This paper proves that the remote-state preparation (RSP) scheme in real Hilbert space can only be implemented when the dimension of the space is 2, 4, or 8. This fact is shown to be related to the parallelizability of the $\left(n-1\right)$-dimensional sphere $S^{n-1}.$ When the dimension is 4 and 8 the generalized scheme is explicitly presented. It is also shown that for a given state with components having the same norm, RSP can be generalized to arbitrary dimension case.

We propose a conditional quantum key distribution scheme with three nonorthogonal states. Combined with the idea presented by Lo et al. (H.-K. Lo, H. F. Chau, and M. Ardehali, e-print arXiv: quant-ph/0011056), the efficiency of this scheme is increased to tend to 100%. Also, such a refined data analysis guarantees the security of our scheme against the most general eavesdropping strategy. Then, based on the scheme, we present a quantum cryptography network with the addition of a device called “space optical switch.” Moreover, we give out a realization of a quantum random number generator. Thus, a feasible experimental scheme of this efficient quantum cryptography network is completely given.

We demonstrate that two coupled Bose-Einstein condensates (BECs) at zero temperature can be used to realize a qubit which is the counterpart of Josephson charge qubits. The two BECs are weakly coupled and confined in an asymmetric double-well trap. When the “charging energy” of the system is much larger than the Josephson energy and the system is biased near a degeneracy point, the two BECs represent a qubit with two states differing only by one atom. The realization of the BEC qubits in realistic BEC experiments is briefly discussed.

We evaluate the performance of a squeezed-state channel, in which classical information is conveyed by squeezed states. The evaluation is carried out by calculating the expurgated lower bound for the reliability function. As a result, we find that using squeezed states improves the channel performance near the zero information rate.

We consider one copy of a quantum system prepared with equal prior probability in one of two nonorthogonal entangled states of multipartite distributed among separated parties. We demonstrate that these two states can be optimally distinguished in the sense of conclusive discrimination by local operations and classical communications alone. This proves strictly the conjecture that Virmani et al. confirmed numerically and analytically. Generally the optimal protocol requires local POVM operations which are explicitly constructed. The result manifests that distinguishable information is obtained only and completely at the last operation and all prior operations give no information about that state.

We investigate the scattering of orthopositronium by atomic sodium target at low and medium energies (up to 50 eV) using target-elastic and projectile-elastic close-coupling approximations (CCA) having different basis sets together with the static-exchange model. The low-energy elastic-scattering parameters are found to be consistent and the estimated binding energy for NaPs (in spin-singlet scattering) runs from 0.0042–0.0052 a.u. compared to value 0.005 892 a.u. obtained by using stochastic variational method [G. Ryzhikh and J. Mitroy, J. Phys. B 31, L401 (1998)]. The elastic cross sections as obtained by different CCA models, except the static-exchange one, differ very marginally among themselves in the energy range 0.017–25 eV and 15 eV, onwards all the partial cross sections are very close to the corresponding exchange plane-wave estimates. At higher energies (above 15 eV), it is found that the major contribution to the total cross section comes from the positronium inelastic channels.

We discuss the calculation of the off-shell T matrix in one, two, and three dimensions using both the Beliaev-Galitskii relation and the inhomogeneous Schrödinger equation. The off-shell T matrix depends, in general, on three quantities: the incoming and outgoing momenta and the energy of the collision, all of which can be chosen independently. We give a simple proof of the fact that for low-momentum scattering the T matrix depends only on the energy of the collision. Using the inhomogeneous Schrödinger equation we derive analytical results for the fully off-shell T matrix in one, two, and three dimensions for hard-sphere central potentials. The usual form of the Beliaev-Galitskii relation is not quite correct for such potentials. We derive the appropriate correction term and show that it corresponds exactly to the contribution to the T matrix from the inhomogeneous term in the inhomogeneous Schrödinger equation.

The absolute single- and double-photoionization cross sections of singly charged sulfur ions have been measured from 20 to 200 eV by merging a 2 keV beam of ${\mathrm{S}}^{+}$ ions with a synchrotron-radiation beam from an undulator. The continuum cross section for single photoionization is strongly influenced by a broad Fano profile near the ${\mathrm{S}}^{+}{\mathrm{}(}^{4}S)$ ionization threshold at 23.34 eV and a Cooper minimum located near 35 eV. Structures resulting from the excitation of $3s$ or $2p$ electrons, either from the ${\mathrm{S}}^{+}\mathrm{}{(}^{4}S)$ ground-state ion or from the ${\mathrm{S}}^{+}{(}^{2}D)$ metastable ion, are resolved, identified, and characterized. Significant deviations exist between the experimental data and the photoionization cross sections calculated as part of the Opacity Project.

The symmetrized gerade and ungerade amplitudes of the helium atom double photoionization are extracted from the convergent close-coupling calculations at fixed excess energies of 9, 20, 40, and 60 eV above the double-ionization threshold, and varying energy-sharing ratios. The amplitudes are fitted with a simple Gaussian ansatz. Although some deviations from this ansatz are clearly visible, especially at small mutual angles, the fully resolved triple differential cross sections generated from the calculated and fitted amplitudes are very close. This observation lends support to the practical parametrization suggested by Cvejanović and Reddish [J. Phys. B 33, 4691 (2000)]. A more thorough testing of the ionization amplitudes can be achieved by performing a complete double-photoionization experiment and by making a comparison with other highly accurate and computationally intensive theories.

The time-dependent close-coupling method is used to obtain absolute results for total integral, single energy, and angle differential cross sections for the double photoionization of beryllium. We compare the total ionization cross section to previous perturbative and recent nonperturbative calculations. We also compare and contrast the triple differential cross sections obtained at 20 eV excess photon energy with those obtained recently for helium [J. Phys. B 34, L457 (2001)].

