A scheme for achieving high temporal resolution in gas electron diffraction is proposed, in which time-dependent electron diffraction patterns can be obtained from energy-resolved angular distributions of electrons scattered by molecules in dynamical processes under the presence of a single-cycle THz-wave pulse. Derived formulae of the differential cross section and numerical simulations of electron signals scattered by Ar atoms and $\mathrm{C}{\mathrm{l}}_2$ molecules show that the temporal resolution of the proposed method can be $<10\mathrm{fs}$ in the pump-probe measurement without scanning the time delay.

We develop an exact approach to study the quench dynamics of hard-core bosons initially in thermal equilibrium in one-dimensional lattices. This approach is used to study the sudden expansion of thermal states after confining potentials are switched off. We find that a dynamical fermionization of the momentum distribution occurs at all temperatures. This phenomenon is studied for low initial site occupations, for which the expansion of the cloud is self-similar. In this regime, the occupation of the natural orbitals allows one to distinguish hard-core bosons from noninteracting fermions. We also study the free expansion of initial Mott insulating domains at finite temperature and show that the emergence of off-diagonal one-body correlations is suppressed gradually with increasing temperature. Surprisingly, the melting of the Mott domain is accompanied by an effective cooling of the system. We explain this phenomenon analytically using an equilibrium description based on an emergent local Hamiltonian.

In this work we discuss the Landau criterion for anisotropic superfluidity. To this end we consider a pointlike impurity moving in a uniform Bose-Einstein condensate with either interparticle dipole-dipole interaction or Raman-induced spin-orbit coupling. In both cases we find that the Landau critical velocity $v_{\mathrm{c}}$ is generally smaller than the sound velocity in the moving direction. Beyond $v_{\mathrm{c}}$, the energy dissipation rate is explicitly calculated via a perturbation approach. In the plane-wave phase of a spin-orbit-coupled Bose gas, the dissipationless motion is suppressed by the Raman coupling even in the direction orthogonal to the recoil momentum. Our predictions can be tested in the experiments with ultracold atoms.

We consider the thermodynamic potential $\mathrm{\Omega }$ of an $N$ component Fermi gas with a short-range interaction obeying $\mathrm{SU}\left(N\right)$ symmetry. We analyze especially the nonanalytic part of $\mathrm{\Omega }$ in the temperature $T$ at low $T$. We examine the temperature range where one can observe this $T^{4}lnT$ contribution and discuss how it can be extracted experimentally.

We study a quasispin-1/2 Bose-Einstein condensate with synthetically generated spin-orbit coupling in a toroidal trap and show that the system has a rich variety of ground states. As the central hole region increases, i.e., the potential changes from harmoniclike to ringlike, the condensate exhibits a variety of structures, such as a modified stripe, an alternately arranged stripe, and countercircling states. In the limit of a quasi-one-dimensional ring, the quantum many-body ground state is obtained, which is found to be the fragmented condensate.

We offer interferometry models for thermal ensembles with one-body losses and the phenomenological inclusion of perturbations covering most of the thermal atom experiments. A possible extension to the many-body case is briefly discussed. The Ramsey pulses are assumed to have variable durations and the detuning during the pulses is distinguished from the detuning during evolution. Consequently, the pulses are not restricted to resonant operation and give more flexibility to optimize the interferometer to particular experimental conditions. On this basis another model is devised in which the contrast loss due to the unequal one-body population decays is canceled by the application of a nonstandard splitting pulse. For the importance of its practical implications, an analogous spin-echo model is also provided. The developed models are suitable for the analysis of atomic clocks and a broad range of sensing applications; they are particularly useful for trapped-atom interferometers.

We show that temporal reflections off a moving refractive index barrier play a major role in the spectral broadening of a dual-wavelength input inside a highly nonlinear, dispersion-decreasing fiber. We also find that a recently developed linear theory of temporal reflections works well in predicting the reflected frequencies. Successive temporal reflections from multiple closely spaced solitons create a blueshifted spectral band, while continuous narrowing of solitons inside the dispersion-decreasing fiber enhances Raman-induced redshifts, leading to supercontinuum generation at relatively low pump powers. We also show how dispersive wave emission can be considered a special case of the more general process of temporal reflections. Hence our findings have implications on all systems able to support solitons.

