Long-range polarization forces between ions and neutral atoms result in large elastic scattering cross sections (e.g., $\sim {10}^6\mathrm{a}.\mathrm{u}.$ for $\mathrm{Na}$-${\mathrm{Na}}^{+}$ or $\mathrm{Na}$-${\mathrm{Ca}}^{+}$ at cold and ultracold temperatures). This suggests that a hybrid ion-neutral trap should offer a general means for significant sympathetic cooling of atomic or molecular ions. We present simion 7.0 simulation results concerning the advantages and limitations of sympathetic cooling within a hybrid trap apparatus consisting of a linear rf Paul trap concentric with a $\mathrm{Na}$ magneto-optical trap (MOT). This paper explores the impact of various heating mechanisms on the hybrid system and how parameters related to the MOT, Paul trap, number of ions, and ion species affect the efficiency of the sympathetic cooling.

We demonstrate two-dimensional robust Raman cooling in a four-level tripod system, in which velocity selective population transfer is achieved by a stimulated Raman adiabatic passage. In contrast to basic two-dimensional Raman cooling with square-envelope pulses [Phys. Rev. A 83, 023407 (2011)], the technique presented here allows for a wide variation in the pulse duration and amplitude once the adiabaticity criterion is satisfied. An efficient population transfer together with attainment of a narrow spread of the resonant-velocity group leads to the narrowing of the velocity-distribution spread down to $0.1v_{\mathrm{rec}}$, corresponding to an effective temperature equal to $0.01T_{\mathrm{rec}}$. This robust method opens additional possibilities for cooling of neutral atoms.

The control of dissociation pathways of ${{\mathrm{H}}_2}^{+}$ through individual manipulations of nuclear vibration and electronic transfers is theoretically studied by using the two-color mid- and near-infrared laser fields. We show that the time-delayed weak near-infrared pulse can steer the electron localization by directly manipulating the electronic transfers in the one-photon coupling region with minor influence on the nuclear motion. Our further results demonstrate that the nuclear and electronic dynamics can be efficiently and individually controlled by changing the relative phase between the two laser pulses. The best balance between the nuclear and the electronic motions and, thus, the further enhancement of electron localization compared to that of a single midinfrared field, are ultimately achieved.

X-ray free-electron lasers (XFELs) are a promising tool for the structural determination of macro- and biomolecules, using coherent diffractive imaging. During imaging, the intense XFEL pulses also efficiently ionize the molecules, so it is important to estimate how the charged environment within the molecule modifies atomic properties, in comparison to the case of an isolated atom. Here, we apply the xatom toolkit to obtain predictions on the modified ionization thresholds and rates of some photoinduced processes in carbon. The Hartree-Fock-Slater model is extended to include the electron screening and ion correlation effects, induced by external charges. With this extended model, we obtain predictions on modifications of orbital energies, photoabsorption cross sections, Auger decay rates, fluorescence emission rates, and atomic scattering factors as a function of the density and temperature of the surrounding charges. Our results have implications for the studies of dynamics within XFEL irradiated samples, in particular for those dedicated to coherent diffraction imaging.

We develop a finite-temperature Hartree theory for the trapped dipolar Bose gas. We use this theory to study thermal effects on the mechanical stability of the system and density oscillating condensate states. We present results for the stability phase diagram as a function of temperature and aspect ratio. In oblate traps above the critical temperature for condensation, we find that the Hartree theory predicts significant stability enhancement over the semiclassical result. Below the critical temperature, we find that thermal effects are well described by accounting for the thermal depletion of the condensate. Our results also show that density oscillating condensate states occur over a range of interaction strengths that broadens with increasing temperature.

The stochastic Gross-Pitaevskii equation and modified Popov theory are shown to provide an excellent ab initio description of finite-temperature, weakly interacting, two-dimensional Bose gas experiments. In particular, we demonstrate that the stochastic Gross-Pitaevskii equation generates excellent agreement with the recent experiment by C.-L. Hung et al. [Nature 470, 236 (2011)], thereby confirming that the observed universality and scale invariance arise naturally within this formalism. This is achieved in an ab initio manner, in which a systematic approach for obtaining a suitable value for the momentum cutoff of interacting classical field theories is proposed by means of the modified Popov theory.

We assess the band structure of arrays of coupled optical waveguides both by ab initio calculations and by experiments, with an excellent quantitative agreement without any adjustable physical parameter. The band structures we obtain can deviate strongly from the expectations of the standard coupled mode theory approximation, but we describe them efficiently by a few parameters within an extended coupled mode theory. We also demonstrate that this description is in turn a firm and simple basis for accurate beam management in functional patterns of coupled waveguides, in full accordance with their design.

