In this paper we describe experimentally and theoretically the increase in the electromagnetically induced absorption (EIA) resonance under the influence of an additional optical field. EIA was observed on the cycling transition of the rubidium ${\text{D}}_2$ line in the Hanle configuration in the presence of the second-laser field acting on an open transition coupled with a cycling one via a common ground state. We present results obtained for different polarization orientations and propagation directions of the two laser beams. A systematic theoretical analysis based on the optical Bloch equations explains most of the experimental data. A lot of attention is also paid to a qualitative understanding of the EIA—its resonance shape, amplitude, and the response to the additional optical fields.

In the investigation of the quantum to classical transition, the Leggett-Garg inequality represents what Bell’s inequality is for the study of entanglement and nonlocality: a definite quantitative test showing the inadequacy of familiar lines of thought in interpreting quantum phenomena. Here we discuss the generalization of the Leggett-Garg inequality to an arbitrary number of measurements in the perspective of its use as a tool for characterizing nondestructive measurement devices.

The decoherence induced on a single qubit by its interaction with the environment is studied. The environment is modeled as a scalar two-level boson system that can go through either first-order or continuous-excited-state quantum phase transitions, depending on the values of the control parameters. A mean-field method based on the Tamm-Damkoff approximation is worked out in order to understand the observed behavior of the decoherence. Only the continuous-excited-state phase transition produces a noticeable effect in the decoherence of the qubit. This is maximal when the system-environment coupling brings the environment to the critical point for the continuous phase transition. In this situation, the decoherence factor (or the fidelity) goes to zero with a finite-size scaling power law.

We propose an experiment utilizing an array of cooled microcantilevers coupled to a sample of ultracold atoms trapped near a microfabricated surface. The cantilevers allow individual lattice site addressing for atomic state control and readout, and potentially may be useful in optical lattice quantum computation schemes. Assuming resonators can be cooled to their vibrational ground state, the implementation of a two-qubit controlled-NOT gate with atomic internal states and the motional states of the resonator is described. We also consider a protocol for entangling two or more cantilevers on the atom chip with different resonance frequencies, using the trapped atoms as an intermediary. Although similar experiments could be carried out with magnetic microchip traps, the optical confinement scheme we consider may exhibit reduced near-field magnetic noise and decoherence. Prospects for using this system for tests of quantum mechanics at macroscopic scales or quantum information processing are discussed.

Recent experimental results demonstrated the generation of a macroscopic quantum superposition (MQS), involving a number of photons in excess of $5\times {10}^4$, which showed a high resilience to losses. In order to perform a complete analysis on the effects of decoherence on these multiphoton fields, obtained through the quantum injected optical parametric amplifier, we investigate theoretically the evolution of the Wigner functions associated to these states in lossy conditions. Recognizing the presence of negative regions in the $W$ representation as an evidence of nonclassicality, we focus our analysis on this feature. A close comparison with the MQS based on coherent $|\alpha ⟩$ states allows us to identify differences and analogies.

Resonance states of unnatural parity $P^e$ and $D^o$ in the positronic atoms $e^{+}\text{H}$ and $e^{+}{\text{He}}^{+}$ are calculated and discussed in detail in terms of the coupled-channel scheme in the hyperspherical coordinate system. New resonances emerge in the analysis of the eigenphase sum. The time-delay matrix eigenvalues prove to be useful in unraveling overlapping resonances. Many resonances are identified as belonging to some infinite series of Feshbach resonances supported by an adiabatic hyperspherical potential. Some Feshbach resonances independent of any infinite series are also identified to be supported by an adiabatic potential. The positions and widths of higher-lying members of the infinite series are known to be expressible as geometric progressions. Their common ratios are obtained theoretically, including those for the series failing to appear in the present calculation. Several resonances unassociated with a minimum in any adiabatic potential are found in the region of avoided crossing. The diabatic picture is invoked for understanding the resonance mechanism, the resonance energy, and the trend in the widths for all those exceptional cases. The lowest-order $2\gamma$ and $3\gamma$ pair annihilation in these unnatural-parity states is shown to be forbidden.

