Exploiting the fact that light propagation in defocusing nonlinear media can mimic the transonic flow of an equivalent fluid, we demonstrate experimentally the formation of an all-optical event horizon in a waveguide structure akin to a hydrodynamic Laval nozzle. The analog event horizon which forms at the nozzle throat is suggested as a platform for analogous gravity experiments.

Simulating a system of two driven coupled qubits, we show that the time-averaged probability to find one driven qubit in its ground or excited state can be controlled by an ac drive in the second qubit. Moreover, off-diagonal elements of the density matrix responsible for quantum coherence can also be controlled via driving the second qubit; that is, quantum coherence can be enhanced by appropriate choice of the biharmonic signal. Such a dynamic synchronization of two differently driven qubits has an analogy with harmonic mixing of Brownian particles forced by two signals through a substrate. Nevertheless, the quantum synchronization in two qubits occurs due to multiplicative coupling of signals in the qubits rather than via a nonlinear harmonic mixing for a classical nanoparticle. Quantum harmonic mixing proposed here can be used to manipulate one driven qubit by applying an additional ac signal to the other qubit coupled with the one we have to control.

We study a recently proposed measure for the quantification of quantum non-Markovianity in the dynamics of open systems which is based on the exchange of information between the open system and its environment. This measure relates the degree of memory effects to certain optimal initial state pairs featuring a maximal flow of information from the environment back to the open system. We rigorously prove that the states of these optimal pairs must lie on the boundary of the space of physical states and that they must be orthogonal. This implies that quantum memory effects are maximal for states which are initially distinguishable with certainty, having a maximal information content. Moreover, we construct an explicit example which demonstrates that optimal quantum states need not be pure states.

The Kochen-Specker theorem shows the impossibility for a hidden variable theory to consistently assign values to certain (finite) sets of observables in a way that is noncontextual and consistent with quantum mechanics. If we require noncontextuality, the consequence is that many observables must not have pre-existing definite values. However, the Kochen-Specker theorem does not allow one to determine which observables must be value indefinite. In this paper we present an improvement on the Kochen-Specker theorem which allows one to actually locate observables which are provably value indefinite. Various technical and subtle aspects relating to this formal proof and its connection to quantum mechanics are discussed. This result is then utilized for the proposal and certification of a dichotomic quantum random number generator operating in a three-dimensional Hilbert space.

In a recent paper, we formulated a theory of nonrelativistic quantum electrodynamics in the presence of an inhomogeneous Huttner-Barnett dielectric. Here we generalize the formalism to anisotropic materials and show how it may be modified to include conducting surfaces. We start with the derivation of the photon propagator for a slab of material and use it to work out the energy-level shift near a medium whose conductivity in the direction parallel to the surface far exceeds that in the direction perpendicular to the surface. We investigate the influence of the anisotropy of the material's electromagnetic response on the Casimir-Polder shifts, both analytically and numerically, and show that it may have a significant impact on the atom-surface interaction, especially in the nonretarded regime, i.e., for small atom-surface separations. Our results for the energy shift may be used to estimate the Casimir-Polder force acting on quantum objects close to multilayers of graphene or graphite. They are particularly important for the case of trapped cold molecules whose dispersive interactions with surfaces often fall within the nonretarded regime where the anisotropy of the material strongly influences the Casimir-Polder force. We also give a formula for the change in the spontaneous decay rate of an excited atom or molecule near an anisotropically conducting surface.

We use perturbed relativistic coupled-cluster (PRCC) theory to calculate the electric dipole polarizability of the noble-gas atoms Ar, Kr, Xe, and Rn. We also provide a detailed description of the nonlinear terms in the PRCC theory and consider the Dirac-Coulomb-Breit atomic Hamiltonian for the calculations. We find that the largest contribution from the Breit interaction to the electric dipole polarizability is 0.1$\%$, in the case of Rn. As we go from Ar to Rn, based on the pattern in the random-phase-approximation effects, the contraction of the outermost $p_{1/2}$ orbitals due to relativistic corrections is discernible without any ambiguity.

The Casimir force between two plates can be controlled using combinations of dispersive metamaterials and nonlinear materials with the optical Kerr effect. The force can be significantly varied and switched between positive and negative values by changing the intensity of a laser pulse. The switching sensitivity increases for small separation between the plates, providing new possibilities of integrating optical devices into nanoelectromechanical systems.

