We propose an efficient quantum-key-distribution scheme based on entanglement. The sender chooses pairs of photons in one of the two equivalent nonmaximally entangled states randomly, and sends a sequence of photons from each pair to the receiver. The sender and receiver choose from the various bases independently but with substantially different probabilities, thus reducing the fraction of discarded data, and a significant gain in efficiency is achieved. We then show that refined data analysis like that proposed by Lo et al. (H. K. Lo, H. F. Chau, and M. Adrehali, e-print quant-ph/0011056) guarantees the security of our scheme against a biased eavesdropping strategy. Remarkably, our scheme is more efficient than distillation of singlets out of the nonmaximally entangled states and biased detection afterward.

The propagation of excitation along a one-dimensional chain of atoms is simulated by means of nuclear magnetic resonance (NMR). The physical system used as an analog quantum computer is a nucleus of ${}^{133}\mathrm{Cs}$ (spin 7/2) in a liquid-crystalline matrix. First, a pseuodpure state of the spin system is prepared. Then, the Hamiltonian of migration is simulated by using a special seven-frequency pulse. The dynamics is monitored by following the transfer of population from one of the eight spin energy levels to the others.

The interaction of ions and molecules with surface modes in cylindrical channels in solids is described on the basis of the dielectric formalism, starting with simple dielectric models and using also more realistic descriptions of the dielectric properties of various materials of experimental interest. The response of the medium is first characterized in terms of the wake potential, calculating the main effects on the moving particles, including self-energy values, stopping and lateral forces. The general scaling properties of the main quantities as well as the effects of plasmon damping in real materials are described. The formulation is extended to ion clusters and molecular composites, introducing new form factors to take into account the cylindrical symmetry of the problem. Applications to simple ions, diclusters, dipoles, and polar molecules channeled in microcapillaries and nanochannels are considered.

We investigate the quantum dynamics for a molecular-state-model system interacting with a resonant two-mode squeezed coherent field, in which each mode is initially correlated. As an example, we consider a three-state molecular model, which mimics the electronic excited states of trans-octatetraene obtained by a full-configuration-interaction calculation using the Pariser-Parr-Pople Hamiltonian, interacting with a two-mode squeezed coherent field. For comparison, we also perform parallel studies, in which the initial fields are prepared in uncorrelated two-mode states, i.e., a two-mode coherent state and a two-mode thermal state. It is well known that the case of two-mode coherent field leads to the usual collapse-revival phenomena of the Rabi oscillations, while the case of a two-mode thermal state leads to an irregular evolution of them. Although the two-mode squeezed coherent field exhibits similar collapse-revival behavior to that for the two-mode coherent field, some differences are detected in the amplitudes and periods of collapse-revival oscillations. Further, the dynamics of off-diagonal molecular density matrices between the ground and second excited states for these two fields are found to be distinctively different from each other. Such attractive behavior is found to be closely associated with the two-mode photon-phase dynamics obtained by the Pegg-Barnett phase operator and the initial quasiprobability (Q function) distribution of photons. These differences in phase properties are also shown to significantly affect the dynamics of the information entropy for a molecule, i.e., molecular entropy, which represents the degree of entanglement between the molecule and photons.

The Hohenberg theorem on the absence of Bose-Einstein condensation (BEC) in homogeneous systems of space dimensions $D<~2$ is based on a well-known Bogoliubov inequality. Applied to an assembly trapped in a harmonic potential we show that the Bogoliubov inequality rules out BEC in all dimensions at finite temperatures. However, this conflicting result with both theory and experiment disappears when the effect of the order parameter is properly taken into account in the boson-field commutation relations. For a hard-sphere Bose gas the theory is consistent with the expansion of the condensate when a positive scattering length is increased, as well as the collapse of the condensate when the sign of the scattering length is reversed and it reaches a minimum critical value.

We consider noncentered vortices and their arrays in a cylindrically trapped Bose-Einstein condensate at zero temperature. We study the kinetic energy and the angular momentum per particle in the Thomas-Fermi regime and their dependence on the distance of the vortices from the center of the trap. Using a perturbative approach with respect to the velocity field of the vortices, we calculate, to first order, the frequency shift of the collective low-lying excitations due to the presence of an off-center vortex or a vortex array, and compare these results with predictions that would be obtained by the application of a simple sum-rule approach, previously found to be very successful for centered vortices. It turns out that the simple sum-rule approach fails for off-centered vortices.

We consider a system of two species of bosons of equal mass, with interactions $U^a\left(|\mathbf{x}|\right)$ and $U^x\left(|\mathbf{x}|\right)$ for bosons of the same and different species, respectively. We present a rigorous proof valid when the Hamiltonian does not include a species-switching term showing that, when $U^x\left(|\mathbf{x}|\right)>\mathrm{}{U}^a\left(|\mathbf{x}|\right),$ the ground state is fully “polarized” (consists of atoms of one kind only). In the unpolarized phase the low-energy excitation spectrum corresponds to two linearly dispersing modes that are even and odd under species exchange. The polarization instability is signaled by the vanishing of the velocity of the odd modes.

Relaxation of a two-level system (TLS) into a resonant infinite-temperature reservoir with a Lorentzian spectrum is studied. The reservoir is described by a complex Gaussian-Markovian field coupled to the nondiagonal elements of the TLS Hamiltonian. The theory can be relevant for electromagnetic interactions in microwave high-Q cavities and muon spin depolarization. Analytical results are obtained for the strong-coupling regime, ${\Omega }_0\gg \nu ,$ where ${\Omega }_0$ is the rms coupling amplitude (Rabi frequency) and $\nu$ is the width of the reservoir spectrum. In this regime, the population difference and half of the initial coherence decay with two characteristic rates: most of the decay occurs at $t\sim {\Omega }_0^{-1},$ the relaxation being reversible for $t\ll ({\Omega }_0^2\nu {)}^{-1/3},$ whereas for $t\gg ({\Omega }_0^2\nu {)}^{-1/3}$ the relaxation becomes irreversible and is practically over. The other half of the coherence decays with a rate on the order of $\nu ,$ which may be slower by orders of magnitude than the time scale of the population relaxation. The above features are explained by the fact that at $t\ll {\nu }^{-1},$ the reservoir temporal fluctuations are effectively one-dimensional (adiabatic). Moreover, we identify the pointer basis, in which the reduction of the state vector occurs. The pointer states depend on the initial phase of the reservoir.

