Both the topics of entanglement and particle statistics have aroused enormous research interest since the advent of quantum mechanics. Using two pairs of entangled particles we show that indistinguishability enforces a transfer of entanglement from the internal to the spatial degrees of freedom without any interaction between these degrees of freedom. Moreover, subensembles selected by local measurements of the path will, in general, have different amounts of entanglement in the internal degrees of freedom depending on the statistics (either fermionic or bosonic) of the particles involved.

We show that by making conditional measurements on the Einstein-Podolsky-Rosen (EPR) squeezed vacuum [T. Opatrný, G. Kurizki, and D.-G. Welsch, Phys. Rev. A 61, 032302 (2000)], one can improve the efficacy of teleportation for both the position-difference, momentum-sum, and number-difference, phase-sum continuous variable teleportation protocols. We investigate the relative abilities of the standard and conditional EPR states, and show that by conditioning we can improve the fidelity of teleportation of coherent states from below to above the $\overline{F}=2/3\mathrm{}$ boundary, thereby achieving unambiguously quantum teleportation.

In this work, we present a joint theoretical-experimental study on electron-${\mathrm{N}}_2\mathrm{O}$ collisions in the intermediate energy range. More specifically, calculated and measured elastic differential, integral, and momentum-transfer cross sections, as well as calculated total and absorption cross sections are reported. The measurements were performed using a crossed electron-beam–molecular-beam geometry. The angular distribution of the scattered electrons was converted to absolute cross sections using the relative-flow technique. Theoretically, a complex optical potential is used to represent the electron-molecule interaction dynamics in the present calculation. The Schwinger variational iterative method combined with the distorted-wave approximation is used to solve the scattering equations. The comparison of the present calculated results with the measured results as well as with the existing experimental and theoretical data shows good agreement.

A rotating-wave approximation (RWA) is developed to describe the interaction of a two-level system, which has permanent dipole moments, with two pulsed lasers. The RWA expressions for the time-dependent populations of the molecular states are applied to model laser-molecule interactions and tested by comparison with exact results. The results are used to discuss the pulsed-laser phase control of molecular excitation through the interplay of competing one- and two-photon resonances involving the effects of a nonzero difference d between the permanent dipoles of the two states involved in the transition; the competition vanishes if $d=0.\mathrm{}$

A theory for the Rabi and internal Josephson effects in an interacting, homogeneous Bose-Einstein gas in the cold collision regime is presented. By using the microscopic transport equation for the density matrix the problem is mapped onto a problem of precession of two coupled classical spins. In the absence of an external excitation field our results agree with the theory of density induced frequency shifts in atomic clocks. In the presence of the external field, the internal Josephson effect occurs in a condensed Bose-Einstein gas as well as in a noncondensed gas. The crossover from Rabi oscillations to Josephson oscillations as a function of interaction strength is studied in detail.

The widely used Gross-Pitaevskii equation treats only coherent aspects of the evolution of a Bose-Einstein condensate. However, inevitably some atoms scatter out of the condensate. We have developed a method, based on the field theory formulation, describing the dynamics of incoherent processes which are due to elastic collisions. We can therefore treat processes of spontaneous emission of atoms into the empty modes, as opposed to stimulated processes, which require nonzero initial occupation. In this paper we study two counterpropagating plane waves of atoms, calculating the full dynamics of mode occupation, as well as the statistics of scattered atoms. The more realistic case of Gaussian wave packets is also analyzed.

We explore the photon statistics of light emitted from a system comprising a single four-level atom strongly coupled to a high-finesse optical cavity mode that is driven by a coherent laser field. In the weak driving regime this system is found to exhibit a photon blockade effect. For intermediate driving strengths we find a sudden change in the photon statistics of the light emitted from the cavity. Photon antibunching switches to photon bunching over a very narrow range of intracavity photon number. It is proven that this sudden change in photon statistics occurs due to the existence of robust quantum interference of transitions between the dressed states of the atom-cavity system. Furthermore, it is shown that the strong photon bunching is a nonclassical effect for certain values of driving field strength, violating classical inequalities for field correlations.

