The communication protocol of Home and Whitaker [Phys. Rev. A 67, 022306 (2003)] is examined in some detail, and found to work equally well using a separable state. The protocol is in fact completely classical, based on postselection of suitable experimental runs. The quantum-cryptography protocol proposed in the same publication is also examined, and this protocol uses entanglement, a strictly quantum property of the system. An individual eavesdropping attack on each qubit pair would be detected by the security test proposed in the mentioned paper. However, the key is provided by groups of qubits, and there exists a coherent attack, internal to these groups, that will go unnoticed in that security test. A modified test is proposed here that will ensure security, even against such a coherent attack.

We describe a scheme for the encoding and manipulation of single photon qubits in optical sideband modes using standard optical elements. We propose and analyze the radio frequency half-wave plate, which may be used to make arbitrary rotations of a state in the frequency basis, and the frequency beamsplitter, which may be used to separate (or combine) photons of different frequencies into (from) different spatial modes.

A density-matrix formalism is developed based on the one-particle density-matrix of a single-determinantal reference state. Unlike traditional density-functional-theory approaches, the $v$-representable problem does not appear in the proposed method, nor the need to introduce functionals defined by a constrained search. The correlation-energy functionals are nonuniversal, in the sense that they depend on the external potential. Nevertheless, model systems can still be used to derive universal energy functionals. Variational and nonvariational energy functionals are introduced that yield the target-state energy when the reference state—or its corresponding one-particle density matrix—is constructed from Brueckner orbitals. Nonvariational energy functionals yield generalized Hartree-Fock equations involving a nonlocal correlation potential and the Hartree-Fock exchange operator; these equations are obtained by imposing the Brillouin-Brueckner condition. The same equations—for the most part—are obtained from variational energy functionals using functional minimization, yielding the (kernel of the) correlation potential as the functional derivative of correlation-energy functionals. Approximations for the correlation-energy functions are introduced, including a one-particle-density-matrix variant of the local-density approximation, a variant of the Colle-Salvetti functional, and a linear combination of the two that is a variant of the correlation-energy functional within the hybrid, three-parameter, Becke-Lee-Yang-Parr density functional (B3LYP).

We investigate the scaling of the phase sensitivity of a nonideal Heisenberg-limited interferometer with the particle number $N$, in the case of the Bayesian detection procedure proposed by Holland and Burnett [Phys. Rev. Lett. 71, 1355 (1993)] for twin boson input modes. Using Monte Carlo simulations for up to $\mathrm{10\hspace{0.17em}000}$ bosons, we show that the phase error of a nonideal interferometer scales with the Heisenberg limit if the losses are of the order of $N^{-1}$. Greater losses degrade the scaling which is then in $N^{-1∕2}$, such as the beam-splitter shot-noise limit, yet the sensitivity stays sub-shot-noise as long as photon correlations are present. These results give the actual limits of Bayesian detection for twin-mode interferometry and prove that it is an experimentally feasible scheme, contrary to what is implied by the coincidence-detection analysis of Kim et al. [Phys. Rev. A 60, 708 (1999)].

We address a systematic approach to the study of the optimum states reaching maximum resolution for interferometers with moving mirrors. We find a correspondence between the optimum states for interferometers with fixed and moving mirrors.

Four problematic circumstances are considered, involving models which describe dynamical wave-function collapse toward energy eigenstates, for which it is shown that wave-function collapse of macroscopic objects does not work properly. In one case, a common particle position measuring situation, the apparatus evolves to a superposition of macroscopically distinguishable states (does not collapse to one of them as it should) because each such particle∕apparatus∕environment state has precisely the same energy spectrum. Second, assuming an experiment takes place involving collapse to one of two possible outcomes which is permanently recorded, it is shown in general that this can only happen in the unlikely case that the two apparatus states corresponding to the two outcomes have disjoint energy spectra. Next, the progressive narrowing of the energy spectrum due to the collapse mechanism is considered. This has the effect of broadening the time evolution of objects as the universe evolves. Two examples, one involving a precessing spin, the other involving creation of an excited state followed by its decay, are presented in the form of paradoxes. In both examples, the microscopic behavior predicted by standard quantum theory is significantly altered under energy-driven collapse, but this alteration is not observed by an apparatus when it is included in the quantum description. The resolution involves recognition that the state vector describing the apparatus does not collapse, but evolves to a superposition of macroscopically different states.

