The sequential inner-shell multiple ionization of krypton was investigated at the Linac Coherent Light Source using ion time-of-flight spectroscopy at photon energies above (2 keV) and below (1.5 keV) the $L$ edge with two x-ray pulse lengths (5 and 80 fs, nominally) and various pulse energies. At 2.5 mJ pulse energy, charge states up to Kr${}^{17+}$ were recorded for $M$-shell ionization and charge states up to Kr${}^{21+}$ for $L$-shell ionization. Comparing the experimental charge state distribution to Monte Carlo rate-equation calculations, we find a strong enhancement of higher charge states at 2 keV photon energy as compared to the theoretical predictions. This enhancement is explained with a resonant ionization pathway where multiple excitations into outer valence and Rydberg orbitals are followed by autoionization. These resonant pathways play an important role for the photoionization of ions with charge higher than Kr${}^{12+}$, for which direct one-photon $L$-shell photoionization is energetically impossible at 2 keV photon energy. Only a small pulse-length dependence of the charge state yield is observed at an x-ray pulse energy of 0.4 mJ.

We perform time-dependent calculations of triply differential cross sections (TDCS) of two-photon double-electron ionization of the aligned H${}_2$ molecule. Our TDCS results for equal energy sharing between photoelectrons agree quite well with a recent time-dependent calculation by Guan et al. [X. Guan, K. Bartschat, and B. I. Schneider, Phys. Rev. A 77, 043421 (2008)] who employed a slightly different numerical technique. We supplement these studies by calculating TDCS at an unequal energy sharing and by generating symmetrized ionization amplitudes.

We study the problem of three ultracold fermions in different hyperfine states loaded into a lattice with spatial dimension $D=1,2$. We consider SU(3)-symmetric attractive interactions and also eventually include a three-body constraint, which mimics the effect of three-body losses in the strong-loss regime. We combine exact diagonalization with the Lanczos algorithm, and evaluate both the eigenvalues and the eigenstates of the problem. In $D=1$, we find that the ground state is always a three-body bound state (trion) for arbitrarily small interaction, while in $D=2$, due to the stronger influence of finite-size effects, we are not able to provide conclusive evidence of the existence of a finite threshold for trion formation. Our data are, however, compatible with a threshold value which vanishes logarithmically with the size of the system. Moreover, we are able to identify the presence of a fine structure inside the spectrum, which is associated with off-site trionic states. The characterization of these states shows that only the long-distance behavior of the eigenstate wave functions provides clear-cut signatures about the nature of bound states and that on-site observables are not enough to discriminate between them. The inclusion of a three-body constraint due to losses promotes these off-site trions to the role of lowest-energy states, at least in the strong-coupling regime.

We study a model of bosons in the lowest Landau level in a rotating trap where the confinement potential is a sum of a quadratic and a quartic term. The quartic term improves the stability of the system against centrifugal deconfinement and allows consideration of rotation frequencies beyond the frequency of the quadratic part. The interactions between particles are modeled by a Dirac $\delta$ potential. We derive rigorously conditions for ground states of the system to be strongly correlated in the sense that they are confined to the kernel of the interaction operator, and thus contain the correlations of the Bose-Laughlin state. Rigorous angular momentum estimates and trial state arguments indicate a transition from a pure Laughlin state to a state containing in addition a giant vortex at the center of the trap (Laughlin quasihole). There are also indications of a second transition where the density changes from a flat profile in a disk or an annulus to a radial Gaussian confined to a thin annulus

We study a system of polar fermions in a two-dimensional optical lattice and show that the multiband Fermi-Hubbard model is necessary to discuss its properties. We take into account both onsite and long-range interactions between different bands, as well as occupation-dependent inter- and intraband tunnelings. For strong-enough dipolar interactions we predict the appearances of phases such as multiband crystals, smectic metal, and exotic $p$-wave supersolids.

In several experiments with ultracold trapped atoms, a narrow loss feature has been observed near an atom-dimer resonance, at which there is an Efimov trimer at the atom-dimer threshold. The conventional interpretation of these loss features is that they are produced by the avalanche mechanism, in which the energetic atom and dimer from three-body recombination undergo secondary elastic collisions that produce additional atoms with sufficient energy to escape from the trapping potential. We use Monte Carlo methods to calculate the average number of atoms lost and the average heat generated by recombination events in a Bose-Einstein condensate and in a thermal gas. We improve on previous models by taking into account the energy dependence of the cross sections, the spatial structure of the atom cloud, and the elastic scattering of the atoms. We show that the avalanche mechanism cannot produce a narrow loss feature near the atom-dimer resonance. The number of atoms lost from a recombination event can be more than twice as large as the three that would be obtained in the absence of secondary collisions. However, the resulting loss feature is broad and its peak is at a scattering length that is larger than the atom-dimer resonance and depends on the trap depth.

