A technique for conditionally creating single-mode or multimode photon-number states is analyzed using Bayesian theory. We consider the heralded $N$-photon states created from the photons produced by an unseeded optical parametric amplifier when the heralding detector is the time-multiplexed photon-number-resolving detector recently demonstrated by Fitch et al. [Phys. Rev. A 68, 043814 (2003)] and simultaneously by Achilles et al. [Opt. Lett. 28, 2387 (2003)]. We find that even with significant loss in the heralding detector, fields with sub-Poissonian photon-number distributions can be created. We also show that heralded multimode fields created using this technique are more robust against detector loss than are single-mode fields.

The nonlinear dynamics of a classical photon pulse in a cavity-QED system is investigated theoretically. It is shown that this system can work as a single-photon filter, which drastically suppresses the multiple-photon probability of the output. The output photon statistics is sensitive to the input pulse length. A suitable choice of pulse length produces a photon pulse with the single-photon probability of 0.32, while the multiple-photon probability is suppressed to 0.01.

Doppler cooling of calcium ions has been experimentally demonstrated using the $S_{1∕2}\to D_{5∕2}$ dipole-forbidden transition. Scattering forces and fluorescence levels a factor of 5 smaller than for the usual Doppler cooling on the dipole-allowed $S_{1∕2}\to P_{1∕2}$ transition have been achieved. Since the light scattered from the ions can be monitored at (violet) wavelengths that are very different from the excitation wavelengths, single ions can be detected with an essentially zero background level. This, as well as other features of the cooling scheme, can be extremely valuable for ion-trap-based quantum-information processing.

We show that temporal two-photon interference effects involving the signal and idler photons created by parametric down-conversion can be fully characterized in terms of the variations of two length parameters—called the biphoton path-length difference and the biphoton path-asymmetry-length difference—which we construct using the six different length parameters that a general two-photon interference experiment involves. We perform an experiment in which the effects of the variations of these two parameters can be independently controlled and studied. In our experimental setup, which does not involve mixing of signal and idler photons at a beam splitter, we further report observations of Hong-Ou-Mandel- (HOM-)like effects both in coincidence and in one-photon count rates. As an important consequence, we argue that the HOM and the HOM-like effects are best described as observations of how two-photon coherence changes as a function of the biphoton path-asymmetry-length difference.

Motivated by a recent proposal by Sushkov and co-workers [Phys. Rev. A 72, 034501 (2005); 73, 022107 (2006)] to search for a $P,T$-violating Schiff moment of the ${}^{207}\mathrm{Pb}$ nucleus in a ferroelectric solid, we have carried out a high-field nuclear magnetic resonance study of the longitudinal and transverse spin relaxation of the lead nuclei from room temperature down to $10\mathrm{K}$ for powder samples of lead titanate (PT), lead zirconium titanate, and a PT monocrystal. For all powder samples and independently of temperature, transverse relaxation times were found to be $T_2\approx 1.5\mathrm{ms}$, while the longitudinal relaxation times exhibited a temperature dependence, with $T_1$ of over an hour at the lowest temperatures, decreasing to $T_1\approx 7\mathrm{s}$ at room temperature. At high temperatures, the observed behavior is consistent with a two-phonon Raman process, while in the low-temperature limit, the relaxation appears to be dominated by a single-phonon (direct) process involving magnetic impurities. We discuss the implications of the results for the Schiff-moment search.

Cooling of molecules via free-space dissipative scattering of photons is thought not to be practicable due to the inherently large number of Raman loss channels available to molecules and the prohibitive expense of building multiple-repumping laser systems. The use of an optical cavity to enhance coherent Rayleigh scattering into a decaying cavity mode has been suggested as a potential method to mitigate Raman loss, thereby enabling the laser cooling of molecules to ultracold temperatures. We discuss the possibility of cavity-assisted laser cooling of particles without closed transitions, identify conditions necessary to achieve efficient cooling, and suggest solutions given experimental constraints. Specifically, it is shown that cooperativities much greater than unity are required for cooling without loss, and that this could be achieved via the superradiant scattering associated with intracavity self-localization of the molecules. Particular emphasis is given to the polar hydroxyl radical (OH), cold samples of which are readily obtained from Stark deceleration.

We present experimental evidence of coherent wave packet motion in dipicolinic acid $\left({\mathrm{C}}_7{\mathrm{H}}_5\mathrm{N}{\mathrm{O}}_4\right)$ which is an important marker molecule for bacterial spores. Resonant impulsive excitation is achieved by applying a uv pump pulse ($267\mathrm{nm}$, $16\mathrm{fs}$) which has a duration that is shorter than the vibrational period of the molecules. The resulting dynamics is then probed with a weaker pulse of the same width and frequency. Evidence of the important “fingerprint” region for this molecule (between $1000{\mathrm{cm}}^{-1}$ and $1500{\mathrm{cm}}^{-1}$) is found in the transient absorption of the probe. We present simulations of the pump-probe experiment, based on the Liouville equation for the density matrix, and predict the optimal pulse width and detuning.