By modeling the interaction of an open quantum system with its environment through a natural generalization of the classical concept of continuous time random walk, we derive and characterize a class of non-Markovian master equations whose solution is a completely positive map. The structure of these master equations is associated with a random renewal process where each event consist in the application of a superoperator over a density matrix. Strong nonexponential decay arise by choosing different statistics of the renewal process. As examples we analyze the stochastic and averaged dynamics of simple systems that admit an analytical solution. The problem of positivity in quantum master equations induced by memory effects [S. M. Barnett and S. Stenholm, Phys. Rev. A 64, 033808 (2001)] is clarified in this context.

We extend an earlier model by Law and Kimble [J. Mod. Opt.44, 2067 (1997)] for a cavity QED based single-photon-gun to atom-photon entanglement generation and distribution. We illuminate the importance of a small critical atom number on the fidelity of the proposed operation in the strong-coupling limit. Our result points to a promisingly high purity and efficiency using currently available cavity QED parameters, and sheds new light on constructing quantum computing and communication devices with trapped atoms, and high-$Q$ optical cavities.

The variational method with multiconfiguration interaction wave function is used to obtain the energies of high-lying core-excited states ${}^{5}P\left(n\right)$ ($n=1–7$) and ${}^{5}S^o\left(m\right)$ ($m=1–5$) in Be-like boron and carbon, including the mass polarization and relativistic corrections. Restricted variational method is carried out to extrapolate a better energy. The oscillator strengths, transition rates, and wavelengths are also calculated. The results are compared with other theoretical and experimental data in the literature. The fine structure and hyperfine structure of core-excited states for this system are also investigated.

In this work, electron emission spectra produced by impact of fast protons on Al(111) surfaces are theoretically and experimentally studied. Contributions coming from the different electronic sources of the metal—atomic inner shells and valence band—are analyzed as a function of the angle of electron emission. In the forward direction, the inner-shell ionization process is the dominant mechanism. The valence emission, instead, becomes important when the ejection angle is separated from the specular-reflection direction. In both angular regions, theoretical and experimental values are in reasonable agreement. The energy shift and broadening of the convoy electron peak at glancing observation angles are well described by the present model, which takes into account the influence of the induced surface field on the ionized electron.

We show that the threshold power density of the intrinsic laser-induced damage in borosilicate glass at $\sim 1\mu \mathrm{m}$ wavelength does not depend on pulse duration from $2\times {10}^{-13}$ to $3\times {10}^{-8}\mathrm{s}$ and has the same value for both single- and multiple-pulse exposure of the sample. This indicates that the mechanism of the intrinsic damage in glasses involves a collective response of a certain volume in the dielectric as a whole, such as “dielectric-metal” phase transition, rather than a process of individual generation and accumulation of electrons, such as multiphoton, tunneling, or avalanche. Also, we demonstrate that under femtosecond exposure the threshold of the plasma formation in transparent glasses is considerably higher than the threshold of the residual change of medium parameters.

Above-threshold ionization of Xe atoms in an infinite sequence of linearly polarized few-cycle pluses is studied. The dependence of photoelectron angular distributions (PADs) on the carrier-envelope (CE) phase in various few-cycle pulses are obtained. Our study shows that PADs in linearly polarized few-cycle pulses vary with the CE phase. The PADs are inversion asymmetric for most CE phases and inversion symmetric for some special CE phases. The PADs are always symmetric about the polarization vector, but the maximal ionization rates vary with the CE phase. Our study also shows the jet structures in PADs arising from the inherent property of photon ionization and the variation of PADs with the number of optical cycles in a single pulse.

The shift of the Bose-Einstein condensation temperature for a homogenous weakly interacting Bose gas in leading order in the scattering length $a$ is computed for given particle density $n$. Variational perturbation theory is used to resum the corresponding perturbative series for $\Delta ⟨{\varphi }^2⟩∕Nu$ in a classical three-dimensional scalar field theory with coupling $u$ and where the physical case of $N=2$ field components is generalized to arbitrary $N$. Our results for $N=1,2,4$ are in agreement with recent Monte Carlo simulations; for $N=2$, we obtain $\Delta T_c∕T_0=\left(1.27\pm 0.11\right)a{n}^{1∕3}$. We use seven-loop perturbative coefficients, extending earlier work by one loop order.

We apply perturbative renormalization-group theory to the symmetric phase of a dilute interacting Bose gas which is trapped in a three-dimensional harmonic potential. Using Wilson’s energy-shell renormalization and the $\epsilon$ expansion, we derive the flow equations for the system. We relate these equations to the flow for the homogeneous Bose gas. In the thermodynamic limit, we apply our approach to examine the transition temperature of the harmonically trapped Bose gas as a function of the scattering length. The results are compared to previous studies of the problem.

