A general theory of optical resonance solitons and breathers in the presence of single-excitonic and biexcitonic transitions in an ensemble of inhomogeneously broadened semiconductor quantum dots is constructed. Optical plane wave solitons ($2\pi$ pulses) are formed in stacked layer structures (many-layered systems) of semiconductor quantum dots. Optical small-amplitude bright breathers ($0\pi$ pulses) in semiconductor quantum dot waveguides are considered. Explicit analytical expressions for the shape and parameters of the solitons and breathers in the regime of self-induced transparency are obtained as well as simulations of the space-time dynamics of two-dimensional breathers presented with realistic parameters which can be reached in current experiments. It is shown that, unlike for plane wave breathers, the parameters additionally depend on the waveguide mode. In the special case of plane wave breathers in semiconductor quantum dots, known analytical results are recovered.

Quantum enhancement of optical pulse timing accuracy is investigated in the Heisenberg picture. Effects of optical loss, group-velocity dispersion, and Kerr nonlinearity on the position and momentum of an optical pulse are studied via Heisenberg equations of motion. Using the developed formalism, the impact of decoherence by optical loss on the use of adiabatic soliton control for beating the timing standard quantum limit [M. Tsang, Phys. Rev. Lett. 97, 023902 (2006)] is analyzed theoretically and numerically. The analysis shows that an appreciable enhancement can be achieved using current technology, despite an increase in timing jitter mainly due to the Gordon-Haus effect. The decoherence effect of optical loss on the transmission of quantum-enhanced timing information is also studied, in order to identify situations in which the enhancement is able to survive.

There exist two distinctive types of soliton-shaping mechanisms in mode-locked fiber lasers: one is the balance between the fiber nonlinear Kerr effect and the linear cavity dispersion, and the other one is that caused by the laser gain saturation and dispersion. We show both numerically and experimentally that as the net cavity group velocity dispersion approaches zero, the two types of soliton-shaping mechanisms coexist in a laser. Depending on the laser operation, either of them could dominate the pulse shaping in a laser. And as a result of their competition, the stable soliton formed in the cavity dispersion regime could have different temporal and spectral profiles.

We propose an entanglement measure to quantify three-qubit entanglement in terms of negativity. A monogamy inequality analogous to the Coffman-Kundu-Wootters inequality is established. This consequently leads to a definition of residual entanglement, which is referred to as the three-$\pi$ in order to distinguish it from the three-tangle. The three-$\pi$ is proved to be a natural entanglement measure. By contrast to the three-tangle, it is shown that the three-$\pi$ always gives greater than zero values for pure states belonging to the $W$ and Greenberger-Horne-Zeilinger classes, implying that three-way entanglement always exists for them; the three-tangle generally underestimates the three-way entanglement of a given system. This investigation will offer an alternative tool to understand genuine multipartite entanglement.

We propose a scheme that allows generation of the Bell and $W$-type entangled states of excitons in quantum dots confined in a single-mode microcavity. It is shown that the entanglement of excitons can be generated when the photons leaking out of the cavity are detected by photon counter. The effect of environmental temperature on the entanglement of excitonic states is also studied.

Dynamical decoupling can be used to preserve arbitrary quantum states despite undesired interactions with the environment, using control Hamiltonians affecting the system only. We present a system-independent analysis of dynamical decoupling based on leading order decoupling error estimates, valid for bounded-strength environments. Using as a key tool a renormalization transformation of the effective system-bath coupling Hamiltonian, we delineate the reliability domain of dynamical decoupling used for quantum state preservation, in a general setting for a single qubit. We specifically analyze and compare two deterministic dynamical decoupling schemes—periodic and concatenated—and distinguish between two limiting cases of fast versus slow environments. We prove that concatenated decoupling outperforms periodic decoupling over a wide range of parameters. These results are obtained for both “ideal” (zero-width) and realistic (finite-width) pulses This work extends and generalizes our earlier work [Phys. Rev. Lett. 95, 180501 (2005)].

The information encoded in the polarization of a single photon can be transferred to a remote location by two-channel continuous-variable quantum teleportation. However, the finite entanglement used in the teleportation causes random changes in photon number. If more than one photon appears in the output, the continuous-variable teleportation accidentally produces clones of the original input photon. In this paper, we derive the polarization statistics of the $N$-photon output components and show that they can be decomposed into an optimal cloning term and completely unpolarized noise. We find that the accidental cloning of the input photon is nearly optimal at experimentally feasible squeezing levels, indicating that the loss of polarization information is partially compensated by the availability of clones.

