Spontaneous scattering noise is incorporated as a build-up source in a fully transient stimulated Brillouin scattering (SBS) model. This powerful simulation tool is successfully applied for a quantitative investigation of the fluctuations in the output pulse duration of SBS pulse compressors. The predictions of this model are experimentally verified in a two-stage SBS compressor.

Specific features of nonlinear interference processes at quantum transitions in near- and fully resonant Doppler-broadened optically dense media are studied. The possibility of all-optical switching of the medium to opaque or, alternatively, to absolutely transparent, or even to strongly amplifying states is explored, which is controlled by a small variation of two driving or probe radiations and does not require any change of the level populations. Optimum conditions for inversionless amplification of short-wavelength radiation above the oscillation threshold at the expense of the longer-wavelength control fields are investigated. The feasibility of overcoming the fundamental limitation on a velocity-interval of resonantly coupled molecules imposed by the Doppler effect is shown, based on quantum coherence.

Near the threshold for electromagnetically induced transparency (EIT) or coherent population trapping (CPT), two-photon-resonance-enhanced self-focusing of a $\Lambda$ system can be exploited to induce spatial confinement in a second, diffracting $\Lambda$ system. The diffracting $\Lambda$ system is characterized by parameters below the EIT or CPT threshold, and the two $\Lambda$ systems must be coupled to form a closed-loop double-$\Lambda$ system. The waveguiding effect is shown to be strongly phase dependent, indicating that it derives from the phase-dependent effective third-order susceptibility rather than the phase-independent effective first-order susceptibility, as is the case in previously studied systems. We also show that when the second $\Lambda$ system initially involves only a single laser beam, the loop is completed by the efficient generation of radiation at the four-wave-mixing frequency, within a propagation distance much shorter than the diffraction length. Both the applied and generated fields exhibit electromagnetically induced waveguiding.

We demonstrate how a general von Neumann–like measurement can be analyzed in terms of histories (paths) constructed for the measured variable $A$. The Schrödinger state of a system in a Hilbert space of arbitrary dimensionality is decomposed into a set of substates, each of which corresponds to a particular detailed history of the system. The coherence between the substates may then be destroyed by meter(s) to a degree determined by the nature and the accuracy of the measurement(s) which may be of von Neumann, finite-time, or continuous type. The cases of a particle described by Feynman paths in the coordinate space and a qubit in a two-dimensional Hilbert space are studied in some detail.

Deterministic discrimination of nonorthogonal states is forbidden by quantum measurement theory. However, if we do not want to succeed all the time, i.e., allow for inconclusive outcomes to occur, then unambiguous discrimination becomes possible with a certain probability of success. A variant of the problem is set discrimination: the states are grouped in sets and we want to determine to which particular set a given pure input state belongs. We consider here the simplest case, termed quantum state filtering, when the $N$ given nonorthogonal states $\{\mid {\psi }_1⟩,\dots ,\mid {\psi }_N⟩\}$ are divided into two sets and the first set consists of one state only while the second consists of all of the remaining states. We present the derivation of the optimal measurement strategy, in terms of a generalized measurement (positive-operator-valued measure), to distinguish $\mid {\psi }_1⟩$ from the set $\{\mid {\psi }_2⟩,\dots ,\mid {\psi }_N⟩\}$ and the corresponding optimal success and failure probabilities. The results, but not the complete derivation, were presented previously [Phys. Rev. Lett. 90, 25901 (2003)] as the emphasis there was on appplication of the results to probabilistic quantum algorithms. We also show that the problem is equivalent to the discrimination of a pure state and an arbitrary mixed state.

We describe generalizations of the Pauli group, the Clifford group, and stabilizer states for qudits in a Hilbert space of arbitrary dimension $d$. We examine a link with modular arithmetic, which yields an efficient way of representing the Pauli group and the Clifford group with matrices over ${\mathbb{Z}}_d$. We further show how a Clifford operation can be efficiently decomposed into one and two-qudit operations. We also focus in detail on standard basis expansions of stabilizer states.

In practical quantum cryptography, the source sometimes produces multiphoton pulses, thus enabling the eavesdropper Eve to perform the powerful photon-number-splitting (PNS) attack. Recently, it was shown by Curty and Lütkenhaus [Phys. Rev. A 69, 042321 (2004)] that the PNS attack is not always the optimal attack when two photons are present: if errors are present in the correlations Alice-Bob and if Eve cannot modify Bob’s detection efficiency, Eve gains a larger amount of information using another attack based on a $2\to 3$ cloning machine. In this work, we extend this analysis to all distances Alice-Bob. We identify a new incoherent $2\to 3$ cloning attack which performs better than those described before. Using it, we confirm that, in the presence of errors, Eve’s better strategy uses $2\to 3$ cloning attacks instead of the PNS. However, this improvement is very small for the implementations of the Bennett-Brassard 1984 (BB84) protocol. Thus, the existence of these new attacks is conceptually interesting but basically does not change the value of the security parameters of BB84. The main results are valid both for Poissonian and sub-Poissonian sources.