The long-standing goal of deterministic quantum interactions between single photons and single atoms was recently realized in various experiments. Among these, an appealing demonstration relied on single-photon Raman interaction (SPRINT) in a three-level atom coupled to a single-mode waveguide. In essence, the interference-based process of SPRINT deterministically swaps the qubits encoded in a single photon and a single atom, without the need for additional control pulses. It can also be harnessed to construct passive entangling quantum gates, and can therefore form the basis for scalable quantum networks in which communication between the nodes is carried out only by single-photon pulses. Here we present an analytical and numerical study of SPRINT, characterizing its limitations and defining parameters for its optimal operation. Specifically, we study the effect of losses, imperfect polarization, and the presence of multiple excited states. In all cases we discuss strategies for restoring the operation of SPRINT.

We theoretically model pair generation and evolution via the nonlinear process of spontaneous four-wave mixing in a coupled-resonator optical waveguide in a photonic crystal slab. Using the adjoint master equation for a system of lossy coupled cavities, we calculate a symmetrized second-order coherence function to determine pair detection probability. We find that the scattering loss can have as large an effect on pair generation as waveguide dispersion. In particular, the wave-vector dependence of the loss can shift the frequency of the maximum detection probability and therefore cannot, in general, be ignored or treated via a simple overall loss factor.

We establish that a connection exists between wave-particle duality of photons and partial polarization of a light beam. We perform a two-path lowest-order (single photon) interference experiment and demonstrate both theoretically and experimentally that the degree of polarization of the light beam emerging from an output of the interferometer depends on path distinguishability. In our experiment, we are able to change the quantum state of the emerging photon from a pure state to a fully mixed state without any direct interaction with the photon. Although most lowest-order interference experiments can be explained by classical theory, our experiment has no genuine classical analog. Our results show that a case exists where the cause of partial polarization is beyond the scope of classical theory.

The paradox of Greenberger-Horne-Zeilinger (GHZ) disproves directly the concept of EPR elements of reality, based on the EPR correlations, in an all-versus-nothing way. A three-qubit experimental demonstration of the GHZ paradox was achieved nearly 20 years ago, followed by demonstrations for more qubits. Still, the GHZ contradictions underlying the tests can be reduced to a three-qubit one. We show an irreducible four-qubit GHZ paradox, and report its experimental demonstration. The bound of a three-setting-per-party Bell-GHZ inequality is violated by $7\sigma$. The fidelity of the GHZ state was around $81\%$, and an entanglement witness reveals a violation of the separability threshold by $19\sigma$.

We study photon creation in a cavity with two perfectly conducting moving mirrors. We derive the dynamic equations of the modes and study different situations concerning various movements of the walls, such as translational or breathing modes. We can even apply our approach to one- or three-dimensional cavities and reobtain well-known results of cavities with one moving mirror. We compare the numerical results with analytical predictions and discuss the effects of the intermode coupling in detail as well as the nonperturbative regime. We also study the time evolution of the energy density and provide analytic justifications for the different results found numerically.

Two recent experiments studying the potential effect of entanglement on the emission properties of excited atoms produced in molecular photodissociation have been interpreted in conflicting ways. We present a theoretical analysis of the problem, showing that the experimental results can be explained by a combination of three processes: entanglement, exchange effects associated with the identical nature of the atoms, and disentanglement by spontaneous emission. According to our approach these experiments provide a verification of the phenomenon of disentanglement by spontaneous emission.

We reported a detailed experimental study of the cold collision of barium monofluoride (BaF) with buffer gas and the high-resolution spectroscopy relevant with direct laser cooling. BaF molecules are efficiently produced with laser ablation and buffer-gas cooled in a cryogenic apparatus. The laser cooling relevant transition $|X{}^2\mathrm{\Sigma },v=0,N=1\rangle$ to $|A{}^2\mathrm{\Pi },v^{\prime }=0,J^{\prime }=1/2\rangle$ is identified. The collision cross section with buffer gas is measured to be $1.4\left(7\right)\times {10}^{-14}{\text{cm}}^{-2}$, which is very suitable for buffer-gas cooling. Both rotational and vibrational temperatures are effectively cooled, and a large amount of molecules are quenched into the desired states. Our study provides an important benchmark for further laser cooling of the BaF molecule.

