In this paper we study the dynamics in the general case for a Tavis-Cummings atom in a nonuniform cavity. In addition to the dynamical Stark shift, the center-of-mass motion of the atom and the recoil effect are considered in both the weak and the strong cavity-atom coupling regimes. It is shown that the spatial motion of the atom inside the cavity in the resonant case leads to a transition between topologically different solutions. This effect is manifested by a singularity in the interlevel transition spectrum. In the nonresonant case, the spatial motion of the atom leads to a switching of the spin orientation. In both effects, the key factor is the relation between the values of the Stark shift and the cavity-field coupling constant. We also investigate the entanglement of an atom in the cavity with the radiation field. It is shown that the entanglement between the atom and the field, usually quantified in terms of purity, decreases with increasing Stark shift.

A quantized vortex dipole is the simplest vortex molecule, comprising two countercirculating vortex lines in a superfluid. Although vortex dipoles are endemic in two-dimensional superfluids, the precise details of their dynamics have remained largely unexplored. We present here several striking observations of vortex dipoles in dilute-gas Bose-Einstein condensates, and develop a vortex-particle model that generates vortex line trajectories that are in good agreement with the experimental data. Interestingly, these diverse trajectories exhibit essentially identical quasiperiodic behavior, in which the vortex lines undergo stable epicyclic orbits.

As stated by S. Ghose et al. [Phys. Rev. Lett. 102, 250404 (2009)], there are certain relationships between tripartite entanglement and tripartite nonlocality for three-qubit Greenberger-Horne-Zeilinger (GHZ) class states. In the present work, we have experimentally demonstrated the theoretical results of Ghose et al. by using both three-photon generalized GHZ (GGHZ) states and maximal slice (MS) states with a count of $\sim$10/s. From the data, we have verified the agreement of the experimental violation of the Svetlichny inequality with the one predicted by quantum mechanics given the reconstructed density matrix. For the MS states, it is demonstrated that the amount of violation increases linearly following the increase of the degree of tripartite entanglement. In contrast, for GGHZ states, there is a minimal value of the violation when the degree of tripartite entanglement is 1/3. Both of the results are consist with the theoretical prediction.

We study a one-parameter family of discrete-time quantum walk models on $\mathbb{Z}$ and ${\mathbb{Z}}^2$ associated with the Hadamard walk. Weak convergence in the long-time limit of all moments of the walker's pseudovelocity on $\mathbb{Z}$ and ${\mathbb{Z}}^2$ is proved. Symmetrization on $\mathbb{Z}$ and ${\mathbb{Z}}^2$ is theoretically investigated, leading to the resolution of the Konno-Namiki-Soshi conjecture in the special case of symmetrization of the unbiased Hadamard walk on $\mathbb{Z}$. A necessary condition for the existence of a phenomenon known as localization is given.

We consider one-photon ionization in a system consisting of two different atoms, $A$ and $B$, which are well separated in space. In such a case, photoionization of the atom with a lower ionization potential can be very strongly influenced, via two-center electron-electron correlations, by resonant electron dipole transitions induced in the other atom. We present a detailed consideration of this resonant photoionization process and discuss a number of effects that are inherent to it.

We discuss an idea to realize a spontaneous current in a superfluid Fermi gas. When a polarized Fermi superfluid ($N_{\uparrow }>N_{\downarrow }$, where $N_{\sigma }$ is the number of atoms in the hyperfine state described by pseudospin $\sigma =\uparrow ,\downarrow$) is loaded onto a ring-shaped trap with a weak potential barrier, some excess atoms ($\Delta N=N_{\uparrow }-N_{\downarrow }$) are localized around the barrier. As shown in our previous paper [T. Kashimura, S. Tsuchiya, and Y. Ohashi, Phys. Rev. A 82, 033617 (2010)], this polarized potential barrier works as a $\pi$ junction in the sense that the superfluid order parameter changes its sign across the barrier. Because of this, the phase of the superfluid order parameter outside the junction is shown to be twisted by $\pi$ along the ring, which naturally leads to a circulating supercurrent. While the ordinary supercurrent state is obtained as a metastable state, this spontaneous current state is shown to be more stable than the case with no current. Our results indicate that localized excess atoms would be useful for the manipulation of the superfluid order parameter in cold Fermi gases.

