We propose a scheme for the generation of various nonclassical states for a cavity field with a driven atom. In the scheme the atom is sent through the cavity and driven by a classical field. The atom interacts dispersively with the cavity field, which induces photon-number dependent Stark shift. Only for a desired Fock state, the atomic transition is tuned in resonance with the classical field. Thus, the measurement of the atomic transition may directly collapse the cavity field onto the desired Fock state. For a two-mode cavity field, pair coherent states and SU(2) coherent states can be generated.

We analyze the consequences of iterative measurement-induced nonlinearity on the dynamical behavior of qubits. We present a one-qubit scheme where the equation governing the time evolution is a complex-valued nonlinear map with one complex parameter. In contrast to the usual notion of quantum chaos, exponential sensitivity to the initial state occurs here. We calculate analytically the Lyapunov exponent based on the overlap of quantum states, and find that it is positive. We present a few illustrative examples of the emerging dynamics.

We propose a framework to construct the ground-state energy and density matrix of an $N$-electron system by solving a self-consistent set of single-particle equations. The method can be viewed as a nontrivial extension of the Kohn-Sham scheme (which is embedded as a special case). It is based on separating the Green’s function into a quasiparticle part and a background part, and expressing only the background part as a functional of the density matrix. The calculated single-particle energies and wave functions have a clear physical interpretation as quasiparticle energies and orbitals.

In this paper we report the theoretical and experimental characterization of a pulsed optically pumped vapor-cell frequency standard based on the detection of the free-induction decay microwave signal. The features that make this standard similar to a pulsed passive maser are presented. In order to predict and optimize the frequency stability, thermal and shot noise sources are analyzed, as well as the conversions of the laser and microwave fluctuations into the output frequency. The experimental results obtained with a clock prototype based on ${}^{87}\mathrm{Rb}$ in buffer gas are compared with the theoretical predictions, showing the practical possibility to implement a frequency standard limited in the medium term only by thermal drift. The achieved frequency stability is ${\sigma }_y\left(\tau \right)=1.2\times {10}^{-12}{\tau }^{-1∕2}$ for measurement times up to $\tau \approx {10}^5\mathrm{s}$. It represents one of the best results reported in literature for gas cell frequency standards and is compliant with the present day requirements for on board space applications.

We use the classical fields approximation to study a translational flow of the condensate with respect to the thermal cloud in a weakly interacting Bose gas confined in a three-dimensional box. We study both subcritical and supercritical relative velocity cases and analyze in detail a state of stationary flow which is reached in the dynamics. This state corresponds to the thermal equilibrium, which is characterized by the relative velocity of the condensate and the thermal cloud. We observe two processes—re-thermalization and drag, both of which lead to a reduction of a relative velocity of the superflow. Yet only the drag process, which is observed above the critical velocity $v_{cr}$, results in transferring a Bose-Einstein condensate to a slower moving mode. In this case the relative velocity of the flow suddenly drops to a value close to zero. Finally, we report the critical velocity to be for our parameters $v_{cr}=0.21c$ for the initial condition and $v_{cr}^{eq}=0.12c$ for re-thermalized superflow ($c$ being the Landau speed of sound), which is strikingly lower than Landau critical velocity, yet consistent with experiments.

We study the properties of ultracold Fermi gases in a three-dimensional optical lattice when crossing a Feshbach resonance. By using a zero-temperature formalism, we show that three-body processes are enhanced in a lattice system in comparison to the continuum case. This poses one possible explanation for the short molecule lifetimes found when decreasing the magnetic field across a Feshbach resonance. Effects of finite temperatures on the molecule formation rates are also discussed by computing the fraction of double-occupied sites. Our results show that current experiments are performed at temperatures considerably higher than expected: lower temperatures are required for fermionic systems to be used to simulate quantum Hamiltonians. In addition, by relating the double occupancy of the lattice to the temperature, we provide a means for thermometry in fermionic lattice systems, previously not accessible experimentally. The effects of ramping a filled lowest band across a Feshbach resonance when increasing the magnetic field are also discussed: fermions are lifted into higher bands due to entanglement of Bloch states, in good agreement with recent experiments.

