We explore the role of laser induced antialignment in enhancing molecular orientation. A field-free enhanced orientation via antialignment scheme is presented, which combines a linearly polarized femtosecond laser pulse with a half-cycle pulse. The laser pulse induces transient antialignment in the plane orthogonal to the field polarization, while the half-cycle pulse leads to the orientation. We identify two qualitatively different enhancement mechanisms depending on the pulse order, and optimize their effects using classical and quantum models both at zero and nonzero temperature.

We propose an on-chip optical waveguide for Bose-Einstein condensates based on the evanescent light fields created by surface states of a photonic crystal. It is shown that the modal properties of these surface states can be tailored to confine the condensate at distances from the chip surface significantly longer that those that can be reached by using conventional index-contrast guidance. We numerically demonstrate that by index-guiding the surface states through two parallel waveguides, the atomic cloud can be confined in a two-dimensional trap at about $1\mu \mathrm{m}$ above the structure using a power of $0.1\mathrm{mW}$.

We present a fully time dependent, adiabatic solution, and steady-state analysis for the ultraslow propagation of the nondegenerate four-wave mixing (NDFWM) signal and the weak probe beam in a closely-cycled double-ladder system. Under appropriate (especially power balance) conditions, the two-mode probe and phase-matched NDFWM pulses, after a characteristic propagation length, evolve into a pair of amplitude and group velocity matched pulses. Double transparency for the probe and NDFWM beams can be achieved owing to an efficient one- and three-photon destructive interference involving the NDFWM beam and its back reaction to the probe beam. The forward and backward configurations are investigated in this double-ladder system, and their efficiencies are calculated and compared.

The Stokes and anti-Stokes components of the spectrum of resonance fluorescence of a single trapped atom, which originate from the mechanical coupling between the scattered photons and the quantized motion of the atomic center of mass, exhibit quantum correlations which are of two-mode-squeezing type. We study and demonstrate the build-up of such correlations in a specific setup, which is experimentally accessible, and where the atom acts as efficient and continuous source of EPR-entangled, two-mode squeezed light.

In this paper we derive an extra class of non-Markovian master equations where the system state is written as a sum of auxiliary matrixes whose evolution involve Lindblad contributions with local coupling between all of them, resembling the structure of a classical rate equation. The system dynamics may develop strong nonlocal effects such as the dependence of the stationary properties with the system initialization. These equations are derived from alternative microscopic interactions, such as complex environments described in a generalized Born-Markov approximation and tripartite system-environment interactions, where extra unobserved degrees of freedom mediates the entanglement between the system and a Markovian reservoir. Conditions that guarantee the completely positive condition of the solution map are found. Quantum stochastic processes that recover the system dynamics in average are formulated. We exemplify our results by analyzing the dynamical action of nontrivial structured dephasing and depolarizing reservoirs over a single qubit.

The anisotropy effect in charge transfer induced by collision of ${\mathrm{C}}^{q+}$ ions $(q=2,4)$ on uracil has been studied theoretically using ab initio molecular methods followed by a semiclassical dynamical treatment. The process depends strongly on the orientation of the projectile in the ${\mathrm{C}}^{4+}+\text{uracil}$ reaction and is favored around the perpendicular direction. The effect is weaker in the ${\mathrm{C}}^{2+}+\text{uracil}$ collision system where cross-sections remain low.

${\mathrm{H}}_2\text{−}\mathrm{Ar}$ scattering processes in the presence of a weak photon field are considered. Calculations are based on an accurate ab initio interaction-induced electric dipole surface and an anisotropic intermolecular potential energy surface. The close-coupled scheme of integrating the Schrödinger equation is employed to calculate the rototranslational absorption spectrum of ${\mathrm{H}}_2\text{−}\mathrm{Ar}$ pairs in the far-infrared region of the electromagnetic spectrum. The results are compared with previous work where the weak anisotropy of the intermolecular interaction potential was suppressed. Under the conditions considered, accounting for the anisotropy modifies the rototranslational absorption spectrum discernibly, but only at some frequency bands where the corrections are negative, typically below 10%.