The temporal dynamics of double ionization of ${\text{H}}_2$ has been investigated both experimentally and theoretically with few-cycle laser pulses. The main observables are the proton spectra associated with the ${\text{H}}^{+}+{\text{H}}^{+}$ fragmentation channel. The model is based on the time-dependent Schrödinger equation and treats the electronic and nuclear coordinates on the same level. Therefore it allows the ultrafast nuclear dynamics to be followed as a function of the laser pulse duration, carrier-envelope phase offset, and peak intensity. We mainly report results in the sequential double-ionization regime above $2\times {10}^{14}\text{W}{\text{cm}}^{-2}$. The proton spectra are shifted to higher energies as the pulse duration is reduced from 40 down to 10 fs. The good agreement between the model predictions and the experimental data at 10 fs permits a theoretical study with pulse durations down to a few femtoseconds. We demonstrate the very fast nuclear dynamics of the ${\text{H}}_2{}_{+}$ ion for a pulse duration as short as 1 fs between the two ionization events, giving $\text{H}_2{}^{+}$ from ${\text{H}}_2$ and ${\text{H}}^{+}+{\text{H}}^{+}$ from $\text{H}_2{}^{+}$. The carrier-envelope phase offset plays a significant role only for pulse durations shorter than 4 fs. At 10 fs, the laser intensity dependence of the proton spectra is fairly well reproduced by the model.

We show an all-optical method of making ${}^7\text{L}\text{i}$ condensates using the tunability of the scattering length in the proximity of a Feshbach resonance. We report the observation of two Feshbach resonances on the $|F=1,m_F=0⟩$ state. A narrow (broad) resonance of 7 G (34 G) width is detected at $831\pm 4\text{G}$ $\left({884}_{-13}^{+4}\text{G}\right)$. The position of the scattering length zero crossing between the resonances is found at $836\pm 4\text{G}$. The broad resonance is shown to be favorable for runaway evaporation which we perform in a crossed-beam optical-dipole trap. Starting directly from the phase-space density of a magneto-optical trap we observe a Bose-Einstein condensation threshold in less than 3 s of forced evaporation.

We study the formation of vortices, vortex necklaces, and vortex ring structures as a result of the interference of higher-dimensional Bose-Einstein condensates (BECs). This study is motivated by earlier theoretical results pertaining to the formation of dark solitons by interfering quasi-one-dimensional BECs, as well as recent experiments demonstrating the formation of vortices by interfering higher-dimensional BECs. Here, we demonstrate the genericness of the relevant scenario, but also highlight a number of additional possibilities emerging in higher-dimensional settings. A relevant example is, e.g., the formation of a “cage” of vortex rings surrounding the three-dimensional bulk of the condensed atoms. The effects of the relative phases of the different BEC fragments and the role of damping due to coupling with the thermal cloud are also discussed. Our predictions should be immediately tractable in currently existing experimental BEC setups.

We propose a scenario to physically implement a kind of topologically decoherence-protected qubit using superconducting devices coupled to a microwave cavity mode with unconventional geometric operations. It is shown that the two needed interactions for selective devices, which are required for implementing such protected qubits, as well as single-qubit gates, can be achieved by using the external magnetic flux. The easy combination of individual addressing with the many-device setup proposed in the system presents a distinct merit in comparison with the implementation of topologically protected qubits in a trapped-ion system.

A computation with unprecedented precision is presented for quasienergies of a two-level atom in a monochromatic radiation on the basis of a recently obtained exact expression [D.-S. Guo et al., Phys. Rev. A 73, 023419 (2006)]. We start with the proof of an expression theorem. With this theorem the quasienergies for any two-level atom can be expressed in terms of the quasienergies for only those with the original energy spacing (per field photon energy) being an integer (preresonances). Then we carry out a numerical evaluation of the quasienergies at the two-photon preresonance, which involves computing an infinite determinant, up to the 18th power of the coupling strength. The theoretical prediction presents an experimental challenge for high-precision tests of quantum mechanics and could be exploited for precise calibration of high laser intensities.