The nonlinear dynamics of $1.6\text{−}\mathrm{\mu }\mathrm{m}\mathrm{fs}$ laser pulses propagating in fused silica is investigated by employing a full-order dispersion model. Different from the x-wave generation in normally dispersive media, a few-cycle spatiotemporally compressed soliton wave is generated with the contrary contributions of anomalous group velocity dispersion (GVD) and self-phase-modulation. However, at the tailing edge of the pulse forms a shock wave which generates separate and strong supercontinuum peaked at $670\mathrm{nm}$. It is also the origin of conical emission formed both in time and frequency domain with the contribution of normal GVD at visible light.

We present a theoretical procedure to estimate with an atomic magnetometer the time dependence of a magnetic field that fluctuates according to an Ornstein-Uhlenbeck process. The magnetometer applies the detected polarization rotation of an optical probe to measure a collective atomic spin, which precesses due to the magnetic field. Based on the noisy optical detection record, our consistent Gaussian update formalism provides an estimator for the magnetic fields, and we identify analytically the steady-state performance of this estimator. We show that the estimate of the current value of the magnetic field is further improved if noisy measurement data obtained also at later times are taken into account.

We propose a scheme to generate four-mode multiphoton entangled states by passing $N$ identical atoms through two high-$Q$ cavities in turn. In the case of $N=1$, the scheme can deterministically generate Einstein-Podosky-Rosen entangled photon pairs. We use adiabatical state evolution under large atom-cavity detuning, so the scheme is insensitive to atomic spontaneous decay. The influence of cavity decay on fidelity and success probability is also discussed.

We map out the phase diagram of strongly interacting fermions in a potential trap with mass and population imbalance between the two components. As a unique feature distinctively different from the equal-mass case, we show that the superfluid here forms a shell structure which is not simply connected in space. Different types of normal states occupy the trap regions inside and outside this superfluid shell. We calculate the atomic density profiles, which provide an experimental signature for the superfluid shell structure.

The zero-temperature dynamical structure factor of the one-dimensional Bose gas with delta-function interaction (Lieb-Liniger model) is computed as a function of momentum and frequency using a hybrid theoretical/numerical method based on the exact Bethe-ansatz solution. This allows one to interpolate continuously between the weakly coupled Thomas-Fermi and strongly coupled Tonks-Girardeau regimes. The results should be experimentally accessible with Bragg spectroscopy.

We present a set of Bell inequalities that gives rise to a finer classification of the entanglement for tripartite systems. These inequalities distinguish three possible biseparable entanglements for three-qubit states. The three Bell operators we employed constitute an external sphere of the separable cube.

We report the observation of emission spectra from ${\mathrm{Rb}}^{*}{\mathrm{He}}_n$ exciplexes in solid ${}^4\mathrm{He}$. Two different excitation channels were experimentally identified, viz., exciplex formation via laser excitation to the atomic $5P_{3∕2}$ and to the $5P_{1∕2}$ levels. While the former channel was observed before in liquid helium, on helium nanodroplets, and in helium gas by different groups, the latter creation mechanism occurs only in solid helium or in gaseous helium above $10\mathrm{K}$. The experimental results are compared to theoretical predictions based on the extension of a model, used earlier by us for the description of ${\mathrm{Cs}}^{*}{\mathrm{He}}_n$ exciplexes. We also report the observation of fluorescence from atomic rubidium in solid helium, and discuss striking differences between the spectroscopic features of $\mathrm{Rb}\text{−}\mathrm{He}$ and $\mathrm{Cs}\text{−}\mathrm{He}$ systems.