The second harmonic (SH) radiation excited in a nonlinear crystal by a Gaussian Schell-model beam with a broad frequency spectrum is analyzed. It is revealed that the frequency spectrum of the SH wave generated depends on the bandwidth of the angular spectrum of the incoherent fundamental wave as well as on its beamwidth. The influence of the fundamental wave beamwidth on the SH wave intensity is discussed. Analytical treatments and numerical simulations of the nonlinear coupling equations are both presented.

Majorana fermions are promising candidates for storing and processing information in topological quantum computation. The ability to control such individual information carriers in trapped ultracold atomic Fermi gases is a novel theme in quantum information science. However, fermionic atoms are neutral and thus are difficult to manipulate. Here, we theoretically investigate the control of emergent Majorana fermions in one-dimensional spin-orbit-coupled atomic Fermi gases. We discuss (i) how to move Majorana fermions by increasing or decreasing an effective Zeeman field, which acts like a solid-state control voltage gate, and (ii) how to create a pair of Majorana fermions by adding a magnetic impurity potential. We discuss the experimental realization of our control scheme in an ultracold Fermi gas of ${}^{40}$K atoms.

The Klein-Gordon equation is used to calculate the Zitterbewegung (ZB, trembling motion) of spin-zero particles in the absence of fields and in the presence of an external magnetic field. Both Hamiltonian and wave formalisms are employed to describe ZB and their results are compared. It is demonstrated that if one uses wave packets to represent particles, then the ZB motion has a decaying behavior. It is also shown that the trembling motion is caused by an interference of two subpackets composed of positive- and negative-energy states, which propagate with different velocities. In the presence of a magnetic field, the quantization of the energy spectrum results in many interband frequencies contributing to ZB oscillations and the motion follows a collapse-revival pattern. In the limit of nonrelativistic velocities, the interband ZB components vanish and the motion is reduced to cyclotron oscillations. The exact dynamics of a charged Klein-Gordon (KG) particle in the presence of a magnetic field is described on an operator level. The trembling motion of a KG particle in the absence of fields is simulated using a classical model proposed by Morse and Feshbach—it is shown that a variance of a Gaussian wave packet exhibits ZB oscillations.

Quenches in an ion chain can create coherent superpositions of motional states across the linear-zigzag structural transition. The procedure has been described in Baltrusch et al. [Phys. Rev. A 84, 063821 (2011)] and makes use of spin-dependent forces, so that a coherent superposition of the electronic states of one ion evolves into an entangled state between the chain's internal and external degrees of freedom. The properties of the crystalline state so generated are theoretically studied by means of Ramsey interferometry on one ion of the chain. An analytical expression for the visibility of the interferometric measurement is obtained for a chain of an arbitrary number of ions and as a function of the time elapsed after the quench. Sufficiently close to the linear-zigzag instability the visibility decays very fast, but exhibits revivals at the period of oscillation of the mode that drives the structural instability. These revivals have a periodicity that is independent of the crystal size, and they signal the creation of entanglement by the quantum quench.

We show a quantum state with explicit local hidden-variable models for correlations between any fixed number of subsystems which cannot be extended to a model simultaneously describing correlations between different numbers of subsystems. The explicit models we discuss may involve several settings per observer and the way to disqualify them involves only two settings per party.

This paper considers a spin-chain model by numerically solving the exact model to explore the nonperturbative dynamical decoupling regime, where an important issue recently arose. Our study has revealed a few universal features of nonperturbative dynamical control irrespective of the types of environments and system-environment couplings. We have shown that, for the spin-chain model, there is a threshold and a large pulse parameter region where the effective dynamical control can be implemented, in contrast to the perturbative decoupling schemes where the permissible parameters are represented by a point or converge to a very small subset in the large parameter region admitted by our nonperturbative approach. An important implication of the nonperturbative approach is its flexibility in implementing the dynamical control scheme in an experimental setup. Our findings have exhibited several interesting features of the nonperturbative regimes such as the chain-size independence, pulse strength upper bound, noncontinuous valid parameter regions, etc. Furthermore, we find that our nonperturbative scheme is robust against randomness in model fabrication and time-dependent random noise.

Genuine multipartite entanglement of a quantum system can be partially destroyed by local decoherence. Is it possible to retrieve the entanglement to some extent by a single local operation? The answer to this question depends very much on the type of initial genuine entanglement. For initially pure $W$ and cluster states and if the decoherence is given by generalized amplitude damping, the answer is shown to be positive. In this case, the entanglement retrieving is achieved just by redistributing the remained entanglement of the system.