In the system with ions confined both in a linear trap and in a high-$Q$ single mode cavity, we present a simple quantum computing scheme by means of lasers and quantized cavity field, based on a previous proposal with bichromatic radiation [Phys. Rev. Lett. $82,$ 1971 (1999)]. The controlled-NOT gate operation can be performed by only three operations and it is independent of motional states of ions. Experimental feasibility for achieving our scheme is discussed.

We propose a scheme for demonstrating entanglement swapping (i.e., teleportation of entanglement) using trapped Bose-Einstein condensates. This is accomplished by detection of the total number of atoms leaking out of two adjacent traps. We describe how this scheme may be used to concentrate entanglement shared between two parties in the form of entangled condensates.

We propose an alternative way to extract single-differential ionization cross sections from time-dependent wave-packet calculations. Using the Temkin-Poet S-wave model for e-H collisions, it is shown that a properly constructed time-dependent wave function “knows” the energy of the one-particle bound or continuum orbital it has been projected on. No explicit symmetrization scheme is therefore needed to ensure a single-differential ionization cross section that accounts for the indistinguishability of the two outgoing electrons.

The retarded van der Waals force between a polarizable particle and a perfectly conducting plate is reexamined. The expression for this force given by Casimir and Polder represents a mean force, but there are large fluctuations around this mean value on short time scales that are of the same order of magnitude as the mean force itself. However, these fluctuations occur on time scales which are typically of the order of the light travel time between the atom and the plate. As a consequence, they will not be observed in an experiment that measures the force averaged over a much longer time. In the large time limit, the magnitude of the mean-squared velocity of a test particle due to this fluctuating van der Waals force approaches a constant, and is similar to a Brownian motion of a test particle in a thermal bath with an effective temperature. However, the fluctuations are not isotropic in this case, and the shift in the mean-square velocity components can even be negative. We interpret this negative shift to correspond to a reduction in the velocity spread of a wave packet. The force fluctuations discussed in this paper are a special case of the more general problem of stress tensor fluctuations. These are of interest in a variety of areas of physics, including gravity theory. Thus the effects of van der Waals force fluctuations serve as a useful model for better understanding quantum effects in gravity theory.

We propose a measurement scheme of generalized Bell states in which, although entanglement is destroyed, the nonlocality feature can be preserved and swapped. Here we consider two quantum systems: one consisting of two-party three-level maximal entangled pairs; the other being three-party two-level maximal entangled pairs. It is found that these local immeasurable states can be prepared without any classical communication and local operation. Moreover, nonmaximal entangled states cannot preserve nonlocality.

The fidelity of continuous-variable teleportation can be optimized by changing the gain in the modulation of the output field. We discuss the gain dependence of fidelity for coherent, vacuum, and one-photon inputs and propose optimal gain tuning strategies for corresponding input selections.

The ground states of Bose-Einstein condensates (BECs) of spin-2 bosons are classified into three distinct phases (ferromagnetic, “antiferromagnetic,” and cyclic) depending on the s-wave scattering lengths of binary collisions for total-spin 0, 2, and 4 channels. Many-body spin correlations and magnetic response of the condensate in each of these phases are studied in a mesoscopic regime, while low-lying excitation spectra are investigated in the thermodynamic regime. In the mesoscopic regime, where the system is so tightly confined that the spatial degrees of freedom are frozen, the exact, many-body ground state for each phase is found to be expressed in terms of the creation operators of pair or trio bosons having spin correlations. These pairwise and trio-wise units are shown to bring about some unique features of spin-2 BECs such as a huge jump in magnetization from minimum to maximum possible values and the robustness of the minimum-magnetization state against an applied magnetic field. In the thermodynamic regime, where the system is spatially uniform, low-lying excitation spectra in the presence of magnetic field are obtained analytically using the Bogoliubov approximation. In the ferromagnetic phase, the excitation spectrum consists of one Goldstone mode and four single-particle modes. In the antiferromagnetic phase, where spin-singlet “pairs” undergo Bose-Einstein condensation, the spectrum consists of two Goldstone modes and three massive ones, all of which become massless when magnetic field vanishes. In the cyclic phase, where boson “trios” condense into a spin-singlet state, the spectrum is characterized by two Goldstone modes, one single-particle mode having a magnetic-field-independent energy gap, and a gapless single-particle mode that becomes massless in the absence of magnetic field.