We investigate the long-range, two-body interactions between rubidium and potassium atoms in highly excited $(n=70)$ Rydberg states. After establishing properly symmetrized asymptotic basis states, we diagonalize an interaction Hamiltonian consisting of the standard Coulombic potential expansion and atomic fine structure to calculate electronic potential energy curves. We find that when both atoms are excited to either the $70s$ state or the $70p$ state, both the $\mathrm{\Omega }=0^{+}$ symmetry interactions and the $\mathrm{\Omega }=0^{-}$ symmetry interactions demonstrate a deep potential well capable of supporting many bound levels; the sizes of the corresponding dimer states are of the order of 2.25 $\mu \mathrm{m}$. We establish $n$-scaling relations for the equilibrium separation $R_e$ and the dissociation energy $D_e$ and find these relations to be consistent with similar calculations involving the homonuclear interactions between rubidium and cesium. We discuss the specific effects of $\ell$ mixing and the exact composition of the calculated potential well via the expansion coefficients of the asymptotic basis states. Finally, we apply a Landau-Zener treatment to show that the dimer states are stable with respect to predissociation.

We report on a realization of sub-Doppler laser cooling of ${}^{39}\mathrm{K}$ atoms using degenerate three-dimensional Raman sideband cooling. We take advantage of the well-resolved excited hyperfine states on the $D_1$ optical transition to produce spin-polarized samples with $1.4\times {10}^8$ atoms at temperatures of 1.8 $\mu \mathrm{K}$. The phase-space densities are $\ge {10}^{-4}$, which significantly improves the initial conditions for a subsequent evaporative cooling step. The presented cooling technique using the $D_1$ line can be adapted to other atomic species and is applicable to high-resolution imaging schemes in far off-resonant optical lattices.

Artificial gauge fields are versatile tools that allow the dynamics of ultracold atoms in Bose-Einstein condensates to be influenced. Here we discuss a method of artificial gauge field generation stemming from the evanescent fields of the curved surface of an optical nanofiber. The exponential decay of the evanescent fields leads to large gradients in the generalized Rabi frequency and therefore to the presence of geometric vector and scalar potentials. By solving the Gross-Pitaevskii equation in the presence of the artificial gauge fields originating from the fundamental Hybrid mode (${\mathrm{HE}}_{11}$) mode of the fiber, we show that vortex rings can be created in a controlled manner. We also calculate the magnetic fields resulting from the higher order ${\mathrm{HE}}_{21}$, Transverse electric mode (${\mathrm{TE}}_{01}$), and Transverse magnetic mode (${\mathrm{TM}}_{01}$) and compare them to the fundamental ${\mathrm{HE}}_{11}$ mode.

We investigate the collective excitations of a Raman-induced spin-orbit coupled Bose-Einstein condensate confined in a quasi-one-dimensional harmonic trap using the Bogoliubov method. By tuning the Raman coupling strength, three phases of the system can be identified. By calculating the transition strength, we are able to classify various excitation modes that are experimentally relevant. We show that the three quantum phases possess distinct features in their collective excitation properties. In particular, the spin dipole and the spin breathing modes can be used to clearly map out the phase boundaries. We confirm these predictions by direct numerical simulations of the quench dynamics that excites the relevant collective modes.

We study the behavior of a single laser-driven trapped ion inside a microscopic optical Fabry-Perot cavity. In particular, we demonstrate a fiber Fabry-Perot cavity operating on the principal $S_{1/2}\to P_{1/2}$ electric dipole transition of an ${\mathrm{Yb}}^{+}$ ion at 369 nm with an ion-cavity coupling strength of $g=2\pi \times 67\left(1\right)$ MHz. We employ the cavity to study the generation of single photons and observe cavity-induced backaction in the Purcell-enhanced emission of photons. Tuning of the amplitude and phase between the driving field and the cavity field built up from photons scattered into the cavity mode by the ion allows us to enhance or suppress the total rate of photon emission from the ion-cavity system.

Similarly to how charged particles experience time-averaged ponderomotive forces in high-frequency fields, linear waves also experience time-averaged refraction in modulated media. Here we propose a covariant variational theory of this ponderomotive effect on waves for a general nondissipative linear medium. Using the Weyl calculus, our formulation accommodates waves with temporal and spatial period comparable to that of the modulation (provided that parametric resonances are avoided). Our theory also shows that any wave is, in fact, a polarizable object that contributes to the linear dielectric tensor of the ambient medium. The dynamics of quantum particles is subsumed as a special case. As an illustration, ponderomotive Hamiltonians of quantum particles and photons are calculated within a number of models. We also explain a fundamental connection between these results and the well-known electrostatic dielectric tensor of quantum plasmas.