We investigate the critical behaviors of the Bell nonlocality, which are measured by the Bell-function values, in conventional and topological quantum-phase transitions. First, by using the quantum-renormalization-group method, we show that the Bell function value of a three-qubit Bell inequality in the anisotropic spin-$1/2$ $XY$ model exhibits singular behaviors at the critical point. Moreover, for topological quantum-phase transitions, we find that the first-order derivative of the Bell-function value of the well-known Clauser-Horne-Shimony-Holt inequality in the Kitaev-Castelnovo-Chamon model behaves singularly at the topological-phase-transition point. Our results have established a link between quantum nonlocality and phase transitions.

We report an experimental implementation of the minimal Deutsch algorithm in an optical setting. In this version, a redundancy is removed from the most famous form of the algorithm. The original version involves manipulation of two qubits, while in its minimal version, only one qubit is used. Our qubit is encoded in the transversal spatial modes of a spontaneous parametric down-converted signal photon, with the aid of a double slit, with the idler photon playing a crucial role in creating a heralded single-photon source. A spatial light modulator (SLM) is programmed to physically generate one-bit functions necessary to implement the algorithm's minimal version, which shows that the SLM can be used in future implementations of quantum protocols.

It is well known that the classification of pure multiparticle entangled states according to stochastic local operations leads to a natural classification of mixed states in terms of convex sets. We present a simple algorithmic procedure to prove that a quantum state lies within a given convex set. Our algorithm generalizes a recent algorithm for proving separability of quantum states [Barreiro et al., Nat. Phys. 6, 943 (2010)]. We give several examples which show the wide applicability of our approach. We also propose a procedure to determine a vicinity of a given quantum state which still belongs to the considered convex set.

We study the structure of logical operators in local $D$-dimensional quantum codes, considering both subsystem codes with geometrically local gauge generators and codes defined by geometrically local commuting projectors. We show that if the code distance is $d$, then any logical operator can be supported on a set of specified geometry containing $\tilde{d}$ qubits, where $\tilde{d}{d}^{1/(D-1)}=O\left(n\right)$ and $n$ is the code length. Our results place limitations on partially self-correcting quantum memories, in which at least some logical operators are protected by energy barriers that grow with system size. We also show that for any two-dimensional local commuting projector code there is a nontrivial logical “string” operator supported on a narrow strip, where the operator is only slightly entangling across any cut through the strip.

The results of ab initio scattering calculations for low-energy electron collisions with the B${}_2$ molecule using the $R$-matrix method have been presented. The differential and momentum-transfer cross sections along with effective collision frequency over a wide electron temperature range (300–30 000 K) are computed at the one-state close-coupling level. In this work, 61-state close-coupling calculations are performed to compute the integral cross sections (elastic and excitations). We investigated five resonances: two in the excitation cross sections of $A{}^3{\Pi }_u$, two in the excitation cross sections of $a{}^5{\Sigma }_u^{-}$ and one in both $b{}^1{\Delta }_g$ and $c{}^1{\Sigma }_g^{+}$ excited states. The Born correction for the dipole-allowed transition ($X{}^3{\Sigma }_g^{-}$ to $A{}^3{\Pi }_u$) has been carried out to account for the contribution of partial waves higher than the $g$ wave ($l=4$) up to which the $R$-matrix scattering calculations are carried. We have detected a stable anionic bound state ${}^2{\Pi }_u$ of B${}_2^{-}$ having the configuration $1{\sigma }_g^2$ $2{\sigma }_g^2$ $1{\sigma }_u^2$ $2{\sigma }_u^2$ $1{\pi }_u^3$. The ionization cross sections are calculated in the binary-encounter Bethe model in which Hartree-Fock molecular orbitals at a self-consistent level are used to calculate kinetic and binding energies of the occupied molecular orbitals. We have also evaluated scattering length for electron-B${}_2$ collisions at the one-state close-coupling level (6.8$a_0$) and at the static exchange plus polarization level (6.2$a_0$).

Atom-atom interactions within a small volume were investigated through the excitation of ultracold Rb atoms. The application of microwaves enhances these interactions, causing the suppression of Rydberg state excitation. The suppression of Rydberg atom excitation was both qualitatively observed and quantitatively analyzed using a universal scaling law, giving a measure of the atom-atom interaction strength in agreement with theoretical prediction.