Atomic clouds prepared in “timed Dicke” states, i.e. states where the phase of the oscillating atomic dipole moments linearly varies along one direction of space, are efficient sources of superradiant light emission [Scully et al., Phys. Rev. Lett. 96, 010501 (2006)]. Here, we show that, in contrast to previous assertions, timed Dicke states are not the states automatically generated by incident laser light. In reality, the atoms act back on the driving field because of the finite refraction of the cloud. This leads to nonuniform phase shifts, which, at higher optical densities, dramatically alter the cooperative scattering properties, as we show by explicit calculation of macroscopic observables, such as the radiation pressure force.

We develop a formalism to incorporate exchange symmetry considerations into the calculation of collisional frequency shifts for atomic clocks using a density-matrix formalism. The formalism is developed for both fermionic and bosonic atomic clocks. Numerical results for a finite-temperature ${}^{87}$Sr ${}^1{S}_0$ ($F=9/2$) atomic clock in a magic wavelength optical lattice are presented.

A system of three spin-$\frac{1}{2}$ atoms allows the construction of a reference-frame-free (RFF) qubit in the subspace with total angular momentum $j=1/2$. The RFF qubit stays coherent perfectly as long as the spins of the three atoms are affected homogeneously. The inhomogeneous evolution of the atoms causes decoherence, but this decoherence can be suppressed efficiently by applying a bias magnetic field of modest strength perpendicular to the plane of the atoms. The resulting lifetime of the RFF qubit can be many days, making RFF qubits of this kind promising candidates for quantum information storage units. Specifically, we examine the situation of three ${}^6\text{Li}$ atoms trapped in a ${\text{CO}}_2$-laser-generated optical lattice and find that, with conservatively estimated parameters, a stored qubit maintains a fidelity of 0.9999 for two hours.

I calculate the fidelity of a $[7,1,3]$ Calderbank-Shor-Steane quantum error correction code logical zero state constructed in a nonequiprobable Pauli operator error environment for two methods of encoding. The first method is to apply fault-tolerant error correction to an arbitrary state of seven qubits utilizing Shor states for syndrome measurement. The Shor states are themselves constructed in the nonequiprobable Pauli operator error environment, and their fidelity depends on the number of verifications done to ensure multiple errors will not propagate into the encoded quantum information. Surprisingly, performing these verifications may lower the fidelity of the constructed Shor states. The second encoding method is to simply implement the $[7,1,3]$ encoding gate sequence also in the nonequiprobable Pauli operator error environment. Perfect error correction is applied after both methods to determine the correctability of the implemented errors. I find that which method attains higher fidelity depends on which of the Pauli operators errors is dominant. Nevertheless, perfect error correction applied after the encoding suppresses errors to at least first order for both methods.

The $A{\hspace{0.5em}}^2\Pi -X{\hspace{0.5em}}^2{\Sigma }^{+}$ (0,0) band system of barium monofluoride (BaF) has been recorded using high-resolution laser-induced fluorescence spectroscopy both field-free and in the presence of static magnetic and electric fields. The field-free spectra for the ${}^{135}\mathrm{Ba}$F, ${}^{137}\mathrm{Ba}$F, and ${}^{138}\mathrm{Ba}$F isotopologues were modeled to generate an improved set of spectroscopic constants for the $A{\hspace{0.5em}}^2$Π($\upsilon$ $=$ 0) and $X{\hspace{0.5em}}^2$Σ${}^{+}$($\upsilon$ $=$ 0) states. The observed optical Stark shifts for the ${}^{138}\mathrm{Ba}$F isotopologue were analyzed to produce the permanent electric dipole moments of 1.50(2) and 1.31(2) D for the $A{\hspace{0.5em}}^2$Π${}_{1/2}$($\upsilon$ $=$ 0) and $A{\hspace{0.5em}}^2$Π${}_{3/2}$ ($\upsilon$ $=$ 0) states, respectively. The observed optical Zeeman shifts for the ${}^{138}\mathrm{Ba}$F isotopologue were analyzed to produce a set of magnetic $g$ factors for the $A{\hspace{0.5em}}^2$Π($\upsilon$ $=$ 0) and $X{\hspace{0.5em}}^2$Σ${}^{+}$($\upsilon$ $=$ 0) states.