We develop a technique of scattering from many-body systems. It is based on the principle of the neutron spin echo (SE), where a neutron wave in the magnetic field splits into two waves, which are separated in space or in time after propagation in this field. The neutron thus prepared as a probe passes through the sample to test its properties on a space $R$ or time $t$ scale. This separation in space or in time can be measured using coherence of these two waves as a phase shift $\varphi$ between them. These two waves are collected or focused and compensated by the SE technique in order to compare their phases after interaction with the sample. In this way one studies interference between these waves and thus can directly measure the pair-correlation function in space or in time. Instead of two-wave SE we propose to realize the four-wave neutron-resonance spin-echo (NRSE). In our experiments, spin precession produced by a couple of the neutron-resonance coils in one arm is compensated by an identical couple of other NR coils in a second arm of a spin-echo machine. The neutron spin-flip probability $\rho$ in the resonance coils is a key parameter of the NRSE arm. The limiting cases, $\rho =0$ and $\rho =1$, provide, in quantum terms, a two-level–two-wave $k$ splitting of the neutron and result in the separation of the split waves into two different lengths in space $(R_1,R_2)$ or in time $(t_1,t_2)$. These two cases correspond to Larmor precession with phase ${\varphi }_1$ in the static magnetic fields of the NR flippers or to NRSE precession with ${\varphi }_2$, respectively. The intermediate case, $0<\rho <1$, provides a four-level–four-wave $k$ splitting of the neutron with the corresponding separations in space $\left(R_1,R_2,R_3\right)$ or in time $\left(t_1,t_2,t_3\right)$. The interference of each pair of waves after compensation results in three different echos with phases ${\varphi }_1$, ${\varphi }_2$, and ${\varphi }_3=({\varphi }_1+{\varphi }_2)∕2$. Focusing or compensating all four waves into a single point of the phase-of-waves diagram produces quantum interference of all newly created waves. This task of focusing is experimentally performed. Different options for the compensation are discussed. The experiment opens the possibility to measure a composite correlation function, combined from several pair-correlation functions.

We set forth a polarization-sensitive quantum-optical coherence tomography (PS-QOCT) technique that provides axial optical sectioning with polarization-sensitive capabilities. The technique provides a means for determining information about the optical path length between isotropic reflecting surfaces, the relative magnitude of the reflectance from each interface, the birefringence of the interstitial material, and the orientation of the optical axis of the sample. PS-QOCT is immune to sample dispersion and therefore permits measurements to be made at depths greater than those accessible via ordinary optical coherence tomography. We also provide a general Jones matrix theory for analyzing PS-QOCT systems and outline an experimental procedure for carrying out such measurements.

A theory of quantum control of short-wavelength sum-frequency generation, which employs the continuum states, is developed. The proposed scheme employs all-resonant coupling and trade-off optimization of the accompanying constructive and destructive quantum interference effects in the lower-order and higher-order polarizations controlled by the overlap of two autoionizinglike laser-induced continuum structures. The scheme does not rely on adiabatic passage, coherent population trapping or maximum atomic coherence as a means to facilitate maximum output. The opportunities for manipulating transparency of the medium and refractive index for the fundamental and generated radiations, as well as nonlinear polarization in the multiple-resonant medium, are shown. This opens the feasibility of creating frequency-tunable narrowband filters, polarization rotators, and dispersive elements for vacuum ultraviolet radiation. The features specific for quantum interference in Doppler-broadened media are investigated. The feasibility of almost complete conversion of long-wavelength fundamental radiation into generated short-wavelength radiation, and of a dramatic decrease in the intensity of required fundamental radiations, is shown.

Cavity solitons are stationary self-organized bright intensity peaks which form over a homogeneous background in the section of broad area radiation beams. They are generated by shining a writing/erasing laser pulse into a nonlinear optical cavity, driven by a holding beam. The ability to control their location and their motion by introducing phase or amplitude gradients in the holding beam makes them interesting as mobile pixels for all-optical processing units. We show the generation of a number of cavity solitons in broad-area vertical cavity semiconductor microresonators electrically pumped above transparency but slightly below threshold. We analyze the switching process in details. The observed spots can be written, erased, and manipulated as independent objects, as predicted by the theoretical model. An especially tailored one is used to simulate the studied phenomena and to compare our simulations to the experimental findings with good agreement.

The general perturbative expression for the lateral Casimir force between two plates covered by longitudinal corrugations of arbitrary shape is obtained. This expression is applicable for corrugation periods larger than the separation distance. The cases of asymmetric corrugations are considered, which allow one to increase the maximum to minimum force ratio and affect the character of equilibrium points. This opens opportunities to control the lateral Casimir forces for use in microelectromechanical devices based entirely on the vacuum fluctuation properties.