We study the quantum dynamics of a one-dimensional gas of hard-core bosons on a ring, and investigate the interference that results when an initially trapped gas localized on one side of the ring is released, split via an optical-dipole grating, and recombined on the other side of the ring. Large visibility interference fringes arise when the wave vector of the optical-dipole grating is larger than the effective Fermi wave vector of the initial gas.

The interaction of an atom with both a quantized cavity field and an external classical driving field, the fields being degenerate in frequency, is studied in the regime where the atom and fields are highly detuned. The atom interacts dispersively with the quantized field but the classical driving field gives rise to the creation or destruction of photons conditional on the state of the atom. We show how this interaction can be used to generate coherent states of the cavity field and various forms of superpositions of macroscopically distinct states. This method is in contrast to the usual method used in microwave cavity QED of injecting a coherent state into a cavity via a waveguide attached to a klystron and where subsequently Schrödinger cat states may be generated by manipulating the field with injected atoms. The method proposed here could possibly be used in the case of an optical cavity. Further, we show that coherent states may be generated in the steady state from the competition between the driven dispersion interaction and dissipative single-photon losses, a form of optical balance.

Quantum interferences in spontaneous emission spectrum from a V-type three-level and a double V-type four-level atom embedded in a double-band photonic band-gap material have been investigated. We demonstrate that there is not only a black dark line, but also a narrow spontaneous line in the spectrum. The dark line results from a destructive interference or singularities of the density of states of the radiation field. The narrow spectral line arises from a population transfer under joint constructive interferences. The spontaneous emission spectrum is compared to the cases of single-band and double-band photonic band-gap reservoirs.

We present a detailed study of how phase-sensitive feedback schemes can be used to improve the performance of optomechanical devices. Considering the case of a cavity mode coupled to an oscillating mirror by the radiation pressure, we show how feedback can be used to reduce the position noise spectrum of the mirror, cool it to its quantum ground state, or achieve position squeezing. Then, we show that even though feedback is not able to improve the sensitivity of stationary position spectral measurements, it is possible to design a nonstationary strategy able to increase this sensitivity.

We develop a dynamical non-Markovian description of quantum computing in the weak-coupling limit, in the lowest-order approximation. We show that the long-range memory of the quantum reservoir (such as the ${1/t}^4$ one exhibited by electromagnetic vacuum) produces a strong interrelation between the structure of noise and the quantum algorithm, implying nonlocal attacks of noise. This shows that the implicit assumption of quantum error correction theory—independence of noise and self-dynamics—fails in long time regimes. We also use our approach to present pure decoherence and decoherence accompanied by dissipation in terms of the spectral density of the reservoir. The so-called dynamical decoupling method is discussed in this context. Finally, we propose a minimal decoherence model, in which the only source of decoherence is vacuum. We optimize the fidelity of quantum-information processing under the trade-off between the speed of the gate and the strength of decoherence.

It is generally accepted that quantum key distribution (QKD) could supply legitimate users with unconditional security during their communication. Quite a lot of satisfactory efforts have been achieved on experimentations with quantum cryptography. However, when the eavesdropper has extra-powerful computational ability, has access to a quantum computer, for example, and can carry into execution any eavesdropping measurement that is allowed by the laws of physics, the security against such attacks has not been widely studied and rigorously proved for most QKD protocols. Quite recently, Shor and Preskill proved concisely the unconditional security of the Bennett-Brassard 1984 (BB84) protocol. Their method is highly valued for its clarity of concept and concision of form. In order to take advantage of the Shor-Preskill technique in their proof of the unconditional security of the BB84 QKD protocol, we introduced in this paper a transformation that can translate the Bennett 1992 (B92) protocol into the BB84 protocol. By proving that the transformation leaks no more information to the eavesdropper, we proved the unconditional security of the B92 protocol. We also settled the problem proposed by Lo about how to prove the unconditional security of the B92 protocol with the Shor-Preskill method.