Relativistic configuration interaction results are presented for the x-ray rates from $2s^{2}3p$ and $1s2s^{2}3p$ configurations of Li- and Be-like ions, respectively. The x-ray energies and rates are calculated for 14 ions in the range $Z=10–54$ using multiconfiguration Dirac-Fock wave functions in the active space approximation. The convergence of the eigenvalues and the effect of different shells on the rates have been studied as the active space is being gradually increased. The influence of configuration mixing on the decay rates of x rays from Li- and Be-like ions are analyzed. Our calculations show that strong configuration mixing reduces substantially the single configuration decay rates from Li-like ions considered in this work whereas it enhances significantly the allowed dipole rates from Be-like ions with $Z<32$. For other ions, the dipole rate decreases. The configuration interaction reduces the spin-forbidden rates of all the ions. The results are compared with other available values.

The generalized oscillator strengths (GOS’s) of discrete excitations $np\to nd$ and $np\to (n+1)s,d$, both dipole $(L=1)$ and octupole $(L=3)$, are studied. We demonstrate that although the relevant transitions in the same atom are closely located in energy, the dependence of their GOS’s on the momentum transfer squared $q^2$ is remarkably different; viz., the GOS’s corresponding to $L=3$ have at least one extra maximum as a function of $q^2$ and dominate over those of the $L=1$, starting from about $q^2=1.5\mathrm{a}.\mathrm{u}.$ Peculiarities are noted in the GOS of only the Ne dipole $2p\to 3s$ transition in contrast to the rest of the atoms; viz., the characteristic minimum and the maximum are not obliterated by the dominance of $2p\to 3d$ GOS dipole and octupole sum as in the other atoms. The depth of the minimum of the GOS for the dipole $2p\to 3s$ transition is reduced considerably by the $2p\to 3d$ dipole and octupole GOS sums, while the corresponding maximum is enhanced. In the GOS sum both the minimum and maximum are still identifiable, permitting experimental observation. The calculations were performed in the one particle Hartree-Fock (HF) approximation and with account of many-electron correlations via the random phase approximation with exchange (RPAE). The GOS’s are studied for values of $q^2$ up to $50\mathrm{a}.\mathrm{u}.$ Our calculated RPAE GOS’s for the $\mathrm{Ar}3p\to 4s$ dipole transition compare very well with the recently measured data of Zhu et al. [Phys. Rev. A 73, 042703 (2006)].

Detection of single molecules with present experimental techniques requires the enhancement of spectral intensities by several orders of magnitude. This is an ambitious goal for analytical techniques of sensors for chemical and biological agents. We have shown using first-principles techniques that the bonding of a benzene molecule to a silicon nanocluster yields at least one of the frontier molecular orbitals delocalized into the entire complex, strongly affecting its vibrational spectrum. This provides a means to detect the presence of the small molecule in the large complex, which can have several orders of magnitude larger cross section to standard chemical radiation techniques than the much smaller single molecule. Thus the transfer of the characteristics of the molecule into the cluster, through molecular orbital mixing, increases tremendously the possibilities of detection of the single molecule as the larger cluster is much more easily detectable and can have its size and gap tailored for the measurement.

In pump-probe experiments with a femtosecond near-uv laser, a wave packet is launched onto an excited electronic state of ${\mathrm{I}}_2^{+}$ through strong field ionization, and starts vibrating in the corresponding potential well. The vibrational wave packet is further ionized by a probe pulse to an excited electronic state of ${\mathrm{I}}_2^{2+}$ that dissociates into ${\mathrm{I}}^{2+}+{\mathrm{I}}^{0+}$. Projecting the vibrational wave packet onto the dissociation channel reveals characteristics of both potential curves via the periodic variations in kinetic energy distribution of the dissociating fragments. The experimental results are consistent with theoretical simulations that are sensitive to the shape of the potential curves. We find that the intermediate state in ${\mathrm{I}}_2^{+}$ is $A^2{\Pi }_{u,3∕2}$ and ionization to this state has an anomalous angular dependence. Moreover, we extract the ${\mathrm{I}}^{2+}+{\mathrm{I}}^{0+}$ potential energy curve and find evidence for a truly bound potential well, in contrast to the metastable ground state potential energy curves.

Approximate momentum distributions of Rydberg electrons in static electric fields have been obtained using an improved impulsive momentum retrieval (IMR) technique. An imaging detector enables the measurement of half-cycle pulse (HCP) ionization probability across the spatial profile of a focused half-cycle pulse beam. By modulating the HCP amplitude we directly measure the derivative of the ionization vs HCP impulse curve, enabling the recovery of momentum distributions with better resolution than previously demonstrated with IMR. For example, for Stark states with small dipole moments, we observe predicted fine-structure in the projection of the momentum distribution along the Stark field axis. We use a semiclassical model to simulate the effect that the nonzero HCP duration has on our measurements. Good agreement between simulated and measured momentum distributions is obtained.

