We consider a class of generalized single mode Dicke Hamiltonians with arbitrary boson coupling in the pseudo-spin $x\text{−}z$ plane. We find exact solutions in the thermodynamic, large-spin limit as a function of the coupling angle, which allows us to continuously move between the simple dephasing and the original Dicke Hamiltonians. Only in the latter case (orthogonal static and fluctuating couplings) does the parity-symmetry induced quantum phase transition occur.

We have studied the evolution of the kinetic energy distribution of thermal electrons emitted from energy-rich clusters as a function of the delay after multiphoton excitation. In this paper we emphasize the description of thermionic emission based on the general formalism of the detailed balance theory. Our description accounts for competing decay channels and underlines the fundamental role of the observation time window. Simulations including two decay channels (namely, delayed electron emission and loss of ${\mathrm{C}}_2$ fragments) are compared with experimental results in the case of hot fullerenes ${\mathrm{C}}_{60}$. The most striking result of the model is the close connection between the effective temperature of the electron distribution and the time delay of observation. Our model is in very good agreement with our experimental measurements of time-dependent photoelectron spectra.

This paper presents the results obtained using a genetic algorithm (GA) to search for stable structures of medium size silicon clusters. In this work the GA uses a semiempirical energy function to find the best cluster structures, which are further optimized using density-functional theory. For small clusters our results agree well with previously reported structures, but for larger ones different structures appear. This is the case of ${\text{Si}}_{36}$ where we report a different structure, with significant lower energy than those previously found using limited search approaches on common structural motifs. This demonstrates the need for global optimization schemes when searching for stable structures of medium-size silicon clusters.

There are several ways to create the vorticity-free solitary waves — rarefaction pulses — in condensates: by the process of strongly nonequilibrium condensate formation in a weakly interacting Bose gas, by creating local depletion of the condensate density by a laser beam, and by moving a small object with supercritical velocities. Perturbations created by such waves colliding with vortices are studied in the context of the Gross-Pitaevskii model. We find that the effect of the interactions consists of two competing mechanisms: the creation of vortex line as rarefaction waves acquire circulation in a vicinity of a vortex core and the loss of the vortex line to sound due to Kelvin waves that are generated on vortex lines by rarefaction pulses. When a vortex ring collides with a rarefaction wave, the ring either stabilizes to a smaller ring after emitting sound through Kelvin-wave radiation or the entire energy of the vortex ring is lost to sound if the radius of the ring is of the order of the healing length. We show that during the time evolution of a tangle of vortices, the interactions with rarefaction pulses provide an important dissipation mechanism enhancing the decay of superfluid turbulence.

We describe a powerful scheme which combines laser cooling on two transitions of metastable helium to obtain a high phase-space density. By running a sequence of a large $1083\text{nm}$ magneto-optical trap (MOT) and a compressed $389\text{nm}$ MOT, a density increase of more than one order of magnitude is achieved within $5\text{ms}$. After compression, $8\times {10}^8$ atoms at a central density of $5\times {10}^{10}{\text{cm}}^{-3}$ remain, while the temperature of the cloud has been reduced from $1\text{mK}$ to $0.4\text{mK}$. The resulting phase-space density $\left(4.1\times {10}^{-6}\right)$ is more than one order of magnitude higher than what we achieved by $1083\text{nm}$ laser cooling only.

We report the results of experimental and theoretical investigations of the two-color, two-photon ionization of Ar atoms, using femtosecond pulses of infrared laser radiation in combination with its extreme-ultraviolet harmonics. It is shown that the intensities of the photoelectron lines resulting from the absorption of photons from both fields strongly depend both on the respective phases of the fields and on atomic quantities such as the asymmetry parameter. These phases, which are notoriously difficult to measure, can be estimated by changing the polarization state of the laser radiation.