A three-level atom interacting with laser fields of different frequencies out of resonance is studied. The corresponding theory of dressed states is applied and extended beyond the leading order to obtain the higher-order corrections. These higher-order corrections can be computed by a dual Dyson series. Rather than to apply perturbation theory, the equations for the probability amplitudes of the transformed Hamiltonian in the dual interaction picture are considered. This gives us the opportunity to study the resonance properties of the quantum dynamics of the three-level atom being driven out of resonance by the external laser fields. In this way, by different resonance conditions due both to the detunings and the couplings, one gets Rabi flopping without resonance of the external fields and the atom levels, with also the Rabi frequency originating both from the detunings and the couplings.

We calculate the influence of excess noise on parametric down conversion in an unstable optical parametric oscillator (OPO), using a quantum quasimode description. We find a strongly enhanced pair photon generation rate below threshold as compared to a conventional stable cavity setup of comparable gain and loss. In addition, the oscillation threshold is lowered due to the influence of the excess noise and the squeezing properties of the emitted light are significantly changed. In general, the maximal quantum-noise suppression in one quadrature component is reduced, which poses strong limitations for the practical usefulness of a geometrically unstable OPO source. The analytical results from our quasimode description are in good agreement with numerical simulations using a positive-P representation of the field in mode space and in position space.

Recently the concept of quantum information has been introduced into game theory. Here we present the first study of quantum games with more than two players. We discover that such games can possess an alternative form of equilibrium strategy, one which has no analog either in traditional games or even in two-player quantum games. In these “coherent” equilibria, entanglement shared among multiple players enables different kinds of cooperative behavior: indeed it can act as a contract, in the sense that it prevents players from successfully betraying one another.

Two-qubit states occupy a large and relatively unexplored Hilbert space. Such states can be succinctly characterized by their degree of entanglement and purity. In this article we investigate entangled mixed states and present a class of states that have the maximum amount of entanglement for a given linear entropy.

An optimum measurement strategy is presented for a new class of quantum states. This provides discrimination among the states with the minimum probability of error.

Asymmetric fragmentation dynamics of ${\mathrm{C}}_{60}$ is studied with ${\mathrm{C}}_{60}^{5+}$ in ${\mathrm{Ar}}^{8+}{-\mathrm{C}}_{60}$ collisions. A sequential “unzipping” emission of light fragments is found to be the principal mechanism in the fullerene to fullerene conversion process. Unstable fullerenes ${\mathrm{C}}_{59}^{4+}$ and ${\mathrm{C}}_{57}^{4+}$ are produced in delayed-then-fast sequential emission of ${\mathrm{C}}^{+}$ and ${\mathrm{C}}_3^{+}.$ Fragmentation of a light fragment is observed for ${\mathrm{C}}_{10}^{2+}$ issued from a previous binary fission of the ${\mathrm{C}}_{60}^{5+}.$

We have systematically studied a simple technique that accurately determines number of atoms in a magneto-optical trap. Absorption energy of a laser field that interacts with cold atoms is a direct measurement of atom number. The measured energy neither depends on the detuning, intensity, and polarization of the laser field nor is affected by other system parameters. Our work also demonstrates that such technique can be applied to study the phenomenon of coherent population trapping.

The atomic triply excited double Rydberg states (TEDRS) of La have been studied both experimentally and theoretically. It is revealed that in such an asymmetric four-body system where three electrons have very different orbital energies, the electronic correlations can be understood by means of decomposing into different dielectronic correlations that play different roles time-dependently. The correlations depend strongly on the relative excited orbital energies of three valence electrons. Our theoretical approach based on a semiclassical model is able to interpret the two observed patterns of ${\mathrm{La}}^{+}$ ion distributions among high Rydberg states resulted from the decay of TEDRS.

A metastable atomic level can be rendered unstable in a controllable way by coupling it to a decaying state. In this work we carry out a full dynamical analysis of the Zeno effect in this kind of unstable systems, comparing it to the inhibition of purely coherent Rabi oscillations. Simple and experimentally feasible measuring strategies involving three atomic levels are considered. It is shown that this induced decay is actually an example of a partial Zeno effect so that the observed evolution results from the competition of two Zeno effects. We also show that a three-level scheme can display both coherent, incoherent, and anti-Zeno effects.

We propose a probabilistic two-party communication complexity scenario with a prior nonmaximally entangled state, which results in less communication than is required with only classical random correlations. A simple all-optical implementation of this protocol is presented and demonstrates our conclusion.

We report the results of our relativistic coupled cluster singles, doubles, and partial triples calculations of the ionization potentials (IP) and excitation energies (EE) for different low-lying levels of ${\mathrm{Ba}}^{+}$. The accuracies of the IP’s and EE’s are approximately 0.2% and 1%, respectively. The inclusion of the triple excitations were crucial to achieve this degree of precision.

X-ray fluorescence signals that provide a localized (atom-specific) probe of core excitations in molecules are calculated for a one-dimensional chain of atoms. The relative and the translational motions of electron-hole pairs are clearly separated in real space and may be directly probed by the frequency and wave-vector dependence of the signals.