I show that the theory developed for quantum delocalization of an excited hydrogen atom in a two-frequency linearly polarized microwave field can be used with a few adaptions to explain the quantum delocalization of alkali-metal atoms in a linearly polarized monochromatic field in the regime where it deviates from hydrogenic behavior. Comparison with numerical and laboratory experiments is satisfactory: apart from a constant factor, independent from the microwave frequency, the same dependence from the initial quantum number is observed.

In the framework of the $\mathrm{O}\left(N\right)$ three-dimensional effective scalar field model for homogeneous dilute weakly interacting Bose gases we use the $1∕N$ expansion, within the large-$N$ limit, to evaluate the parameter $r_c$ which is directly related to the critical chemical potential ${\mu }_c$. This quantity enters the order-$a^2{n}^{2∕3}$ coefficient contributing to the critical temperature shift $\Delta T_c$ where $a$ represents the $s$-wave scattering length and $n$ represents the density. Compared to the recent precise numerical lattice simulation results, our calculation suggests that the large-$N$ approximation performs rather well even for the physical case $N=2$. We then calculate the same quantity but using different forms of the optimized perturbative (variational) method, showing that these produce excellent results for both the finite-$N$ and large-$N$ cases.

We present a mean-field model of a continuous-wave atom laser with Raman output coupling. The noncondensate is pumped at a fixed input rate which, in turn, pumps the condensate through a two-body scattering process obeying the Fermi golden rule. The gas is then coupled out by a Gaussian beam from the system, and the temperature and particle number are self-consistently evaluated against equilibrium constraints. We observe the dependence of the second-order coherence of the output upon the width of the output-coupling beam, and note that even in the presence of a highly coherent trapped gas, perfect coherence of the output matter wave is not guaranteed.

We demonstrate an efficient fiber-coupled source of nondegenerate polarization-entangled photons at 795 and $1609\mathrm{nm}$ using bidirectionally pumped parametric down-conversion in bulk periodically poled lithium niobate. The single-mode source has an inferred bandwidth of $50\mathrm{GHz}$ and a spectral brightness of $300\text{pairs}∕\left(\mathrm{s}\mathrm{GHz}\mathrm{mW}\right)$ of pump power that is suitable for narrowband applications such as entanglement transfer from photonic to atomic qubits.

We study the absorption spectrum of a weak probe field near resonant to an atomic transition, the upper level of which is strongly coupled to a third level by the interaction with a Lorentzian bosonic reservoir, such as, e.g., a mode of a high-$Q$ cavity or a local vibration in a solid. The reservoir coupling is approximated by the interaction with a classical complex Gaussian-Markovian random process (control field), which is justified when the reservoir temperature exceeds significantly the mode frequency or when the high-$Q$ cavity is pumped by broadband incoherent radiation. The present theory is applicable also when the control field is chaotic laser light. We assume that the rms control-field Rabi frequency $V_0$ is much greater than the field detuning ${\Delta }_c$, which, in turn, is much greater than the material relaxation constants. We reveal and describe analytically all qualitatively different regimes of the spectrum modification and obtain their validity conditions. The analytical results are verified by numerical calculations using the exact continued-fraction solution. The analytical formulas obtained allow one to perform fast computer calculations for arbitrarily small values of the reservoir (control-field) bandwidth $\nu$, in contrast to the known numerical methods, which require sharply increasing computational resources with a decrease of $\nu$. In the most interesting case $\nu ⪡V_0$, the spectrum consists of two peaks, the nonvanishing bandwidth and material relaxation affecting mainly the dip between the peaks. The results obtained in the static limit (i.e., a very narrow reservoir) are independent of the reservoir band shape. We reveal reservoir-induced transparency (RIT)—i.e., absorption reduction due to the reservoir coupling. Moreover, two unexpected, remarkable features are uncovered in a range of intermediate values of $\nu$ and $V_0$, ${\Gamma }^2\mid {\Delta }_c\mid ⪡V_0^2\nu ⪡{\mid {\Delta }_c\mid }^3$ ($\Gamma$ is the spectral width in the absence of the control field): an extra peak in the dip and a resonance-absorption decrease by a half (i.e., an increase of RIT) relative to the static limit. The existence of the above spectral features depends on the statistical details of the control-field temporal behavior.

A quantum system in interaction with a repeatedly measured one undergoes a nonunitary time evolution and is pushed into a subspace substantially determined by the two-system coupling. The possibility of suitably modifying such an evolution through quantum Zeno dynamics (i.e., the generalized quantum Zeno effect) addressing the system toward an a priori decided target subspace is illustrated. Applications and their possible realizations in the context of trapped ions are also discussed.