The complex nature of electron-electron correlations is made manifest in the very simple but nontrivial problem of two electrons confined within a sphere. The description of highly nonlocal correlation and self-interaction effects by widely used local and semilocal exchange-correlation energy density functionals is shown to be unsatisfactory in most cases. Even the best such functionals exhibit significant errors in the Kohn-Sham potentials and density profiles.

The Kohn variational method is used with a configuration-interaction-type wave function to determine the $J=0$ and $J=1$ phase shifts and annihilation parameter $Z_{\text{eff}}$ for positron-hydrogenic ion scattering. The phase shifts are within $1–2\%$ of the best previous calculations. The values of $Z_{\text{eff}}$ are small and do not exceed unity for any of the momenta considered. At thermal energies $Z_{\text{eff}}$ is minute with a value of order ${10}^{-50}$ occurring for ${\text{He}}^{+}$ at $k=0.05a_0^{-1}$. In addition to the variational calculations, analytic expressions for the phase shift and annihilation parameters within the Coulomb wave Born approximation are derived and used to help elucidate the dynamics of positron collisions with positive ions.

We propose a feasible scheme of conditional quantum partial teleportation of a qubit as optimal asymmetric cloning at a distance. In this scheme, Alice preserves one imperfect clone whereas other clone is teleported to Bob. Fidelities of the clones can be simply controlled by an asymmetry in Bell-state measurement. The optimality means that tightest inequality for the fidelities in the asymmetric cloning is saturated. Further we design a conditional teleportation as symmetric optimal $N\to N+1$ cloning from $N$ Alice’s replicas on single distant clone. We shortly discussed two feasible experimental implementations, first one for teleportation of polarization state of a photon and second one for teleportation of a time-bin qubit.

The existence of long-lived states of ${\mathrm{N}}^{-}$ has been a topic investigated with conflicting experimental results. Highly excited spin-aligned states, however, were predicted to have long lifetimes and even be stable against autodetachment. We repeated the measurements of ${\mathrm{N}}^{-}$ formation in $0.9\text{MeV}$ ${\mathrm{N}}_2+\text{Ar}$ charge-exchange collisions and found that the ions reported previously as ${\mathrm{N}}^{-}$ [Heber et al., Phys. Rev. A 38, 4504 (1988)] are an ${\mathrm{O}}^{-}$ fragment from a $0.9\text{MeV}$ ${\mathrm{H}}_2{\text{NO}}^{+}$ impurity beam. This result adds to the comulating data indicating that ${\mathrm{N}}^{-}$ is not going to affect carbon dating measurements using accelerator mass spectrometry.

We present an experimental and theoretical analysis of the energy-time entanglement properties of a two-photon state generated by pulsed spontaneous parametric down-conversion. Different methods of spectral or temporal shaping of one of the two photons have allowed the observation of nonlocal modulations in the autocorrelation or in the spectrum of the other, by coincidence measurements. Either a monochromator or etalon filters have been used to shape the idler photon while observing the conditioned signal autocorrelation with a Michelson interferometer. Alternatively, the unbalanced interferometer has been used to shape the signal photon while measuring the conditioned idler spectrum with the monochromator. The role of the pump spectrum has been investigated by shaping it with additional filters. A theoretical treatment has allowed us to completely describe all the observed features as a function of the different experimental parameters.

We consider $N$ two-level atoms in a ring cavity interacting with a broadband squeezed vacuum centered at frequency ${\omega }_s$ and an input monochromatic driving field at frequency $\omega$. We show that, besides the central mode (at $\omega$), an infinity of sideband modes are produced at the output, with frequencies shifted from $\omega$ by multiples of $2\left(\omega -{\omega }_s\right)$. We analyze the optical bistability of the two nearest sideband modes, red shifted and blue shifted.

Quantum-information processing systems are often operated through time-dependent controls; choosing these controls in a way that makes the resulting operation insensitive to variations in unknown or uncontrollable system parameters is an important prerequisite for obtaining high-fidelity gate operations. In this article we present a numerical method for constructing such robust control sequences for a quite general class of quantum-information processing systems. As an application of the method we have designed a robust implementation of a phase-shift operation central to rare-earth-metal quantum computing, an ensemble quantum computing system proposed by Ohlsson et al. [Opt. Commun. 201, 71 (2002)]. In this case the method has been used to obtain a high degree of insensitivity with respect to differences between ensemble members, but it is equally well suited for quantum computing with a single physical system.