We discuss the canonical form for a pure state of three identical bosons in two modes, and classify its entanglement correlation into two types, the analogous Green-Horne-Zeilinger and the $W$ types as well known in systems of three distinguishable qubits. We have performed a detailed study of two important entanglement measures for such a system, the concurrence $\mathcal{C}$ and the triple entanglement measure $\tau$. We have also calculated explicitly the spin squeezing parameter $\xi$ and the result shows that the $W$ state is the most “antisqueezing” state, for which the spin-squeezing parameter cannot be regarded as an entanglement measure.

We consider the ground state of an XX chain that is constrained to carry a current of energy. The von Neumann entropy of a block of $L$ neighboring spins, describing entanglement of the block with the rest of the chain, is computed. Recent calculations have revealed that the entropy in the XX model diverges logarithmically with the size of the subsystem. We show that the presence of the energy current increases the prefactor of the logarithmic growth. This result indicates that the emergence of the energy current gives rise to an increase of entanglement.

The applicability of an approach to the calculation of tunneling splittings in a one-dimensional potential with symmetrically disposed minima is presented. The method is based on the concept of contraction of a Lie algebra [Mol. Phys. 93, 441 (1998)], recently extended to encompass the case where more than one symmetry-related minima is present. The method is validated against highly precise published calculations using other approaches and also against the tunneling energy levels of lower inversion vibrational levels in $\mathrm{N}{\mathrm{H}}_3$.

Ab initio calculations of dielectronic recombination processes from the ground state $\left[\mathrm{Ne}\mathit{\text{core}}\right]3s^{2}3p^{6}3d^9$ of Co-like ${\mathrm{Pd}}^{19+}$ ion through doubly excited states $\left[\mathrm{Ne}\mathit{\text{core}}\right]3s^{2}3p^{6}3d^{8}4ln{l}^{\prime }$, $\left[\mathrm{Ne}\mathit{\text{core}}\right]3s^{2}3p^{5}3d^{9}4ln{l}^{\prime }$, and $\left[\mathrm{Ne}\mathit{\text{core}}\right]3s3p^{6}3d^{9}4ln{l}^{\prime }$ ($n=4$ or 5) of Ni-like ${\mathrm{Pd}}^{18+}$ ions are performed using the multiconfiguration Hartree-Fock method with relativistic correction. Total and state-to-state rate coefficients are obtained in the temperature range from $0.1\text{to}{10}^4\mathrm{eV}$. Comparison of the rate coefficients from $3s$, $3p$, and $3d$ subshell excitations shows $3d$ subshell excitation dominates over the others in the whole energy region, but $3p$ and $3s$ subshell excitation cannot be neglected in high temperature; when electron temperature $T>500\mathrm{eV}$, the contributions from $3p$ and $3s$ subshells are more than 40% and 10% of the dielectronic recombination (DR) rate coefficients from $3d$ subshell excitations; especially for the DR processes through the doubly excited states $n=5$, the contribution from $3p$ subshell excitations is approximately 60% of the rate coefficients from $3d$ subshell excitations. The two-electron radiative transitions, such as $3d^{8}4p4d\to 3d^{9}4s$ and $3d^{8}4d^2\to 3d^{9}4p$, are found to be important due to the strong configuration interactions, which contribute more than 10% of the total rate coefficients through the doubly excited states $\left[\mathrm{Ne}\mathit{\text{core}}\right]3s^{2}3p^{6}3d^{8}4l4l^{\prime }$.

We study the dynamics of Bose-Einstein condensates in symmetric double-well potentials following a sudden change of the potential from the Mott-insulator to the superfluid regime. We introduce a continuum approximation that maps that problem onto the wave-packet dynamics of a particle in an anharmonic effective potential. For repulsive two-body interactions the visibility of interference fringes that result from the superposition of the two condensates following a stage of ballistic expansion exhibits a collapse of coherent oscillations onto a background value whose magnitude depends on the amount of squeezing of the initial state. Strong attractive interactions are found to stabilize the relative number dynamics. We visualize the dynamics of the system in phase space using a quasiprobability distribution that allows for an intuitive interpretation of the various types of dynamics.