Second-order interference and Hanbury-Brown and Twiss–type experiments can provide an operational framework for the construction of witness operators that can test classical and nonclassical properties of a Gaussian squeezed state, and provide entanglement witness operators to study the separability properties of correlated Gaussian squeezed sates.

In this paper we explore the possibility of fundamental tests for coherent-state optical quantum computing gates [T. C. Ralph et al., Phys. Rev. A 68, 042319 (2003)] using sophisticated but not unrealistic quantum states. The major resource required in these gates is a state diagonal to the basis states. We use the recent observation that a squeezed single-photon state $\left[S\left(r\right)\mid 1⟩\right]$ approximates well an odd superposition of coherent states $(\mid \alpha ⟩-\mid -\alpha ⟩)$ to address the diagonal resource problem. The approximation only holds for relatively small $\alpha$, and hence these gates cannot be used in a scalable scheme. We explore the effects on fidelities and probabilities in teleportation and a rotated Hadamard gate.

Absolute double differential cross section spectra of ${\mathrm{O}}_2$ at scattering angles of 0°, 2°, 4°, 6°, and 8° below 119 eV have been determined by the angle-resolved electron-energy-loss spectrometer at an incident electron energy of 2500 eV. Some features above the first ionization threshold stand out as the momentum transfer square $K^2$ increases. Based on our experimental results and theoretical analysis, they are assigned to the optically forbidden Rydberg series of $2{\sigma }_u^{-1}\left(c{}^4\mathrm{\Sigma }_u^{-}\right)np{\sigma }_u{}^3\mathrm{\Sigma }_g^{-}$ $(n=2\text{–}6)$ and the inner-valence transition of ${\left(2{\sigma }_g\right)}^{-1}{\left(1{\pi }_g\right)}^3{}^3\mathrm{\Pi }_g\leftarrow X{}^3\mathrm{\Sigma }_g^{-}$, respectively. Furthermore, the observed energy positions of $2{\sigma }_u^{-1}\left(c{}^4\mathrm{\Sigma }_u^{-}\right)np{\sigma }_u{}^3\mathrm{\Sigma }_g^{-}$ $(n=2\text{–}6)$ confirm a theoretical prediction [Zhong and Li, J. Phys. B 37, 735 (2004)] that the eigenquantum defects show an evident oscillation due to the presence of shape resonance.

Absolute differential cross sections are reported for electron capture and loss by $1–5\mathrm{keV}$ H atoms incident on CO and $\mathrm{C}{\mathrm{O}}_2$ for laboratory scattering angles up to 1.73°, and for charge transfer of $1–5\mathrm{keV}$ ${\mathrm{H}}^{+}$ with CO and $\mathrm{C}{\mathrm{O}}_2$ for scattering angles up to 2.51°. To our knowledge, the H-atom differential electron-capture and -loss cross sections presented here are the first of their kind for CO and $\mathrm{C}{\mathrm{O}}_2$. The differential electron-loss cross sections are very similar to one another, and to previous measurements with other molecular targets, suggesting that some aspects of these collisions may be amenable to a relatively basic theoretical model. The differential measurements reported here significantly advance our knowledge of these collision processes and very good agreement is observed between the corresponding integral cross sections and prior work.

Simple electron capture processes are studied using an orthonormal two state continuum-distorted-wave (CDW) basis. The suitability of the basis set is tested by comparing predictions for total and differential cross sections with available experimental data. Overall good agreement is obtained and the authors conclude that a relatively small CDW basis set may be suitable to model a wide variety of low-energy collisions if the members of this extended set are astutely chosen.

Energies and density distributions of the helium atoms in ${\mathrm{Br}}_2\left(X\right)-{\left({}^4\mathrm{He}\right)}_N$ clusters are calculated using a quantum “Hartree-like” approach in which the dopant molecule and the ${}^4\mathrm{He}$ atoms play the role of the nuclei and electrons, respectively, of the original Hartree formulation. A detailed generalization of the methodology is presented. The validity of this treatment is assessed by comparing energies and density distributions for $N=2$ up to $N=18$ with those obtained by performing quantum diffusion Monte Carlo (DMC) calculations. The present Hartree model shows good agreement with the DMC calculations, the main difference being that the DMC density distributions of the He atoms are more isotropic than those generated via the model. The treatment is extended to larger (up to $N=60$) clusters and saturation effects are analyzed and discussed.

We demonstrate a method of inducing an optical Feshbach resonance based on a coherent free-bound stimulated Raman transition. In our experiment atoms in a ${}^{87}\mathrm{Rb}$ Bose-Einstein condensate are exposed to two phase-locked Raman laser beams which couple pairs of colliding atoms to a molecular ground state. By controlling the power and relative detuning of the two laser beams, we can change the atomic scattering length considerably. The dependence of scattering length on these parameters is studied experimentally and modeled theoretically.