Features of incoherent x-ray radiation, produced by relativistic electrons with energy of several tens of MeV in crystals, are studied for observation angles exceeding ${\gamma }^{-1}$ (γ is the projectile Lorentz factor) and for all possible momentum transfer. It is shown that the fivefold cross section has a pronounced structure, with sharp maxima and a minimum. A Monte Carlo procedure is developed to obtain a threefold cross section and, then, contributions of different radiation mechanisms are analyzed. The results of numerical integration are compared with experimental ones, and a good agreement between them is verified. The obtained results can be used both in experimental investigations dealing with interaction of charged particle beams with crystals, and to optimize parameters of x-ray sources based on the coherent x-radiation process.

We examine the adiabatic limit of population transfer in two-level models driven by a chirped laser field. We show that the nonadiabatic correction is minimized when the adiabatic eigenenergies associated to the dynamics are parallel. In the diagram of the difference of the eigenenergy surfaces as a function of the parameters, this corresponds to an adiabatic passage along a level line. The analytical arguments are based on the Dykhne-Davis-Pechukas treatment. We illustrate this behavior with various examples.

We present an experimental study of the loading of a magneto-optical trap (MOT) from a brightened and slowed beam of metastable neon atoms. The unprecedented high numbers of $9\times {10}^{920}\mathrm{Ne}$ and $3\times {10}^9$ ${}^{22}\mathrm{Ne}$ metastable atoms are trapped under unconventional trap conditions as compared to metastable helium traps, such as low intensity and small detuning. These cause the MOT to have an extraordinarily large volume on the order of $1\hspace{0.5em}{\mathrm{cm}}^3$ and a typical peak density of ${10}^{10}$ ${\mathrm{a}\mathrm{t}\mathrm{o}\mathrm{m}\mathrm{s}/\mathrm{c}\mathrm{m}}^3.$ A simple Doppler model is discussed which explains why the optimum is found under these conditions. The model includes the seventh beam necessary for the last slowing step before loading.

We have observed alignment-to-orientation conversion in the $\mathrm{}\left(5d6p\right)\mathrm{}$ ${}^{1}P$ state of atomic barium due to the combined effects of a static Zeeman shift and a dynamic Stark shift associated with the electric field of a pulsed laser beam. The measurements yield a value for the frequency-dependent tensor polarizability of the state in reasonable agreement with a simple perturbation theory calculation. With a tunable laser producing the dynamic Stark shift, we can both enhance the magnitude of the effect by tuning close to a resonance and reverse the sign of the orientation by tuning above or below the resonance. This method of producing an oriented atomic state is quite general, and with easily available field strengths can produce large orientations.

We study a single-mode laser system, whose active medium consists of molecules with a large difference between excited and ground electrical permanent-dipole moments. In this case, the Maxwell-Bloch equations are further coupled by nonlinear terms involving the ratio between this difference between the dipoles and the transition dipole moment. It is found that these new terms lead to multiple stationary solutions. From the linear stability analysis, we demonstrate the bistable (or multistable) character of the lasing solutions.

A fully quantum-mechanical Heisenberg description of the quantum-nondemolition measurements via soliton collision in an optical fiber is provided. Solutions to the equations of motion for meaningful soliton collective operators such as photon number, phase, position, and frequency (momentum) are applied to quantifying the error of inference of photon number and frequency of the signal soliton by performing a measurement on the probe soliton. A detection method based on measuring a four-component quadrature amplitude is proposed and proved to give the best accuracy.

We investigate quantum effects in pattern formation for a degenerate optical parametric oscillator with walk-off. This device has a convective regime in which macroscopic patterns are both initiated and sustained by quantum noise. Familiar methods based on linearization about a pseudoclassical field fail in this regime and new approaches are required. We employ a method in which the pump field is treated as a c-number variable but is driven by the c-number representation of the quantum subharmonic signal field. This allows us to include the effects of the fluctuations in the signal on the pump, which in turn act back on the signal. We find that the nonclassical effects, in the form of squeezing, survive just above the threshold of the convective regime. Further, above threshold, the macroscopic quantum noise suppresses these effects.

The temporal evolutions of electromagnetically induced transparency (EIT) and absorption (EIA) were observed in a Hanle absorption experiment carried on the $D_2$ lines of ${}^{87}\mathrm{Rb}$ vapor by suddenly turning the magnetic field on or off. The main features of the experimental observations are well reproduced by a theoretical model based on Bloch equation where the atomic level degeneracy has been fully accounted for. Similar (opposite phase) evolutions were observed at low optical-field intensities for Hanle-EIT or Hanle-EIA resonances. Unlike the Hanle-EIA transients that are increasingly shorter for driving field intensities approaching saturation, the $B\ne 0$ transient of the Hanle-EIT signal at large driving field intensities present a long decay time approaching the atomic transit time. Such counterintuitive behavior is interpreted as a consequence of the Zeno effect.

The Larmor precession of a relativistic neutral spin-$\frac{1}{2}$ particle in a uniform constant magnetic field confined to the region of a one-dimensional arbitrary potential barrier is investigated. The spin precession serves as a clock to measure the time spent by a quantum particle traversing a potential barrier. With the help of a general spin coherent state, it is explicitly shown that the precession time is equal to the dwell time.

The problem of inferring entanglement on the basis of incomplete prior information [R. Horodecki et al., Phys. Rev. A 59, 1799 (1999)] is reexamined with regard to operators of a more general character than the ones discussed by these authors. We show that different inference schemes are affected by the problem of overestimating the amounts of entanglement (AOE). For bipartite quantum systems, states containing the maximum and minimum possible AOE are derived explicitly.