We present a strategy for postpulse molecular orientation aiming both at efficiency and maximal duration within a rotational period. We first identify the optimally oriented states which fulfill both requirements. We show that a sequence of half-cycle pulses of moderate intensity can be devised for reaching these target states.

An optical source that produces single-photon pulses on demand has potential applications in linear optics quantum computation, provided that stringent requirements on indistinguishability and collection efficiency of the generated photons are met. We show that these are conflicting requirements for anharmonic emitters that are incoherently pumped via reservoirs. As a model for a coherently pumped single photon source, we propose cavity-assisted spin-flip Raman transitions in a single electron charged quantum dot embedded in a microcavity. We demonstrate that using such a source, arbitrarily high collection efficiency and indistinguishability of the generated photons can be obtained simultaneously with increased cavity coupling. We analyze the role of errors that arise from distinguishability of the single-photon pulses in linear optics quantum gates by relating the gate fidelity to the strength of the two-photon interference dip in photon cross-correlation measurements. We find that performing controlled phase operations with error $<1\%$ requires nanocavities with Purcell factors $F_P⩾40$ in the absence of dephasing, without necessitating the strong coupling limit.

The dynamics of a Bose-Einstein condensate in a harmonic trap with a nonadiabatically loaded optical lattice is studied analytically. As the global effect of the optical potential can be described in terms of a renormalized interaction coupling constant and of an effective mass in the laser direction, a fast loading can be understood as a sudden change of those characteristic parameters. In this approach, a standard scaling transformation is applied to study the resulting dynamics, in particular, the generation of collective modes. The relevance of the excited modes to the interference patterns obtained after free expansion is analyzed. The applicability of trap-frequency adjustments as a strategy for suppressing the loading induced excitations is discussed.

The dynamics of a two-level atom driven by a single laser beam and three-level atom (Lambda configuration) irradiated by two laser beams are studied taking into account of the quantized center-of-mass motion of the atom. It is shown that the trapped atom system under appropriate resonance condition exhibits the large time-scale revivals when the index of the vibrational sideband responsible for the atomic electronic transition is greater than unity. The revival times are shown to be dependent on the initial number of vibrational excitations and the magnitude of the Lamb-Dicke parameter. The sub-Poissonian statistics in vibrational quantum number is observed at certain time intervals. The minimum time of interaction for which the squeezed states of motional quadrature are generated is found to be decreasing with the increase in the Lamb-Dicke parameter.

Cesium vapor interacting with a tightly focused, intense midinfrared $\left(3.4\mu \mathrm{m}\right)$ source produces high harmonic radiation in the visible∕UV spectral range. The measured yields of harmonic orders 9–17 are found to yield good quantitative agreement with numerical simulations that include both the single-atom and the macroscopic response. The $5\text{th}–9\text{th}$ harmonic orders are generated with sufficient pulse energies $(\sim 100\text{pJ})$ for direct temporal measurements using an autocorrelation method and when correlated with bandwidth measurements are found not to be transform limited. A blueshift connected to a strong time-dependent ionization appears to be the cause of this spectral broadening.

We study the dynamics of a Heisenberg $XY$ spin chain with an unknown state coded into one qubit or a pair of entangled qubits, with the rest of the spins being in a polarized state. The time evolution involves magnon excitations, and through them the entanglement is transported across the channel. For a large number of qubits, explicit formulas for the concurrences, measures for two-qubit entanglements, and the fidelity for recovering the state some distance away are calculated as functions of time. Initial states with an entangled pair of qubits show better fidelity, which takes its first maximum value at earlier times, compared to initial states with no entangled pair. In particular initial states with a pair of qubits in an unknown state $\alpha \uparrow \uparrow +\beta \downarrow \downarrow$ are best suited for quantum-state transport.

The application of dynamical decoupling pulses to a single qubit interacting with a linear harmonic oscillator bath with $1/f$ (more generally, strong low-frequency) spectral density is studied, and compared to the Ohmic case. Decoupling pulses that are slower than the fastest bath time scale are shown to drastically reduce the decoherence rate in the $1/f$ case. Contrary to conclusions drawn from previous studies, this shows that dynamical decoupling pulses do not always have to be ultrafast. Our results explain the recent experiment in which dephasing due to $1/f$ charge noise affecting a charge qubit in a small superconducting electrode was successfully suppressed using spin-echo-type gate-voltage pulses.

We investigate a quasi-one-dimensional system of trapped cold bosonic atoms in an optical lattice by using the density-matrix renormalization group to study the Bose-Hubbard model at $T=0$ for experimentally realistic numbers of lattice sites. It is shown that a properly rescaled one-particle density matrix characterizes superfluid versus insulating states just as in the homogeneous system. For typical parabolic traps we also confirm the widely used local-density approach for describing correlations in the limit of weak interaction. Finally, we note that the superfluid to Mott-insulating transition is seen most directly in the half-width of the interference peak.

What correlations are present in the ground state of a many-body Hamiltonian? We study the relationship between ground-state correlations, especially entanglement, and the energy gap between the ground and first excited states. We prove several general inequalities which show quantitatively that ground-state correlations between systems not directly coupled by the Hamiltonian necessarily imply a small energy gap.

Let $S$ be the von Neumann entropy of a finite ensemble $\mathcal{E}$ of pure quantum states. We show that $S$ may be naturally viewed as a function of a set of geometrical volumes in Hilbert space defined by the states and that $S$ is monotonically increasing in each of these variables. Since $S$ is the Schumacher compression limit of $\mathcal{E}$, this monotonicity property suggests a geometrical interpretation of the quantum redundancy involved in the compression process. It provides clarification of previous work in which it was shown that $S$ may be increased while increasing the overlap of each pair of states in the ensemble. As a by-product, our mathematical techniques also provide an interpretation of the subentropy of $\mathcal{E}$.