Polarized ${}^3\text{H}\text{e}$ produced by spin-exchange optical pumping (SEOP) has potential as a neutron spin filter for polarization and polarization analysis in many neutron-scattering and neutron particle physics applications. The advantage of the SEOP method is its suitability for providing continuous stable polarization over the course of long experiments. However, we have discovered that exposure to high neutron flux leads to additional strong relaxation mechanisms in the optically polarized alkali-metal vapor used to polarize the ${}^3\text{H}\text{e}$. At a neutron flux density of $4.7\times {10}^9{\text{cm}}^{-2}{\text{s}}^{-1}$, the alkali-metal relaxation rate increased from 100 to $1000{\text{s}}^{-1}$ leading to reduced alkali-metal polarization. Other effects such as time dependence and gas composition dependence were explored to help understand the processes. In this paper we discuss our observations and present possible solutions for practical use of SEOP as a neutron spin filter for high-flux density applications.

We consider the phase coherent transport of a quasi-one-dimensional beam of Bose-Einstein condensed particles through a disordered potential of length $L$. Among the possible different types of flow we identified [T. Paul, P. Schlagheck, P. Leboeuf, and N. Pavloff, Phys. Rev. Lett. 98, 210602 (2007)], we focus here on the supersonic stationary regime where Anderson localization exists. We generalize the diffusion formalism of Dorokhov-Mello-Pereyra-Kumar to include interaction effects. It is shown that interactions modify the localization length and also introduce a length scale $L^{\ast }$ for the disordered region, above which most of the realizations of the random potential lead to time-dependent flows. A Fokker-Planck equation for the probability density of the transmission coefficient that takes this effect into account is introduced and solved. The theoretical predictions are verified numerically for different types of disordered potentials. Experimental scenarios for observing our predictions are discussed.

We study the dynamics of a two-species Bose-Einstein condensate in a double well. Such a system is characterized by the intraspecies and interspecies $s$-wave scattering as well as the Josephson tunneling between the two wells and the population transfer between the two species. We investigate the dynamics for some interesting regimes and present numerical results to support our conclusions. In the case of vanishing intraspecies scattering lengths and a weak interspecies scattering length, we find collapses and revivals in the population dynamics. A possible experimental implementation of our proposal is briefly discussed.

We present a theoretical analysis of dilute gas Bose-Einstein condensates with dipolar atomic interactions under rotation in elliptical traps. Working in the Thomas-Fermi limit, we employ the classical hydrodynamic equations to first derive the rotating condensate solutions and then consider their response to perturbations. We thereby map out the regimes of stability and instability for rotating dipolar Bose-Einstein condensates and, in the latter case, discuss the possibility of vortex lattice formation. We employ our results to propose several routes to induce vortex lattice formation in a dipolar condensate.

Within the framework of quantum field theory, we show that the photon system in a blackbody whose interior is filled by a Kerr nonlinear crystal is in a condensation state. In the condensation state, bare photons with opposite wave vectors and helicities are bound into pairs and unpaired bare photons are transformed into a new kind of quasiparticle, the nonpolariton. The photon-pair system is a condensate and the nonpolariton system is a boson gas. At zero temperature the condensate possesses a largest persistent energy density. The persistent energy density of the condensate is a monotonically decreasing function of temperature and Kerr nonlinear coefficient. The $Q$ function of a Kerr nonlinear blackbody at any temperature is derived analytically. In the transition from the normal to the condensation state, the phase symmetry of the photon system is spontaneously broken.

Homodyne detection is a well-known technique in measuring phase-dependent properties of optical radiation. We apply balanced homodyne detection to ghost imaging with thermal light, and analyze how to obtain the amplitude and phase information about an object by optimizing the optical setups and controlling the local oscillators. Compared with ghost imaging using direct detection, the homodyne detection scheme is very sensitive to phase changes of optical fields so that it can be used to image a pure phase object. The analytical results are backed up by numerical simulations based on statistical optics.

It is well known that, under the rotating-wave approximation, the Rabi Hamiltonian, which describes the interaction between a two-level atom and a single field mode, reduces to the Jaynes-Cummings model through exclusion of its counter-rotating term. In this work, it is found that, since photons in the mode are indistinguishable, there exist two conditions where the counter-rotating term can never be ignored. These conditions, all subject to the requirements that the atom-mode coupling is weak and the atom is initially in its ground state, are obtained in the stationary perturbation theory and confirmed through an analysis of the atom-mode system’s evolution.