We present a theoretical study of a type of Josephson device, the so-called “optical Josephson junction” [Y. Shin et al. Phys. Rev. Lett. 95, 170402 (2005).]. In this device, two condensates are optically coupled through a waveguide by a pair of Bragg beams. This optical Josephson junction differs from the usual Josephson junction where condensates are weakly coupled by tunneling through a barrier. We discuss the use of this optical Josephson junction, for making precision measurements.

We describe an experiment based on atoms trapped close to a macroscopic surface, to study the interactions between the atoms and the surface at very small separations $(0.6–10\mu \mathrm{m})$. In this range the dominant potential is the QED interaction (Casimir-Polder and van der Waals) between the surface and the atom. Additionally, several theoretical models suggest the possibility of Yukawa-type potentials with sub-millimeter range, arising from new physics related to gravity. The proposed setup is very similar to neutral atom optical lattice clocks, but with the atoms trapped in lattice sites close to the reflecting mirror. A sequence of pulses of the probe laser at different frequencies is then used to create an interferometer with a coherent superposition between atomic states at different distances from the mirror (in different lattice sites). Assuming atom interferometry state-of-the-art measurement of the phase difference and a duration of the superposition of about $0.1\mathrm{s}$, we expect to be able to measure the potential difference between separated states with an uncertainty of $\approx {10}^{-4}\mathrm{Hz}$. An analysis of systematic effects for different atoms and surfaces indicates no fundamentally limiting effect at the same level of uncertainty, but does influence the choice of atom and surface material. Based on those estimates, we expect that such an experiment would improve the best existing measurements of the atom-wall QED interaction by ⩾ 2 orders of magnitude, while gaining up to four orders of magnitude on the best present limits on new interactions in the range between $100\mathrm{nm}$ and $100\mu \mathrm{m}$.

The transitional behavior of the two-photon Kerr effect between the self-Kerr and cross-Kerr cases is investigated. To this end, we have developed a semiclassical method for evaluating the two-photon Kerr effect that is applicable to any two-photon input state. It is revealed that the maximum Kerr effect is obtained when the second photon is input with a delay time that corresponds to the absorption time of the first photon by the optical material.

The energy-loss straggling of protons channeled in gold crystals in the ⟨100⟩ direction has been experimentally and theoretically studied at the low-velocity limit. The experimental results show a threshold effect in the straggling values, which is similar to that observed earlier in the energy loss. This effect is described by a theoretical model which explains the velocity dependence by considering the individual features of the contributions of free electrons in the conduction band and the nearly free $\left(5d^{10}\right)$ electrons which generate a threshold effect at low energies due to their binding energy. The model predicts a deviation of proportionality with ion velocity of the energy-loss spread and yields a good description of the obtained experimental data.

We considered a scenario recently suggested by Augusiak and Horodecki [Phys. Rev. A 73, 012318 (2006)], which used Smolin bound entangled states to share secrets. It is shown that a dishonest party can fully access secret information by an intercept-resend strategy without being detected.

A method for producing an upper bound for all multipartite purification protocols is devised, based on knowing the optimal protocol for purifying bipartite states. When applied to a range of noise models, both local and correlated, the optimality of certain protocols can be demonstrated for a variety of graph and valence bond states.

Using the Hellmann-Feynman theorem for quasistationary quasienergy states of a quantum system in an intense laser field, we present the high-order harmonic generation (HHG) amplitude in terms of the complex quasienergy of a bound electron, thereby avoiding the necessity for an explicit form for the electron wave function in HHG calculations. This formulation for the HHG amplitude confirms use of the dual dipole moment (instead of the dipole moment expectation value) in wave-function-based HHG calculations in order to properly account for both the ionization and the Stark shift of the initial bound state in a strong laser field.

A formulation for the high-order harmonic generation (HHG) amplitude [M. V. Frolov et al., Phys. Rev. A 75, 063407 (2007), preceding paper] is employed to provide analytical results for HHG rates within our recently developed time-dependent effective range (TDER) theory (for time-dependent problems involving weakly bound electron systems). Exact and approximate (including quasiclassical) TDER HHG rates are employed to analyze the accuracy of common approximate methods for HHG calculations. For various specific negative ions with $s$ and $p$ outer electrons, numerical results for HHG spectra are presented over a wide interval of laser frequencies (extending from the tunneling to the multiphoton regimes). The role of initial bound state symmetry effects on the HHG spectra is also analyzed. Finally, Coulomb corrections to TDER results for HHG rates are introduced and discussed.