Variational calculations of the ground state of positronium hydride (HPs) are reported, including various expectation values, electron-positron annihilation rates, and leading relativistic corrections to the total and dissociation energies. The calculations have been performed using a basis set of 4000 thoroughly optimized explicitly correlated Gaussian basis functions. The relative accuracy of the variational energy upper bound is estimated to be of the order of 2$\times$10${}^{-10}$, which is a significant improvement over previous nonrelativistic results.

The single- and multiple-electron loss of fast highly charged projectiles in collisions with neutral molecules is studied within the framework of a nonperturbative approach. The cross sections for single-, double-, and triple-electron losses are calculated for the collision system ${\text{Fe}}^{q+}\to {\text{N}}_2$ ($q=24$, 25, 26) at the collision energies 10, 100, and 1000 MeV/nucleon. The effects caused by the collision multiplicity and the orientation of the axis of the target molecule are treated. It is shown that the collision multiplicity effect leads to considerable differences for the cases of perpendicular and parallel orientations of the molecular axes with respect to the direction of the projectile motion, while for chaotic orientation such an effect is negligible.

We show that semiclassical states adapted to the symmetry of the Hamiltonian are an excellent approximation to the exact quantum solution of the ground and first excited states of the Dicke model. Their overlap with the exact quantum states is very close to 1 except in a close vicinity of the quantum phase transition. Furthermore, they have analytic forms in terms of the model parameters and allow us to calculate analytically the expectation values of field and matter observables. Some of these differ considerably from results obtained via the standard coherent states and by means of Holstein-Primakoff series expansion of the Dicke Hamiltonian. Comparison with exact solutions obtained numerically supports our results. In particular, it is shown that the expectation values of the number of photons and of the number of excited atoms have no singularities at the phase transition. We comment on why other authors have previously found otherwise.

Soliton dynamics in a large variety of longitudinally modulated lattices are studied in terms of phase-space analysis for an effective particle approach and direct numerical simulations. Complex soliton dynamics are shown to depend strongly on both their power and width and their initial momentum as well as on lattice parameters. A rich set of qualitatively distinct dynamical features of soliton propagation that have no counterpart in longitudinally uniform lattices is illustrated. This set includes cases of enhanced soliton mobility, dynamical switching, extended trapping in several transverse lattice periods, and quasiperiodic trapping, which are promising for soliton control applications.

We report a precision measurement of the cesium $6P_{3/2}$ excited-state lifetime. Two collimated, counterpropagating thermal Cs beams cross perpendicularly to femtosecond pulsed laser beams. High timing accuracy is achieved from having excitation and ionization laser pulses which originate from the same mode-locked laser. Using pulse selection we vary the separation in time between excitation and ionization laser pulses while counting the ions produced. We obtain a Cs $6P_{3/2}$ lifetime of 30.460(38) ns, which is a factor of two improvement from previous measurements and with an uncertainty of 0.12%, is one of the most accurate lifetime measurements on record.

We consider parity nonconservation (PNC) in singly ionized ytterbium (Yb${}^{+}$) arising from the neutral current weak interaction. We calculate the PNC electric dipole transition amplitude ($E{1}_{\mathrm{PNC}}$) and the properties associated with it using relativistic coupled-cluster theory. $E{1}_{\mathrm{PNC}}$ for the $\left[4f^{14}\right]{}^{2}6s\to \left[4f^{14}\right]{}^{2}5d_{3/2}$ transition in Yb${}^{+}$ has been evaluated to within an accuracy of 5%. The improvement of this result is possible. It therefore appears that this ion is a promising candidate for testing the standard model of particle physics.

When a resonant laser sent on an optically thick cold atomic cloud is abruptly switched off, a coherent flash of light is emitted in the forward direction. This transient phenomenon is observed due to the highly resonant character of the atomic scatterers. We analyze quantitatively its temporal properties and show very good agreement with theoretical predictions. Based on complementary experiments, the phase of the coherent field is reconstructed without interferometric tools.

We propose an implementation of the two-qubit gate in a quantum dot spin qubit system which is immune to charge noise problems. Our proposed implementation, if it could be realized in a physical system, would have the advantage of being robust against uncertainties and fluctuations in the tunnel coupling and barrier gate voltage pulse area. The key idea is to introduce an auxiliary dot and use an analog to the stimulated Raman adiabatic passage pulse sequence in three-level atomic systems, often referred to in the context of electron transport in quantum dot systems as Coherent Tunneling by Adiabatic Passage. Spin-dependent tunneling opens the possibility of performing two-qubit gate operations by this method.