We present the theory of a pair of atoms in a one-dimensional optical lattice interacting via a narrow Feshbach resonance. Using a two-channel description of the resonance, we derive analytic results for the scattering states inside the continuum band and the discrete bound states outside the band. We identify a Fano resonance profile, and the survival probability of a molecule when swept through the Bloch band of scattering states by varying an applied magnetic field. We discuss how these results may be used to investigate the importance of the structured nature of the continuum in experiments.

We numerically study coherent tunneling oscillations of the particles between two levels in a double-well potential in the presence of anharmonic periodic potentials. Extremely short driving pulses modify the tunneling coefficient to ${\kappa }_{\mathit{eff}}=\kappa \cos A$, where $\kappa$ is the bare tunneling coefficient without the driving field and $A$ is the pulse area of the driving wave form. The modulation amplitude of the ${\kappa }_{\mathit{eff}}$ gradually decreases as the driving wave form becomes broad and is given by ${\kappa }_{\mathit{eff}}=\kappa J_0\left(A\right)$ for the sinusoidal modulation, where $J_0\left(x\right)$ denotes the ordinary Bessel function of order zero. Theoretical derivation of the effective tunneling coefficient ${\kappa }_{\mathit{eff}}=\kappa \cos A$ is also shown for a periodic $\delta$ kick with alternating sign by means of the transfer matrix formula.

In contrast to the seminal entanglement-separability paradigm widely used in quantum information theory, we introduce a quantum-classical dichotomy in order to classify and quantify statistical correlations in bipartite states. This is based on the idea that while in the classical description of nature measurements can be carried out without disturbance, in the quantum description, generic measurements often disturb the system and the disturbance can be exploited to quantify the quantumness of correlations therein. It turns out that certain separable states still possess correlations of a quantum nature and indicates that quantum correlations are more general than entanglement. The results are illustrated in the Werner states and the isotropic states, and are applied to quantify the quantum advantage of the model of quantum computation proposed by Knill and Laflamme [Phys. Rev. Lett. 81, 5672 (1998)].

The relativistic distorted-wave (RDW) procedures developed by Chen [Phys. Rev. A 53, 3227 (1996)] have been extended to include pseudostates. The RDW procedures with pseudostates are applied to the calculation of electron impact excitation (EIE) of $\mathrm{Fe}\mathrm{XVII}$. Pseudostates are shown to have a significant effect on the cross sections for important transitions such as $3C$ $(\lambda =15.015\mathrm{\AA })$ and $3D$ $(\lambda =15.262\mathrm{\AA })$ of $\mathrm{Fe}\mathrm{XVII}$. The convergence of the RDW calculations has been carefully investigated. The present RDW calculations of $\mathrm{Fe}\mathrm{XVII}$ are further compared with our previous close coupling results using the fully relativistic Dirac $R$-matrix (DRM) method, where resonance and more complete channel coupling effects were included in addition to the background direct excitations. The present RDW calculations are in good agreement to the background cross sections from the DRM calculations. This agreement indicates the mutual confirmation of the validity of both the RDW and the DRM calculations of the direct or background EIE cross sections.

We present an attempt at the theoretical interpretation of recent experimental results on multiphoton ionization of neon by photons of energies 38.4 and 42.8 eV at the free electron laser facility [A. A. Sorokin et al., Phys. Rev. A 75, 051402(R) (2007)]. Although, given the photon energies, the intensities employed place the process within the regime of perturbation theory, effects such as the pulsed nature of the field and expansion of the interaction volume need to be and are taken into consideration. The values of some of the cross sections are obtained through scaling and given the semiquantitative nature of the calculations, overall reasonable agreement with the experimental data is found, although some issues pertaining to details remain as questions.

We study the phase diagram of the zero-temperature, one-dimensional Bose-Fermi-Hubbard model for fixed fermion density in the limit of small fermionic hopping. This model can be regarded as an instance of a disordered Bose-Hubbard model with dichotomic values of the stochastic variables. Phase boundaries between compressible, incompressible (Mott-insulating), and partially compressible phases are derived analytically within a generalized strong-coupling expansion and numerically using density matrix renormalization group (DMRG) methods. We show that first-order correlations in the partially compressible phases decay exponentially, indicating a glass-type behavior. Fluctuations within the respective incompressible phases are determined using perturbation theory and are compared to DMRG results.

We develop a computationally tractable method for calculating correlation functions of the finite temperature trapped Bose gas that includes the effects of $s$-wave interactions. Our approach uses a classical field method to model the low energy modes and treats the high energy modes using a Hartree-Fock description. We present results of first and second order correlation functions, in position and momentum space, for an experimentally realistic system in the temperature range of $0.6T_c$ to $1.0T_c$. We also characterize the spatial coherence length of the system. Our theory should be applicable in the critical region where experiments are now able to measure first and second order correlations.