We propose schemes that can perform perfect time-reversal operation of the tunneling dynamics of Bose-Einstein condensates in optical lattices. One of the time-reversal schemes is to reverse the sign of the tunneling coefficient by a time-periodic potential, maintaining its magnitude at a constant value. The other scheme utilizes a staggered $0\text{−}\pi$ phase imprinting on the optical lattices. If the reversal of the tunneling coefficient or the phase imprinting is performed at $\tau ={\tau }_p$, the initial distribution of the condensates is regained at $\tau =2{\tau }_p$. Although the nonlinear interatomic interaction deteriorates the fidelity of the time-reversal operation, if the sign of the nonlinearity is reversed simultaneously with the phase imprinting at $\tau ={\tau }_p$, the time-reversal operation at $\tau =2{\tau }_p$ becomes nearly perfect.

We study the generation of spin squeezing by the repeated action of the angular momentum (Dicke) lowering operator on an atomic coherent state prepared for a collection of $N$ two-level atoms or ions. The atoms or ions of the atomic coherent state are not entangled, but the action of the lowering operator (or similarly of the raising operator) generates entanglement among the atoms, and spin squeezing occurs for some ranges of the relevant parameter. Spin squeezing in a collection of two-level atoms or ions is of importance for precision spectroscopy. We discuss methods by which our spin-squeezed states could be generated in the contexts of cavity QED and trapped ions.

The combination of polarization gating and two-color gating is a promising approach to generate single isolated attosecond pulses with multicycle pump lasers. We searched the phase-matching conditions for high-order harmonic generation with such a double optical gating using $0.85\mathrm{mJ}$, $8\mathrm{fs}$ laser pulses centered at $790\mathrm{nm}$. At the optimized gas target location and pressure, the single extreme ultraviolet pulse energy is $6.5\mathrm{nJ}$ for argon and 170 pJ for neon, which are much higher then those generated with conventional polarization gating. In addition, the carrier-envelope phase effects with a $2\pi$ periodicity were attained when the highest extreme ultraviolet photon flux was produced, indicating the robustness of the double optical gating.

In a recent paper [J. K. Asboth et al., Phys. Rev. Lett. 98, 203008 (2007)] we have shown that traveling density wavelike collective oscillations can arise in an asymmetrically pumped optical lattice, and by increasing the lattice size or pump asymmetry, these waves can destabilize the structure even in the overdamped limit. The long-range interaction giving rise to collective motion stems from the back-action of the atoms on the field creating the lattice. In this paper we present a detailed description of these phenomena. We derive the force on a disk-shaped cloud of trapped particles including the back-action on the trapping light, and analyze its relation to the standard perturbative approach giving the “dipole force” and “radiation pressure.” We calculate the self-consistent lattice constant for both red and blue detuned lattices and find that it decreases—by the same amount in the two cases—as the pump asymmetry is increased. We present the detailed derivation of the lattice vibration eigenmodes using the transfer matrix method, which reveals that the instability is enhanced resonantly at certain settings of the asymmetry.

We present deterministic algorithms for the simultaneous control of an arbitrary number of quantum observables. Unlike optimal control approaches based on cost function optimization, quantum multiobservable tracking control (MOTC) is capable of tracking predetermined homotopic trajectories to target expectation values in the space of multiobservables. The convergence of these algorithms is facilitated by the favorable critical topology of quantum control landscapes. Fundamental properties of quantum multiobservable control landscapes, including the MOTC Gramian matrix, are introduced. The effects of multiple control objectives on the structure and complexity of optimal fields are examined. With minor modifications, the techniques described herein can be applied to general quantum multiobjective control problems.

We investigate the dynamics of an unstable vortex ring in a pancake-shaped Bose-Einsten condensate by solving the Gross-Pitaevskii equation numerically. It is found that a quasisteady turbulent state with long relaxation time can be achieved through the disruption of a perturbed vortex ring in the condensate owing to the bending-wave instability. We verify that this quantum turbulent state is characterized by Kolmogorov energy spcetrum.

We study non-Gaussian states generated by two-photon subtraction from a continuous-wave (cw) squeezed light source. In a cw scheme, one can subtract two photons from the source with a designated time separation and can generate temporally multiplexed superposition states of continuous variables. We numerically study the properties of these states in light of bosonic interference in the time domain. In an appropriate temporal mode, an amplified superposition state of continuous variable is produced in a region where the time separation is comparable with the coherence time of squeezed packets.

Positrons can attach to molecules via vibrational Feshbach resonances, leading to very large annihilation rates. The predictions of a recent theory for this process are validated for deuterated methyl halides where all modes are dipole coupled to the incident positron. Data and analysis are presented for methanol and ethylene, demonstrating the importance of combination and overtone resonances and the ability of the theory to account for these features. The mechanism for these resonances and criteria for their occurrence as well as outstanding questions are discussed.

We present a systematic comparison of the most recent thermodynamic measurements of a trapped Fermi gas at unitarity with predictions from strong-coupling theories and quantum Monte Carlo (MC) simulations. The accuracy of the experimental data, of the order of a few percent, allows a precise test of different many-body approaches. We find that a Nozières and Schmitt-Rink treatment of fluctuations is in excellent agreement with the experimental data and available MC calculations at unitarity.