We study the structure, stability, and dynamics of dark solitary waves in parabolically trapped, collisionally inhomogeneous Bose-Einstein condensates (BECs) with spatially periodic variations of the scattering length. This collisional inhomogeneity yields a nonlinear lattice, which we tune from a small-amplitude, approximately sinusoidal structure to a periodic sequence of densely spaced spikes. We start by investigating time-independent inhomogeneities, and we subsequently examine the dynamical response when one starts with a collisionally homogeneous BEC and then switches on an inhomogeneity either adiabatically or nonadiabatically. Using Bogoliubov-de Gennes linearization as well as direct numerical simulations of the Gross-Pitaevskii equation, we observe dark solitary waves, which can become unstable through oscillatory or exponential instabilities. We find a critical wavelength of the nonlinear lattice that is comparable to the healing length. Near this value, the fundamental eigenmode responsible for the stability of the dark solitary wave changes its direction of movement as a function of the strength of the nonlinearity. When it increases, it collides with other eigenmodes, leading to oscillatory instabilities; when it decreases, it collides with the origin and becomes imaginary, illustrating that the instability mechanism is fundamentally different in wide-well versus narrow-well lattices. When starting from a collisionally homogeneous setup and switching on inhomogeneities, we find that dark solitary waves are preserved generically for aligned lattices. We briefly examine the time scales for the onset of solitary-wave oscillations in this scenario.

We investigate the quantum anomalous Hall effect in a mixture of ultracold neutral bosons and fermions held on a hexagonal optical lattice. In the strong atom-atom interaction limit, composite fermions composed of one fermion with bosons or bosonic holes in the mixture are formed. Such composite fermions have already been generated successfully in experiment [Nat. Phys. 7, 642 (2011)]. Here we predict that this kind of composite fermion may provide a realization of the quantum anomalous Hall effect by tuning the atom-atom interaction or the filling of the bosons in the mixture. We also discuss the corresponding experimental signatures of the quantum anomalous Hall effect in the Bose-Fermi mixture on hexagonal optical lattice.

We study normal modes propagating on top of the stable uniform background in arrays of dipolar Bose-Einstein condensate (BEC) droplets trapped in a deep optical lattice. Both the on-site mean-field dynamics of the droplets and their displacement due to the repulsive dipole-dipole interactions (DDIs) are taken into account. Dispersion relations for two modes, viz., high- and low- frequency counterparts of optical and acoustic phonon modes in condensed matter, are derived analytically and verified by direct simulations, for both cases of the repulsive and attractive contact interactions. The (counterpart of the) optical-phonon branch does not exist without the DDIs. These results are relevant in the connection to emerging experimental techniques enabling real-time imaging of the condensate dynamics and direct experimental measurement of phonon dispersion relations in BECs.

We study the quantum evolution of a cloud of hard-core bosons loaded on a one-dimensional optical lattice after its sudden release from a harmonic trap. Just after the trap has been removed, a linear ramp potential is applied, mimicking the so-called Galileo ramp experiment. The nonequilibrium expansion of the bosonic cloud is elucidated through a hydrodynamical description which is compared to the exact numerical evolution obtained by exact diagonalization on finite lattice sizes. The system is found to exhibit a rich behavior, showing, in particular, Bloch oscillations of a self-trapped condensate and an ejected particle density leading to two diverging entangled condensates. Depending on the initial density of the gas different regimes of Josephson-like oscillations are observed. At low densities, the trapped part of the cloud is in a superfluid phase that oscillates in time as a whole. At higher densities, the trapped condensate is in a mixed superfluid–Mott-insulator phase that show a breathing regime for steep enough potential ramps.

The concept of performing quantum lithography with classical, instead of entangled, light is of great interest for practical implementation. One recent proposal [Hemmer, Muthukrishnan, Scully, and Zubairy, Phys. Rev. Lett. 96, 163603 (2006)] to accomplish this was based on the use of frequency selective multiphoton transitions in a two-level atom. To analyze this possibility we investigate a four-level system theoretically and show that a high-resolution lithography is possible. We quantitatively identify conditions under which high-contrast lithography can be performed without sacrificing high resolution. The approach is a perturbative density matrix method. This model identifies the tradeoff between resolution and contrast as being due to higher-order terms from intermediate-level Rabi oscillations. We propose using positive and negative one-photon detunings to suppress this effect. Allowing higher Rabi frequencies means that shorter interaction time can be used, and this overcomes the degradation in contrast due to population decay. Finally, we generalize our scheme to $\mathit{N}$-photon lithography.