Attosecond spectroscopy is currently restricted to photon energies around 100 eV. We show that under these conditions, electron-electron scatterings, as the photoelectrons leave the metal, give rise to a tail of secondary electrons with lower energies and hence a significant background. We develop an analytical model based on an approximate solution to Boltzmann’s transport equation to account for the amount and energy distribution of these secondary electrons. Our theory is in good agreement with the electron spectrum found in a recent attosecond streaking experiment. To suppress the background and gain higher-energy resolution, photon sources of higher energy could be advantageous.

We investigate numerically the appearance and evolution of entanglement in spin systems prepared initially in a pure state. We consider the dipolar coupling spin systems of different molecular structures: benzene ${\text{C}}_6{\text{H}}_6$, cyclopentane ${\text{C}}_5{\text{H}}_{10}$, sodium butyrate ${\text{CH}}_3{\left({\text{CH}}_2\right)}_2{\text{CO}}_2\text{Na}$, and calcium hydroxyapatite ${\text{Ca}}_5\left(\text{OH}\right){\left({\text{PO}}_4\right)}_3$. Numerical simulations show that the close relationship exists between the intensity of second order (2Q) coherences and concurrences of nearest spins in a cyclopentane molecule.

We present a combined theoretical and experimental study of molecular field effects on molecular core levels. Polarization-dependent resonant inelastic x-ray scattering is observed experimentally after resonant $K$-shell excitation of ${\text{CF}}_3\text{Cl}$ and HCl. We explain the linear dichroism observed in spin-orbit level intensities as due to molecular field effects, including singlet-triplet exchange, and interpret this behavior in terms of population differences in the $2p_{x,y,z}$ inner-shell orbitals. We investigate theoretically the different factors that can affect the electronic populations and the dynamical $R$ dependence of the spin-orbit ratio. Finally, the results obtained are used to interpret the $L$-shell absorption spectra of the two molecules.

Relativistic configuration-interaction calculations have been performed for anion binding energies of the entire actinide row using a corelike treatment of the $5f$ subshell: the same occupancy and universal $jls$ restrictions are applied to the subshell throughout the basis of each neutral and anion calculation. We predict bound $7p$ attachments to all actinide ground state configurations except Fm, Md, and No. Additional anion bound states are formed by $7p$ attachment to excited thresholds in Pa and Lr as well as $7s$ attachments relative to excited open-$s$ thresholds in Th, Pa, U, and Np. Of the 41 bound actinide anion states presented, over half are characterized here for the first time. The most unusual case is ${\text{Pa}}^{-}$, where these ab initio calculations predict five bound anion states arising from four different configurations.

We present an explicit analytic calculation of the energy-level shift of an atom in front of a nondispersive and nondissipative dielectric slab. We work with the fully quantized electromagnetic field, taking retardation into account. We give the shift as a two-dimensional integral and use asymptotic analysis to find expressions for it in various retarded and nonretarded limiting cases. The results can be used to estimate the energy shift of an atom close to layered microstructures.

A scheme for coherent population transfer via four quantum states in atomic or molecular systems of high-energy transitions by induced multiphoton adiabatic passage (IMAP) is proposed. The nonperturbative multiphoton resonant theory of interaction of such quantum systems with the two strong laser fields of different frequencies is developed and the possibility of coherent control of population in high-energy quantum structures by IMAP is shown.

Spectral properties of the Bose-Hubbard model and a recently proposed coupled-cavity model are studied by means of quantum Monte Carlo simulations in one dimension. Both models exhibit a quantum phase transition from a Mott insulator to a superfluid phase. The dynamic structure factor $S\left(k,\omega \right)$ and the single-particle spectrum $A\left(k,\omega \right)$ are calculated, focusing on the parameter region around the phase transition from the Mott insulator with density one to the superfluid phase, where correlations are important. The strongly interacting nature of the superfluid phase manifests itself in terms of additional gapped modes in the spectra. Comparison is made to recent analytical work on the Bose-Hubbard model. Despite some subtle differences due to the polaritonic particles in the cavity model, the gross features are found to be very similar to the Bose-Hubbard case. For the polariton model, emergent particle-hole symmetry near the Mott lobe tip is demonstrated and temperature and detuning effects are analyzed. A scaling analysis for the generic transition suggests mean-field exponents, in accordance with field-theory results.