We propose a protocol for perfect quantum state transfer that is resilient to a broad class of realistic experimental imperfections, including noise sources that could be modeled either as independent Markovian baths or as certain forms of spatially correlated environments. We highlight interesting connections between the fidelity of state transfer and quantum stochastic resonance effects. The scheme is flexible enough to act as an effective entangling gate for the generation of genuine multipartite entanglement in a control-limited setting. Possible experimental implementations using superconducting qubits are also briefly discussed.

Recently, de Visser and Blaauboer [Phys. Rev. Lett. 96, 246801 (2006)] proposed the most efficient deterministic teleportation protocol $\mathcal{T}$ for electron spins in a semiconductor nanostructure consisting of a single and a double quantum dot. However, it is as yet unknown if $\mathcal{T}$ can be completed before decoherence sets in. In this paper we analyze the detrimental effect of nuclear spin baths, the main source of decoherence, on $\mathcal{T}$. We show that nonclassical teleportation fidelity can be achieved with $\mathcal{T}$ provided certain conditions are met. Our study indicates that realization of quantum computation with quantum dots is indeed promising.

We study the two-particle quantum Zeno dynamics with a type of nondeterministic collective measurement whose outcome indicates whether the two-particle state has been collapsed to $\mid 11⟩$. Such a threshold detection, when used continuously, can lead to nontrivial quantum dynamics. We show that such type of dynamics can be used to produce quantum entanglement almost deterministically. We then numerically show the robustness of the method and we find that the operational errors of the small-angle rotations do not accumulate. We also propose a possible implementation using superconducting flux qubits.

Necessary and sufficient conditions for deterministic remote extraction and destruction of qubit information encoded in bipartite states using only local operations and classical communications (LOCC) are presented. The conditions indicate that there is a way to asymmetrically spread qubit information between two parties such that it can be remotely extracted with unit probability at one of the parties but not at the other as long as they are using LOCC. Remote destruction can also be asymmetric between the two parties, but the conditions are incompatible with those for remote extraction.

The energy law giving the eigenenergies of a $-c_n/R^n-c_m/R^m$ potential (studied by LeRoy in 1980), is revisited. For $n=3$, $m=6$, an analytical law giving the density of states is deduced. In the context of weakly bound levels an energy law giving the vibrational number $\mathit{v}$ versus the binding energy $ϵ$ is given. We show that the well-known LeRoy-Bernstein formula (1970) has to be corrected by additional terms, with the first one varying as $ϵ$, the second one as ${ϵ}^2$, and the third as ${ϵ}^{7/6}$. The use of such a law is discussed in the context of the photoassociation spectroscopy of long-range molecular levels.

We investigate the role of electron correlation effects in the electric-dipole polarizabilities of the ground states of the alkaline earth metals, helium, and ytterbium by employing the relativistic coupled-cluster (RCC) theory. These effects are incorporated via the residual Coulomb interaction to all orders in the RCC singles and doubles approximations. The perturbed wave functions used in the calculations of the polarizabilities are obtained by directly solving the first-order perturbed RCC equations, thereby avoiding the sum-over-states approach. Our results are compared with other calculations and available experimental data.

Light-induced forces can bind dielectric spheres into extended structures through two mechanisms, each with its own length scale which can be adjusted by the configuration of the external light source. By manipulating the commensurability of the two length scales, these two mechanisms can cooperate to bind a large number of spheres. When the two length scales become incommensurate for some particular incident angle, the competition between the two mechanisms can lead to modulated structures and other complex phenomena such as reentrant stability.

We investigate localized defect states near the edge of a band gap in a two-dimensional photonic crystal. An asymptotic approach based on Green’s functions leads to analytical results both for the frequency and for the spatial behavior of the defect states. In particular, we find a simple exponential law which relates the change in frequency of the defect states to the relative change in electrical energy of the Bloch modes on the band edge, and to the density of states in the photonic crystal. We find that the symmetries of the Bloch modes at band extrema play an important role in the manifestation and evolution of defect states. We confirm the analysis with numerical simulations based on the fictitious source superposition method.

In many physical systems it is expected that environmental decoherence will exhibit an asymmetry between dephasing and relaxation that may result in qubits experiencing discrete phase errors more frequently than discrete bit errors. In the presence of such an error asymmetry, an appropriately asymmetric quantum code—that is, a code that can correct more phase errors than bit errors—will be more efficient than a traditional, symmetric quantum code. Here we construct fault tolerant circuits to convert between an asymmetric subsystem code and a symmetric subsystem code. We show that, for a moderate error asymmetry, the failure rate of a logical circuit can be reduced by using a combined symmetric asymmetric system and that doing so does not preclude universality.

We propose a universal quantum computation scheme for trapped ions in thermal motion via the technique of adiabatic passage, which incorporates the advantages of both the adiabatic passage and the model of trapped ions in thermal motion. Our scheme is immune from the decoherence due to spontaneous emission from excited states as the system in our scheme evolves along a dark state. In our scheme the vibrational degrees of freedom are not required to be cooled to their ground states because they are only virtually excited. It is shown that the fidelity of the resultant gate operation is still high even when the magnitude of the effective Rabi frequency moderately deviates from the desired value.