We present a theory for ultracold atomic gases in an optical lattice near a Feshbach resonance. In the single-band approximation the theory describes atoms and molecules that can both tunnel through the lattice. Moreover, an avoided crossing between the two-atom and the molecular states occurs at every site. We determine the microscopic parameters of the generalized Hubbard model that describes this physics, using the experimentally known parameters of the Feshbach resonance in the absence of the optical lattice. As an application we also calculate the zero-temperature phase diagram of an atomic Bose gas in an optical lattice.

Spontaneous parametric processes pumped transversely with short pulses are studied under a unified framework, which proves that such processes can efficiently generate entangled photon pairs with time anticorrelation and frequency correlation. Improvements upon previously proposed schemes can be made by the use of quasi-phase-matching, four-wave mixing, and cross-phase-modulation compensation. The use of frequency-correlated photons in the Hung-Ou-Mandel interferometer is studied.

We investigate theoretically and experimentally a nondestructive interferometric measurement of the state population of an ensemble of laser-cooled and trapped atoms. This study is a step toward generation of (pseudo)spin squeezing of cold atoms targeted at the improvement of the cesium clock performance beyond the limit set by the quantum projection noise of atoms. We calculate the phase shift and the quantum noise of a near-resonant optical probe pulse propagating through a cloud of cold ${}^{133}\mathrm{Cs}$ atoms. We analyze the figure of merit for a quantum nondemolition (QND) measurement of the collective pseudospin and show that it can be expressed simply as a product of the ensemble optical density and the pulse-integrated rate of the spontaneous emission caused by the off-resonant probe light. Based on this, we propose a protocol for the sequence of operations required to generate and utilize spin squeezing for the improved atomic clock performance via a QND measurement on the probe light. In the experimental part we demonstrate that the interferometric measurement of the atomic population can reach a sensitivity of the order of $\sqrt{N_{\mathrm{at}}}$ in a cloud of $N_{\mathrm{at}}$ cold atoms, which is an important benchmark toward the experimental realization of the theoretically analyzed protocol.

We propose a scheme for periodic generation of subfemtosecond pulses on the basis of modulated Raman coherence. The detailed theoretical analyses and numerical simulations show that an oscillatory Raman coherence with periodically changed phase is established. It leads to periodic compression of the generated broadband sidebands in the same medium. As compared with the scheme by using nonmodulated driving fields where subfemtosecond pulses can be generated only with negative Raman detunings, the modulated Raman coherence scheme supports subfemtosecond pulse generation with positive, negative, or even opposite Raman detunings around the Raman resonance.

Published experimental results show that single-mode laser light is characterized in the microwave range by a frequency noise which essentially includes a white part and a $1∕f$ (flicker) part. We theoretically show that the spectral density (the line shape) which is compatible with these results is a Voigt profile whose Lorentzian part or homogeneous component is linked to the white noise and the Gaussian part to the $1∕f$ noise. We measure semiconductor laser line profiles and verify that they can be fit with Voigt functions. It is also verified that the width of the Lorentzian part varies like $1∕P$ where $P$ is the laser power while the width of the Gaussian part is more of a constant. Finally, we theoretically show from first principles that laser line shapes are also described by Voigt functions where the Lorentzian part is the laser Airy function and the Gaussian part originates from population noise.

Based on a quantum secure direct communication (QSDC) protocol [Phys. Rev. A 69 052319 (2004)], we propose a $(n,n)$-threshold scheme of multiparty quantum secret sharing of classical messages (QSSCM) using only single photons. We take advantage of this multiparty QSSCM scheme to establish a scheme of multiparty secret sharing of quantum information (SSQI), in which only all quantum information receivers collaborate can the original qubit be reconstructed. A general idea is also proposed for constructing multiparty SSQI schemes from any QSSCM scheme.

We present a classical analysis of a recently predicted phenomena that a pair of bare nuclei of like charge can “bind” in the sense that Coulomb explosion is slowed down when the system is subject to a strong external electromagnetic field [O. Smirnova, M. Spanner, and M. Ivanov et al., Phys. Rev. Lett. 90, 243001 (2003)]. We show that this effect is described by classical mechanics and obtain trapping when the external field is treated in the dipole approximation. When the full nondipole interaction is used trapping is less likely.

We obtain a query lower bound for quantum algorithms solving the phase estimation problem. Our analysis generalizes existing lower-bound approaches to the case where the oracle $Q$ is given by controlled powers $Q^p$ of $Q$, as it is, for example, in Shor’s order-finding algorithm. In this setting we will prove a $\Omega (\log 1∕ϵ)$ lower bound for the number of applications of $Q^{p_1}$, $Q^{p_2},\dots$. This bound is tight due to a matching upper bound. We obtain the lower bound using a technique based on frequency analysis.