We develop a model for a noisy communication channel in which the noise affecting consecutive transmissions is correlated. This model is motivated by fluctuating birefringence of fiber optic links. We analyze the role of entanglement of the input states in optimizing the classical capacity of such a channel, with a constraint that in every use of the channel at most one photon is being transmitted. Assuming a general form of the ensemble for two consecutive transmissions, we derive tight bounds on the classical channel capacity depending on whether the input states used for communication are separable or entangled across different temporal slots. This result demonstrates that by an appropriate choice, the channel capacity may be notably enhanced by exploiting entanglement.

Resonant charge-exchange cross sections and the relevant transport (diffusion) cross sections for excited states of nitrogen and oxygen atoms have been calculated. The calculation is performed using the asymptotic approach, based on the single-electron asymptotic representation of the electron wave function. The ground-state cross sections are in a good agreement with those calculated via comprehensive quantum chemical approach. The results of calculations demonstrate a reasonable accuracy and a high convenience of this approach in determination of cross sections for the manifold of excited states of atoms.

The predictions of the Bohmian and the decoherent (or consistent) histories formulations of the quantum mechanics of a closed system are compared for histories—sequences of alternatives at a series of times. For certain kinds of histories, Bohmian mechanics and decoherent histories may both be formulated in the same mathematical framework within which they can be compared. In that framework, Bohmian mechanics and decoherent histories represent a given history by different operators. Their predictions for the probabilities of histories of a closed system therefore generally differ. However, in an idealized model of measurement, the predictions of Bohmian mechanics and decoherent histories coincide for the probabilities of records of measurement outcomes. The formulations are thus difficult to distinguish experimentally. They may differ in their accounts of the past history of the Universe in quantum cosmology.

A method for performing quantum state tomography for quadrupole nuclei is presented in this paper. First, it is shown that upon appropriate phase cycling, the nuclear-magnetic-resonance (NMR) intensities of quadrupole nuclei depend only on diagonal elements of the density-matrix. Thus, a method for obtaining the density-matrix elements, which consists of dragging off-diagonal elements into the main diagonal using fine phase-controlled selective radio-frequency pulses, was derived. The use of the method is exemplified through ${}^{23}\mathrm{Na}$ NMR (nuclear spin $I=3∕2$) in a lyotropic liquid crystal at room temperature, in three applications: (a) the tomography of pseudopure states, (b) the tomography of the quadrupole free evolution of the density matrix, and (c) the unitary state evolution of each qubit in the system over the Bloch sphere upon the application of a Hadamard gate. Further applications in the context of pure NMR and in the context of quantum information processing, as well as generalizations for higher spins, are discussed.

We theoretically investigate quadrupolar collective excitations in two-component Bose-Einstein condensates and their nonlinear dynamics associated with harmonic generation and mode coupling. Under the Thomas-Fermi approximation and the quadratic polynomial ansatz for density fluctuations, the linear analysis of the superfluid hydrodynamic equations predicts excitation frequencies of three normal modes constituted from monopole and quadrupole oscillations, and those of three scissors modes. These six modes are bifurcated into in-phase and out-of-phase modes by the intercomponent interaction, yielding the nonlinear dynamics that are absent in a single-component condensate. We obtain analytically the resonance conditions for the second-harmonic generation in terms of the trap aspect ratio and the strength of intercomponent interaction. The numerical simulation of the coupled Gross-Pitaevskii equations vindicates the validity of the analytical results and reveals the dynamics of the second-harmonic generation and nonlinear mode coupling that lead to nonlinear oscillations of the condensate with damping and recurrence reminiscent of the Fermi-Pasta-Ulam problem.