{mK}$. The decay of the population of trapped atoms does not follow a single-exponential behavior and has contributions from radiative and collisional processes.

We study the continuous accumulation of cold atoms from a magneto-optical trap (MOT) into a finite depth trap, consisting in a magnetic quadrupole trap dressed by a radiofrequency (rf) field. Chromium atoms $\left({}^{52}\mathrm{Cr}\right)$ in a MOT are continuously optically pumped by the MOT lasers to metastable dark states. In the presence of a rf field, the temperature of the metastable atoms that remain magnetically trapped can be as low as $25\mathrm{\mu }\mathrm{K}$, with a density of ${10}^{17}\mathrm{atoms}{\mathrm{m}}^{-3}$, resulting in an increase of the phase-space density, still limited to $7.0\times {10}^{-6}$ by inelastic collisions. To investigate the thermalization issues in the truncated trap, we measure the free evaporation rate in the rf-truncated magnetic trap, and deduce the average elastic cross section for atoms in the ${}^{5}D_4$ metastable states, ${\sigma }_{el}=7.0\times {10}^{-16}{\mathrm{m}}^2$.

We derive the frequency shift for a forbidden optical transition $J=0\to J^{\prime }=0$ caused by the simultaneous actions of an elliptically polarized lattice field and a static magnetic field. We find that a simple configuration of lattice and magnetic fields leads to a cancellation of this shift to first order in lattice intensity and magnetic field. In this geometry, the second-order lattice intensity shift can be minimized as well by use of optimal lattice polarization. Suppression of these shifts could considerably enhance the performance of the next generation of atomic clocks.

It is shown by computer simulations that superfluid parahydrogen clusters of more than 22 molecules can be turned insulating and “solidlike” by the replacement of as few as one or two molecules, with ones of the heavier orthodeuterium isotope. A much smaller effect is observed with substitutional orthohydrogen. Substitutional orthodeuterium molecules prevalently sit in the inner part of the cluster, whereas orthohydrogen impurities reside primarily in the outer shell, near the surface. Implications on the superfluidity of pure parahydrogen clusters are discussed.

We demonstrate an experiment for remote preparation of arbitrary photon polarization states. With local operations, polarization measurement, and one way classical communication, any states lying on and inside the Poincaré sphere can be remotely prepared. For arbitrary pure states, the efficiency is 100% with a communication cost of one entanglement bit and two classical bits. For mixed states, polarization insensitive measurement is introduced and the classical communication cost is one bit. Spontaneous parametric down-conversion (SPDC) is employed as an entanglement source. The remotely prepared qubits are estimated via quantum tomography process and fidelity between the experimentally prepared state and the expected state is considered to test the data. We achieve remote preparation of 13 states with fidelities all above 0.994.

We summarize the results of numerical calculations of ${\mathrm{HD}}^{+}$ in a $7.1\mathrm{fs}$ (intensity full width at half maximum) $790\mathrm{nm}$ laser pulse. The molecule is assumed to be aligned with the linearly polarized laser field and includes two electronic and one nuclear degrees of freedom. We report total dissociation and ionization probabilities from the lowest 10 vibrational states for a range of intensities from ${10}^{13}$ to $7\times {10}^{14}\mathrm{W}∕{\mathrm{cm}}^2$. The conditions for the observability of carrier-envelope phase (CEP) effects for a mixed initial state and for intensity averaging over the laser focal volume are discussed in detail.

Individual Ba ions are trapped in a gas-filled linear ion trap and observed with a high signal-to-noise ratio by resonance fluorescence. Single-ion storage times of $\sim 5\min$ $(\sim 1\min )$ are achieved using He (Ar) as a buffer gas at pressures in the range $8\times {10}^{-5}–4\times {10}^{-3}\text{torr}$. Trap dynamics in buffer gases are experimentally studied in the simple case of single ions. In particular, the cooling effects of light gases such as He and Ar and the destabilizing properties of heavier gases such as Xe are studied. A simple model is offered to explain the observed phenomenology.

It is shown that inhomogeneous nonlinear interactions in a Bose-Einstein condensate loaded in an optical lattice can result in a delocalizing transition in one dimension, which sharply contrasts to the known behavior of discrete and periodic systems with homogeneous nonlinearity. The transition can be originated either by decreasing the amplitude of the linear periodic potential or by the change of the mean value of the periodic nonlinearity. The dynamics of the delocalizing transition is studied.

The high-order harmonic generation was analyzed by interaction of the femtosecond pulses with the laser plasma produced on the surfaces of various targets. The plasma formation was accomplished by the interaction of the prepulse radiation of different pulse duration ($160\mathrm{fs}$, $1.5\mathrm{ps}$, $210\mathrm{ps}$, and $20\mathrm{ns}$) with the low-$Z$ (lithium, boron, carbon), medium-$Z$ (manganese, zinc, nickel), and high-$Z$ (silver, barium) targets. We showed that plasma formation conditions play a crucial role in harmonic generation and the optimization of this process mostly depends on the energy of prepulse rather than its intensity at the target surface. These studies also demonstrated that the delay between the prepulse and femtosecond pulse is another important parameter, which distinguishes harmonic generation in the cases of the low- and high-$Z$ targets.

We propose a scheme for implementing unconventional geometric quantum computation by using the interaction of two atoms with a two-mode cavity field. The evolution of the system results in a nontrivial two-qubit phase gate. The operation of the proposed gate involves only metastable states of the atom and hence is not affected by spontaneous emission. The effect of cavity decay on the gate is investigated. It is shown that the evolution time of the gate in the two-mode case is less than that in the single-mode case proposed by Feng et al. [Phys. Rev. A 75, 052312 (2007)]. Thus the gate can be more decay tolerant than the previous one. The scheme can also be generalized to a system consisting of two atoms interacting with an $N$-mode cavity field.

We derive a capacity formula for attenuation channels assisted by quantum linear amplifiers when the input signal states are restricted to Gaussian states with the same correlation matrix. A phase insensitive linear amplifier and a phase sensitive linear amplifier are compared by using this formula.