We investigate the dynamical properties of a superfluid gas of trapped fermionic atoms in the BCS phase. As a simple example we consider the reaction of the gas to a slow rotation of the trap. It is shown that the currents generated by the rotation can be understood within a two-fluid model similar to the one used in the theory of superconductors, but with a position-dependent ratio of normal and superfluid densities. The rather general result of this paper is that already at very low temperatures, far below the critical one, an important normal-fluid component appears in the outer regions of the gas. This renders the experimental observation of superfluidity effects more difficult and indicates that reliable theoretical predictions concerning other dynamical properties, like the frequencies of collective modes, can only be made by taking into account temperature effects.

We present the results of an experiment on light scattering from an elongated Bose-Einstein condensate (BEC) interacting with a far-off-resonant pump laser. By collective atomic recoil lasing (CARL) a coherent superposition of two atomic wave packets with different momenta is created. Varying the intensity of a weak counterpropagating laser beam we observe the transition from the pure superradiant regime to the Bragg scattering regime, where Rabi oscillations in a two-level system are observed. The process is limited by the decoherence between the two atomic wave packets. In the superradiant regime the experiment gives evidence of a contribution to decoherence which depends on the initial velocity of the condensate. The system is described by the CARL-BEC model, which is a generalization of the Gross-Pitaevskii model to include the self-consistent evolution of the scattered field and a phase-diffusion decoherence process, which accounts for the observed damping.

We present a theoretical analysis of optical parametric oscillations through four-wave mixing in a nonlinear high-$Q$ whispering-gallery-mode resonator. It is shown that even a small flux of pump photons is sufficient to reach the threshold of the oscillations. We demonstrate that due to narrow bandwidth of the resonator modes as well as the high efficiency of the resonant frequency conversion the oscillator produces a stable narrow-band beat note of the pump, signal, and idler waves making an all-optical secondary frequency reference feasible.

From the viewpoint of quasinormal modes, we describe a decoherence mechanism of charge qubit of Josephson junctions (JJ) in a lossy microcavity, which can appear in a realistic experiment for quantum computation based on a JJ qubit. We show that nonlinear coupling of a charge qubit to the quantum cavity field can result in additional dissipation of the resonant mode due to the effective interaction between those nonresonant modes and the resonant mode, which is induced by the charge qubit itself. We calculate the characteristic time of the decoherence by making use of the system plus bath method.

We explicitly show a protocol in which an arbitrary two qubit state $\mid \varphi ⟩=a\mid 00⟩+b\mid 01⟩+c\mid 10⟩+d\mid 11⟩$ is faithfully and deterministically teleported from Alice to Bob. We construct the 16 orthogonal generalized Bell states that can be used to teleport the two qubits. The local operations Bob must perform on his qubits in order to recover the teleported state are also constructed. They are restricted only to single-qubit gates. This means that a controlled-NOT gate is not necessary to complete the protocol. A generalization where $N$ qubits are teleported is also shown. We define a generalized magic basis, which possesses interesting properties. These properties help us to suggest a generalized concurrence from which we construct a measure of entanglement that has a clear physical interpretation: A multipartite state has maximum entanglement if it is a genuine quantum teleportation channel.

Within the theory of Siegert pseudostates, it is possible to accurately calculate bound states and resonances. The energy continuum is replaced by a discrete set of states. Many questions of interest in scattering theory can be addressed within the framework of this formalism, thereby avoiding the need to treat the energy continuum. For practical calculations it is important to know whether a certain subset of Siegert pseudostates comprises a basis. This is a nontrivial issue, because of the unusual orthogonality and overcompleteness properties of Siegert pseudostates. Using analytical and numerical arguments, it is shown that the subset of bound states and outgoing Siegert pseudostates forms a basis. Time evolution in the context of Siegert pseudostates is also investigated. From the Mittag-Leffler expansion of the outgoing-wave Green’s function, the time-dependent expansion of a wave packet in terms of Siegert pseudostates is derived. In this expression, all Siegert pseudostates—bound, antibound, outgoing, and incoming—are employed. Each of these evolves in time in a nonexponential fashion. Numerical tests underline the accuracy of the method.

We present a statistical fragmentation study of the ${\mathrm{C}}_5$, ${\mathrm{C}}_7$, and ${\mathrm{C}}_9$ carbon clusters using the Metropolis Monte Carlo and Weisskopf methods. We show that inclusion of several isomeric forms as well as rotational effects is essential to reproduce the experimental observations. We have found that, for cluster excitation energies around $10\mathrm{eV}$, several fragmentation channels are efficiently populated, but the dominant one always corresponds to ${\mathrm{C}}_{n-3}∕{\mathrm{C}}_3$. For high enough excitation energies, we observe first-order phase transitions corresponding to a complete breakup of the cluster.