We have analyzed our recently measured three-body loss rate coefficient for a Bose-Einstein condensate of spin-polarized metastable triplet ${}^4$He atoms in terms of Efimov physics. The large value of the scattering length for these atoms, which provides access to the Efimov regime, arises from a nearby potential resonance. We find the loss coefficient to be consistent with the three-body parameter (3BP) found in alkali-metal experiments, where Feshbach resonances are used to tune the interaction. This provides evidence for a universal 3BP outside the group of alkali-metal elements. In addition, we give examples of other atomic systems without Feshbach resonances but with a large scattering length that would be interesting to analyze once precise measurements of three-body loss are available.

An analytic three-channel model is developed for the description of simultaneous direct and indirect dissociative recombination (DR) cross sections of molecular ions with electrons. The model is formulated in terms of three parameters describing the interactions between the electron-ion entrance channel $e$, the dissociation channel $d$, and an “active” quasibound molecular Rydberg channel $a$ and yields resonance-averaged cross sections for low incident electron energies. The relative magnitudes of the parameters determine (i) the height of the downward steps at the ion vibrational thresholds and (ii) the relative contributions of the direct and indirect processes. The model is intended to serve for the empirical analysis of observed DR cross sections, and in particular it allows the assessment of the extent to which reionization of the capture complex competes with dissociation. The model is applied to the DR of H${}_3$${}^{+}$ and HCO${}^{+}$.

Laser-dressed absorption in atomic helium is studied, both theoretically and experimentally, by transient absorption spectroscopy using isolated 400-as pulses centered at 22 eV and 12-fs near-infrared (NIR) pulses with 780-nm central wavelength. Multiple features in the helium singly excited bound-state spectrum are observed only when the NIR and attosecond pulses are overlapped in time. Theoretical analysis indicates that these light-induced structures (LISs) are the intermediate states in resonant, second-order processes that transfer population to multiple dipole forbidden states. The use of broadband, coherent extreme ultraviolet (XUV) radiation allows observation of these LISs without specifically tuning to a two-photon resonance, as would be required with narrowband XUV light. The strength and position of the LISs depend strongly on the NIR intensity and the pump-probe delay.

We propose a single-site addressing implementation based on the subwavelength localization via adiabatic passage (SLAP) technique. We consider a sample of ultracold neutral atoms loaded into a two-dimensional optical lattice with one atom per site. Each atom is modeled by a three-level $\Lambda$ system in interaction with a pump and a Stokes laser pulse. Using a pump field with a node in its spatial profile, the atoms at all sites are transferred from one ground state of the system to the other via stimulated Raman adiabatic passage, except the one at the position of the node that remains in the initial ground state. This technique allows for the preparation, manipulation, and detection of atoms with a spatial resolution better than the diffraction limit, which either relaxes the requirements on the optical setup used or extends the achievable spatial resolution to lattice spacings smaller than accessible to date. In comparison to techniques based on coherent population trapping, SLAP gives a higher addressing resolution and has additional advantages such as robustness against parameter variations, coherence of the transfer process, and the absence of photon induced recoil. Additionally, the advantages of our proposal with respect to adiabatic spin-flip techniques are highlighted. Analytic expressions for the achievable addressing resolution and efficiency are derived and compared to numerical simulations for ${}^{87}$Rb atoms in state-of-the-art optical lattices.

The $3p$ photoionization and subsequent Auger decay of initially neutral atomic germanium is studied both experimentally and theoretically. The binding energies and relative intensities of the $3p$ photoelectron spectrum are given. The $M_{2,3}{M}_{3,4}{M}_{3,4}$ and $M_{2,3}{M}_{3,4}N$ Auger electron spectrum leading to doubly ionized final states is presented. The photoelectron and Auger electron spectra were interpreted using multiconfiguration Dirac-Fock calculations that provided the identification for the main spectral features.

We present a fully ab initio calculations for attosecond transient absorption spectroscopy of atomic krypton with overlapping pump and probe pulses. Within the time-dependent configuration interaction singles (TDCIS) approach, we describe the pump step (strong-field ionization using a near-infrared pulse) as well as the probe step (resonant electron excitation using an extreme-ultraviolet pulse) from first principles. We extend our TDCIS model and account for the spin-orbit splitting of the occupied orbitals. We discuss the spectral features seen in a recent attosecond transient absorption experiment [A. Wirth et al., Science 334, 195 (2011)]. Our results support the concept that the transient absorption signal can be directly related to the instantaneous hole population even during the ionizing pump pulse. Furthermore, we find strong deformations in the absorption lines when the overlap of pump and probe pulses is maximum. These deformations can be described by relative phase shifts in the oscillating ionic dipole. We discuss possible mechanisms contributing to these phase shifts. Our finding suggests that the nonperturbative laser dressing of the entire $N$-electron wave function is the main contributor.