We investigate the double K-shell ionization of heliumlike ions caused by the absorption of a single photon with energies being much smaller than the rest energy of an electron. In the near-threshold region, differential and total cross sections of the process are calculated for light ions, taking into account the leading orders of the $1/Z$ and $\alpha Z$ expansions. QED perturbation theory with respect to the parameter $1/Z$ exhibits a fast convergence in the entire nonrelativistic domain for moderate nuclear charge numbers $Z>~2.$ Going beyond the electric dipole approximation leads to a forward/backward asymmetry in the angular distributions for the ejected electrons with respect to the incident photon beam. A comparison of theoretical predictions for the ratio of double-to-single photoionization cross sections with available experimental data for a number of neutral atoms is also presented.

In this paper we propose to control the bond length of a diatomic molecule and its vibrational coherent motion over arbitrary distances. The method requires (a) mixing a dissociative potential with bound electronic configurations in order to stabilize the continuum, and (b) manipulating the wave-packet transfer below the threshold of adiabatic passage. Using asymmetric pulses it is possible to implement sudden-adiabatic transitions to stop or induce the motion in the light-induced potential, controlling the nonadiabatic dynamics in a most efficient way.

It has been suggested by Timmermans [Phys. Rev. Lett. 87, 240403 (2001)] that loss of fermions in a degenerate system causes strong heating. We address the fundamental limit imposed by this loss on the temperature that may be obtained by sympathetic cooling of fermions by bosons. Both a quantum Boltzmann equation and a quantum Boltzmann master equation are used to study the evolution of the occupation number distribution. It is shown that, in the thermodynamic limit, the Fermi gas cools to a minimal temperature $k_{\mathrm{B}}T∕\mu \propto {({\gamma }_{\text{loss}}∕{\gamma }_{\text{coll}})}^{0.44}$, where ${\gamma }_{\text{loss}}$ is a constant loss rate, ${\gamma }_{\text{coll}}$ is the bare fermion-boson collision rate not including the reduction due to Fermi statistics, and $\mu \sim k_{\mathrm{B}}{T}_{\mathrm{F}}$ is the chemical potential. It is demonstrated that, beyond the thermodynamic limit, the discrete nature of the momentum spectrum of the system can block cooling. The unusual nonthermal nature of the number distribution is illustrated from several points of view: the Fermi surface is distorted, and in the region of zero momentum the number distribution can descend to values significantly less than unity. Our model explicitly depends on a constant evaporation rate, the value of which can strongly affect the minimum temperature.

We propose quantum devices that can realize probabilistically different projective measurements on a qubit. The desired measurement basis is selected by the quantum state of a program register. First we analyze the phase-covariant multimeters for a large class of program states and then the universal multimeters for a special choice of program. In both cases we start with deterministic but error-prone devices and then proceed to devices that never make a mistake but from time to time give an inconclusive result. These multimeters are optimized (for a given type of program) with respect to the minimum probability of an inconclusive result. This concept is further generalized to multimeters that minimize the error rate for a given probability of an inconclusive result (or vice versa). Finally, we propose a generalization for qudits.

We show that a variety of cosmologically motivated effective quasiparticle space-times can be produced in harmonically trapped superfluid Bose and Fermi gases. We study the analog of cosmological particle production in these effective space-times, induced by trapping potentials and coupling constants possessing an arbitrary time dependence. The WKB probabilities for phonon creation from the superfluid vacuum are calculated, and an experimental procedure to detect quasiparticle production by measuring density-density correlation functions is proposed.

We propose a scheme to generate maximally entangled states (MESs) of multiple three-level atoms in microwave cavity QED based on the resonant atom-cavity interaction. In the scheme, multiple three-level atoms initially in their ground states are sequently sent through two suitably prepared cavities. After a process of appropriate atom-cavity interaction, a subsequent measurement on the second cavity field projects the atoms onto the MESs. The practical feasibility of this method is also discussed.

We consider sets of quantum observables corresponding to eutactic stars. Eutactic stars are systems of vectors which are the lower-dimensional “shadow” image, the orthogonal view, of higher-dimensional orthonormal bases. Although these vector systems are not comeasurable, they represent redundant coordinate bases with remarkable properties. One application is quantum secret sharing.

We combine the ideas of qubit encoding and dispersive dynamics to enable robust and easy quantum-information processing on paired superconducting charge boxes sharing a common bias lead. We establish a decoherence free subspace on these and introduce universal gates by dispersive interaction with a $\mathrm{LC}$ resonator and inductive couplings between the encoded qubits. These gates preserve the code space and only require the established local symmetry and the control of the voltage bias. The principle of dispersive manipulation of encoded qubits is general and beneficial to other physical systems.

Quantum algorithms are built enabling Poincaré recurrence times and periodic orbits of classical dynamical systems to be found. It is shown that exponential gain compared to classical algorithms can be reached for a restricted class of systems. Quadratic gain can be achieved for a larger set of dynamical systems. The simplest cases can be implemented with a small number of qubits.

We present multiply differential cross sections for electron-impact ionization of the water molecule. The experimental results are compared with theoretical cross sections calculated using a recently developed distorted-wave Born approach for molecules. The experimental cross sections exhibit a very large recoil scattering, which is not predicted by the theory. This has implications for applications of this theoretical approach in areas such as modeling of ionization in biological systems.