The recently introduced correlation function [M. Schulz et al., Phys. Rev. Lett. 84, 863 (2000)] for the analysis of double ionization processes is reconsidered. We present a model based on the frozen-correlation and continuum distorted-wave with eikonal initial-state approximations, and compare numerical results with experimental data for double ionization in $100\mathrm{MeV}∕\mathrm{amu}$ ${\mathrm{C}}^{6+}\text{−}\mathrm{He}$ and $3.6\mathrm{MeV}∕\mathrm{amu}$ ${\mathrm{Au}}^{53+}\text{−}\mathrm{He}$ collisions. Our calculations confirm earlier conclusions about the primary and secondary roles of final-state and initial-state correlations, respectively. However, we show that the usual definition of the correlation function on the level of cross sections is problematic, since even a fully uncorrelated calculation gives rise to a nontrivial result. The intended limit of a zero-valued function for an independent electron calculation is achieved only if the analysis is performed on the level of impact-parameter dependent transition probabilities. Consequently, the workings of initial- and final-state correlations are reflected unambiguously only on this level.

Electron-impact ejected electron spectra of He below the ${\mathrm{He}}^{+}$ $N=2$ threshold have been measured for all combinations of incident energies 550, 150, and $75\mathrm{eV}$ and ejected electron directions 60°, 90°, and 120° with respect to the incident electron beam direction. Autoionizing levels from the four main series ${}^{1}S_0^e$, ${}^{3}P_1^o$, ${}^{1}D_2^e$, and ${}^{1}P_1^o$, are observed up to $n=7$, and resolved up to $n=5$. In addition several weak members of other series are observed. The positions, and in some cases the widths, are tabulated for 30 levels, and these are compared with theoretical and other experimental values.

We propose a Bragg spectroscopy experiment to measure the onset of superfluid pairing in ultracold trapped Fermi gases. In particular, we study two-component Fermi gases in the weak coupling BCS and Bose Einstein condensation limits as well as in the strong coupling unitarity limit. The low-temperature Bragg spectrum exhibits a gap directly related to the pair-breaking energy. Furthermore, the Bragg spectrum has a large maximum just below the critical temperature when the gas is superfluid in the BCS limit. In the unitarity regime, we show how the pseudogap in the normal phase leads to a significant suppression of the low-frequency Bragg spectrum.

We study the back action of a single-photon detector on the electromagnetic field upon a photodetection by considering a microscopic model in which the detector is constituted of a sensor and an amplification mechanism. Using the quantum trajectories approach we determine the quantum jump superoperator (QJS) that describes the action of the detector on the field state immediately after the photocount. The resulting QJS consists of two parts: the bright-count term, representing the real photoabsorptions, and the dark-count term, representing the amplification of intrinsic excitations inside the detector. First we compare our results for the counting rates to experimental data, showing good agreement. Then we point out that by modifying the field frequency one can engineer the form of the QJS, obtaining the QJS’s proposed previously in an ad hoc manner.

We have studied the carrier-envelope phase-dependent above-threshold-ionization processes by solving the time-dependent Schrödinger equation nonperturbatively. Our simulated carrier-envelope phase-dependent above-threshold-ionization spectra are in good agreement with the experimental measurement of Paulus et al. [Phys. Rev. Lett. 91, 253004 (2003)] if we shift the experimental carrier-envelope phase by $0.15\pi$. This means that we have recalibrated the carrier-envelope phase more accurately by comparing our simulations with the experimental measurements. Furthermore, by tracing back the origin of the ionized electrons, we have clearly identified that the low-energy electrons come from the direct ionization by the laser field and the high-energy electrons come from the recollision with the parent core after being bounced back by the laser field.

We present two cavity quantum electrodynamics proposals that, sharing the same basic elements, allow for the deterministic generation of entangled photon pairs by means of a three-level atom successively coupled to two single longitudinal mode high-$Q$ optical resonators presenting polarization degeneracy. In the faster proposal, the three-level atom yields a polarization-entangled photon pair via two truncated Rabi oscillations, whereas in the adiabatic proposal, a counterintuitive stimulated Raman adiabatic passage process is considered. Although slower than the former process, this second method is very efficient and robust under fluctuations of the experimental parameters and, particularly interesting, almost completely insensitive to atomic decay.