We present an explicit analytical analysis of the ground state of vortex lattice structures, based on a minimization of the Gross-Pitaevskii energy functional in a fast rotating Bose-Einstein condensate between the Thomas-Fermi and the lowest-Landau-level regimes. By a Bogoliubov-like transformation we find that the coarse-grained average of the atomic density varies as ${[1-(x^2+y^2)∕R_{\perp }^2]}^{3∕2}$ in the $xy$ plane, and that the vortices form an almost regular triangular lattice. Our variational calculation is quantitatively and qualitatively consistent with recent analytical, numerical, and experimental data.

We compare the results of two strong field coherent control experiments: one which optimizes multi-photon population transfer in atomic sodium (from the 3s to the 4s state, measured by spontaneous emission from the 3p-3s transition) with one that optimizes stimulated emission on the 3p-3s transition in an ensemble of sodium atoms. Both experiments make use of intense, shaped ultrafast laser pulses discovered by a Genetic Algorithm inside a learning control loop. Optimization leads to improvements in the spontaneous and stimulated emission yields of about $4$ and ${10}^4$, respectively, over an unshaped pulse. We interpret these results by modeling both the single atom dynamics as well as the stimulated emission buildup through numerical integration of Schrödinger’s and Maxwell’s equations. Our interpretation leads to the conclusion that modest yields for controlling single quantum systems can lead to dramatic effects whenever an ensemble of such systems acts collectively following controlled impulsive excitation.

Synthesizing an effective identity evolution in a target system subjected to unwanted unitary or nonunitary dynamics is a fundamental task for both quantum control and quantum information processing applications. Here, we investigate how single-bit, discrete-time feedback capabilities may be exploited to enact or to enhance quantum procedures for effectively suppressing unwanted dynamics in a finite-dimensional open quantum system. An explicit characterization of the joint unitary propagators correctable by a single-bit feedback strategy for arbitrary evolution time is obtained. For a two-dimensional target system, we show how by appropriately combining quantum feedback with dynamical decoupling methods, concatenated feedback-decoupling schemes may be built, which can operate under relaxed control assumptions and can outperform purely closed-loop and open-loop protocols.

Recent calculations of the response of a single atom subjected to a two-color laser pulse with the higher frequency being resonant with an excitation of the target atom revealed a significant enhancement of photoionization as well as high-order harmonic generation [K. Ishikawa, Phy. Rev. Lett. 91, 043002 (2003)]. We investigate the problem in the framework a fully quantum-mechanical pulse propagation algorithm and perform calculations for rare gases in the single-active-electron approximation. The enhancement of harmonic output compared to the corresponding one-color pulse remains intact for short propagation lengths, promising the feasibility of experimental realization. We also study weak second colors resonant via a two-photon transition where significant enhancements in harmonic yields can be observed as well.

A theoretical investigation for implementing a scheme of forced evaporative cooling in radio-frequency (rf) adiabatic potentials is presented. Supposing the atoms to be trapped in a combination of a dc magnetic field and a rf field at frequency ${\omega }_1$, the cooling procedure is facilitated using a second rf source at frequency ${\omega }_2$. This second rf field produces a controlled coupling between the spin states dressed by ${\omega }_1$. The evaporation is then possible in a pulsed or continuous mode. In the pulsed case, atoms with a given energy are transferred into untrapped dressed states by abruptly switching off the ${\omega }_2$ coupling. In the continuous case, it is possible for energetic atoms to adiabatically follow the doubly dressed states and escape out of the trap. Our results also show that when ${\omega }_1$ and ${\omega }_2$ are separated by at least the Rabi frequency associated with ${\omega }_1$, additional evaporation zones appear which can make this process more efficient.