We study the phase diagram of the Dicke model in terms of the excitation energy and the radiation-matter coupling constant $\lambda$. Below a certain critical value ${\lambda }_c$, all the energy levels have a well-defined parity. For $\lambda >{\lambda }_c$ the energy spectrum exhibits two different phases separated by a critical energy $E_c$ that proves to be independent of $\lambda$. In the upper phase, the energy levels have also a well-defined parity, but below $E_c$ the energy levels are doubly degenerated. We show that the long-time behavior of appropriate parity-breaking observables distinguishes between these two different phases of the energy spectrum. Steady states reached from symmetry-breaking initial conditions restore the symmetry only if their expected energies are above the critical. This fact makes it possible to experimentally explore the complete phase diagram of the excitation spectrum of the Dicke model.

Ghost imaging has been receiving increasing interest for possible use as a remote-sensing system. There has been little comparison, however, between ghost imaging and the imaging laser radars with which it would be competing. Toward that end, this paper presents a performance comparison between a pulsed, computational ghost imager and a pulsed, floodlight-illumination imaging laser radar. Both are considered for range-resolving (three-dimensional) imaging of a collection of rough-surfaced objects at standoff ranges in the presence of atmospheric turbulence. Their spatial resolutions and signal-to-noise ratios are evaluated as functions of the system parameters, and these results are used to assess each system's performance tradeoffs. Scenarios in which a reflective ghost-imaging system has advantages over a laser radar are identified.

We provide a formalism to describe deterministic emission of single photons with tailored spatial and temporal profiles from a regular array of multilevel atoms. We assume that a single collective excitation is initially shared by all the atoms in a metastable atomic state and that this state is coupled by a classical laser field to an optically excited state which rapidly decays to the ground atomic state. Our model accounts for the different field polarization components via reabsorption and emission of light by the Zeeman manifold of optically excited states.

We study the photon blockade of two-photon scattering in a one-dimensional waveguide, which contains two atoms coupled via the Rydberg interaction. We obtain the analytic scattering solution of photonic wave packets with the Laplace transform method. We examine the photon correlation by addressing the two-photon relative wave function and the second-order correlation function in the single- and two-photon resonance cases. It is found that, under the single-photon resonance condition, photon bunching and antibunching can be observed in the two-photon transmission and reflection, respectively. In particular, the bunching and antibunching effects become stronger with the increasing of the Rydberg coupling strength. In addition, we find a phenomenon of bunching-antibunching transition caused by the two-photon resonance.

We propose a theoretical scheme to realize a sensitive amplification of quantum discord (QD) between two atomic qubits via a cavity–Bose-Einstein-condensate (BEC) system which was used to realize the Dicke quantum phase transition (QPT) for the first time [K. Baumann, C. Guerlin, F. Brennecke, and T. Esslinger, Nature (London) 464, 1301 (2010)]. It is shown that influence of the cavity-BEC system upon the two qubits is equivalent to a phase decoherence environment. It is found that QPT in the cavity-BEC system is the physical mechanism of the sensitive QD amplification.

Quantum noiseless linear amplification (NLA) is an important tool for long-distance optical quantum communication. In this paper, we show that NLA with linear optics and photon counts can be further improved by applying quadrature squeezing. Moreover, we find that such enhancement through squeezing can be observed even when a photon dichotic on-off detector and a pseudo-single-photon state are applied to the amplification process. Such a result could find more applications in already-known NLA-related quantum information tasks.

The ten-electron generalized relativistic effective core potential and the corresponding correlation spin-orbital basis sets are generated for the Ra atom and the relativistic coupled-cluster calculations for the RaO molecule are performed. The main goal of the study is to evaluate the $T,P$-odd parameter $X$ characterized by the molecular electronic structure and corresponding to a “volume effect” in the interaction of the ${}^{225}$Ra nucleus Schiff moment with electronic shells of RaO. Our final result for $X{(}^{225}$RaO$)$ is $-7532$ which is surprisingly close to that in ${}^{205}$TlF but has a different sign. The obtained results are discussed and the quality of the calculations is analyzed. The value is of interest for a proposed experiment on RaO [Phys. Rev. A 77, 024501 (2008)] due to a very large expected Schiff moment of the ${}^{225}$Ra nucleus.

Choosing a specific way of dividing a closed system into parts is a starting point for the decoherence program and for the quantum thermalization program. It is shown that one can always choose a way of partitioning so that decoherence-assisted classicality does not emerge and thermalization does not occur. Implications of this result are discussed.