The dynamics of atomic levels resonantly coupled by a coherent and intense short high-frequency laser pulse is discussed and it is advocated that this dynamics is sensitively probed by measuring the spectra of the particles emitted. It is demonstrated that the time envelope of this laser pulse gives rise to two waves emitted with a time delay with respect to each other at the rising and falling sides of the pulse, which interfere in the time domain. By computing numerically and analyzing explicitly analytically a showcase example of sequential two-photon ionization of an atom by resonant laser pulses, we argue that this dynamic interference should be a general phenomenon in the spectroscopy of strong laser fields. The emitted particles do not have to be photoelectrons. Our results allow us also to interpret the already studied resonant Auger effect of an atom by intense free-electron laser pulses, and also to envisage experiments in which photons are emitted.

We propose an experimental scheme to simulate the fractionalization of the particle number by using a one-dimensional spin-orbit-coupled ultracold fermionic gas. The desired spin-orbit coupling, a kinklike potential, and a conjugation-symmetry-breaking mass term are properly constructed by laser-atom interactions, leading to an effective low-energy relativistic Dirac Hamiltonian with a topologically nontrivial background field. The designed system supports a localized soliton excitation with a fractional particle number that is generally irrational and experimentally tunable, providing a direct realization of the celebrated generalized Su-Schrieffer-Heeger model. In addition, we elaborate on how to detect the induced soliton mode with the fractional particle number in the system.

We discuss the effects of quantum fluctuations on the properties of vortex lattices in rapidly rotating ultracold atomic gases. We develop a variational method that goes beyond the Bogoliubov theory by including the effects of interactions between the quasiparticle excitations. These interactions are found to have significant quantitative effects on physical properties even at relatively large filling factors. We use our theory to predict the expected experimental signatures of quantum fluctuations of vortices and to assess the competition of the triangular vortex lattice phase with other phases in finite-sized systems.

Dynamics of skyrmionic spin texture in the spin-1 Bose-Einstein condensate is examined by analytical and numerical means. We show the skyrmion (coreless vortex) to be inherently unstable in the sense that the state initially prepared purely within the antiferromagnetic (ferromagnetic) order parameter manifold inevitably evolves into a mixture of both. The vorticity-dependent drift in the presence of the trapping potential also contributes to the disintegration of the initial spin texture manifold. We argue that the notion of a skyrmion as a topologically protected entity becomes ill defined during the dynamical evolution process.

In this paper we develop a quantum field approach to reveal the thermodynamic properties of the Bose-Einstein condensate with equal Rashba and Dresselhaus spin-orbit couplings. In the experimentally feasible regime, the phase transition from the separate phase to the single-minimum phase can be driven well by the tunable temperature. Moreover, the critical temperature, which is independent of the trap frequency, can be derived exactly. At the critical point, the specific heat has a large jump and can be thus regarded as a promising candidate to detect this temperature-driven phase transition. In addition, we obtain the analytical expressions for the specific heat and the entropy in the different phases. In the single-minimum phase, the specific heat and the entropy are governed only by the Rabi frequency. However, in the separate phase with lower temperature, we find that they are determined only by the strength of the spin-orbit coupling. Finally, the effect of the effective atom interaction is also addressed. In the separate phase, this effective atom interaction affects dramatically the thermodynamic properties, including the critical temperature.

We study the transmission spectra of a one-dimensional photonic crystal with a defect containing a four-level atomic medium that exhibits a greatly enhanced third-order susceptibility while having a vanishing linear susceptibility dependent on the electromagnetically induced transparency. Two ways of controlling the transmission of a photonic crystal are discussed: via absorption, i.e., nonlinear (two-photon) absorption of the probe field enhanced by constructive quantum interference, and via dispersion, which comes down to shifting the resonance frequency of the defect mode for the probe field by varying the refractive index based on the giant Kerr nonlinearity (cross-phase modulation). We demonstrate that such systems enable nonlinear all-optical switching at ultralow intensities of the coupling and switching laser fields.