We present the Bogoliubov theory for the Bose-Einstein condensation of Feshbach molecules in a balanced Fermi mixture. Because the Bogoliubov theory includes (Gaussian) fluctuations, we can in this manner accurately incorporate both the two-body and many-body aspects of the BEC-BCS crossover that occurs near a Feshbach resonance. We apply the theory in particular to the very broad Feshbach resonance in atomic ${}^6\mathrm{Li}$ at a magnetic field of $B_0=834\mathrm{G}$ and find good agreement with experiments in that case. The BEC-BCS crossover for more narrow Feshbach resonances is also discussed.

The dynamics and quantum statistical properties of the three-level laser ladder model are investigated. A comparison between this model to the other models, V and $\Lambda$ ones, is presented. We found that the ladder-pumped lasers have both the highest intensity and the lowest linewidth, whereas the maximum noise reduction is more pronounced in the V and $\Lambda$ models. We also show that at high pumping rate the ladder scheme behaves as the two-level one.

We present detailed experimental and numerical studies of random lasing in weakly scattering systems. The interference of scattered light, which is weak in the passive systems, is greatly enhanced in the presence of high gain, providing coherent and resonant feedback for lasing. The lasing modes are confined in the vicinity of the pumped volume due to absorption of emitted light outside it. In the ballistic regime where the size of the gain volume is less than the scattering mean free path, lasing oscillation occurs along the direction in which the gain volume is most extended, producing directional laser output. The feedback for lasing originates mainly from backscattering of particles near the boundaries of the pumped region. It results in nearly constant frequency spacing of lasing modes, which scales inversely with the maximum dimension of the gain volume.

We obtain the ground-state energy level and associated geometric phase in the Dicke model analytically by means of the Holstein-Primakoff transformation and the boson expansion approach in the thermodynamic limit. The nonadiabatic geometric phase induced by the photon field is derived with the time-dependent unitary transformation. It is shown that the quantum phase transition characterized by the nonanalyticity of the geometric phase is remarkably of the first order. We also investigate the scaling behavior of the geometric phase at the critical point, which can be measured in a practical experiment to detect the quantum phase transition.

We consider interaction-free measurement (IFM) with imperfect interaction. In the IFM proposed by Kwiat et al., we assume that the interaction between an absorbing object and a probe photon is imperfect, so that the photon is absorbed with probability $1-\eta$ $(0⩽\eta ⩽1)$ and it passes by the object without being absorbed with probability $\eta$ when it approaches close to the object. We derive the success probability $P$ that we can find the object without the photon absorbed under the imperfect interaction as a power series in $1∕N$ and show the following result: Even if the interaction between the object and photon is imperfect, we can let the success probability $P$ of the IFM get close to unity arbitrarily by making the reflectivity of the beam splitter larger and increasing the number of the beam splitters. Moreover, we obtain an approximating equation of $P$ for large $N$ from the derived power series in $1∕N$.

What can we learn about entanglement between individual particles in macroscopic samples by observing only the collective properties of the ensembles? Using only a few experimentally feasible collective properties, we establish an entanglement measure between two samples of spin-1/2 particles (as representatives of two-dimensional quantum systems). This is a tight lower bound for the average entanglement between all pairs of spins in general and is equal to the average entanglement for a certain class of systems. We compute the entanglement measures for explicit examples and show how to generalize the method to more than two samples and multipartite entanglement.

Exact and closed-form expressions of the particle density and the momentum distribution are derived for a coherent state of a noninteracting Fermi gas, while such a state can be obtained from the ground state in a $d$-dimensional isotropic harmonic trap by modulating the trap frequency and shifting the trap center. The profile of the momentum distribution turns out to be identical in shape with that of the particle density, however, as a manifestation of the Heisenberg uncertainty principle, the dispersion of the distribution increases (decreases) when that of the particle density is decreased (increased). The expressions are also applicable for a sudden and total opening of the trap, and it is shown that, after the opening, the gas has a stationary momentum distribution whose dispersion could be arbitrarily large or small.