We introduce a scheme to perform the cooling algorithm, first presented by Boykin et al. in 2002, for an arbitrary number of times on the same set of qbits. We achieve this goal by adding an additional SWAP gate and a bath contact to the algorithm. This way one qbit may repeatedly be cooled without adding additional qbits to the system. By using a product Liouville space to model the bath contact we calculate the density matrix of the system after a given number of applications of the algorithm.

A theoretical model has been developed to describe the probability of charge transfer from helium cations to dopant molecules inside helium nanodroplets following electron-impact ionization. The location of the initial charge site inside helium nanodroplets subject to electron impact has been investigated and is found to play an important role in understanding the ionization of dopants inside helium droplets. The model is consistent with a charge migration process in small helium droplets that is strongly directed by intermolecular forces originating from the dopant, whereas for large droplets (tens of thousands of helium atoms and larger) the charge migration increasingly takes on the character of a random walk. This suggests a clear droplet size limit for the use of electron-impact mass spectrometry for detecting molecules in helium droplets.

The harmonic emission from cluster nanoplasmas subject to short, intense infrared laser pulses is analyzed by means of particle-in-cell simulations. A pronounced resonant enhancement of the low-order harmonic yields is found when the Mie plasma frequency of the ionizing and expanding cluster resonates with the respective harmonic frequency. We show that a strong, nonlinear resonant coupling of the cluster electrons with the laser field inhibits coherent electron motion, suppressing the emitted radiation and restricting the spectrum to only low-order harmonics. A pump-probe scheme is suggested to monitor the ionization dynamics of the expanding clusters.

The electric field gradient in late transition metal compounds is incorrectly determined by most density functionals. We show that the coupling of short-range density functional based with long-range wave function based methods using a reparametrization of the Coulomb-attenuated Becke three-parameter Lee-Yang-Parr approximation gives reliable results for the electric field gradients of copper and gold for a series of compounds. This results in nuclear quadrupole moments of $-0.208\mathrm{b}$ for ${}^{63}\mathrm{Cu}$ and $+0.526\mathrm{b}$ for ${}^{197}\mathrm{Au}$ in good agreement with experimental values of $-0.220\left(15\right)$ and $+0.547\left(16\right)\mathrm{b}$, respectively.

Through multiphoton ionization measurements, the polarization effects in destructive quantum interference under three-photon resonant excitation have been studied. Recent observations [V. Peet, Phys. Rev. A 74, 033406 (2006)] have indicated that contrary to the well-known pattern of a total suppression of resonance excitation, the destructive interference becomes incomplete if three-photon transition is driven by crossed beams with orthogonal polarization planes. These observations have been tested for a more general case of two-color excitation and very similar polarization-dependent anomalies in the interference character have been registered. It has been shown that the destructive interference is modified and the resonance excitation does occur if two crossed laser beams have opposite circular polarizations. The pressure-induced evolution of the uncanceled ionization peaks has the ratio of blue shift to width close to 0.5 exactly as it is known for resonance ionization peaks registered under excitation by counterpropagating laser beams.

When an entangled state evolves under local unitaries, the entanglement in the state remains fixed. Here we show that the dynamical phase acquired by an entangled state in such a scenario can always be understood as the sum of the dynamical phases of its subsystems. In contrast, the equivalent statement for the geometric phase is not generally true unless the state is separable. For an entangled state an additional term is present, the mutual geometric phase, that measures the change the additional correlations present in the entangled state make to the geometry of the state space. For $N$ qubit states we find that this change can be explained solely by classical correlations for states with a Schmidt decomposition and solely by quantum correlations for $W$ states.

A semiclassical Bohr-Sommerfeld approximation is derived for an $N$-particle, two-mode Bose-Hubbard system modeling a Bose-Einstein condensate in a double-well potential. This semiclassical description is based on the “classical” dynamics of the mean-field Gross-Pitaevskii equation and is expected to be valid for large $N$. We demonstrate the possibility to reconstruct quantum properties of the $N$-particle system from the mean-field dynamics. The resulting semiclassical eigenvalues and eigenstates are found to be in very good agreement with the exact ones, even for small values of $N$, both for subcritical and supercritical particle interaction strength where tunneling has to be taken into account.

The electronic state spectroscopy of perfluorocyclobutane $\left(c\text{−}{\mathrm{C}}_4{\mathrm{F}}_8\right)$ has been investigated using high resolution vacuum ultraviolet (vuv) photoabsorption spectroscopy in the energy range $6.0–11\mathrm{eV}$. The electron energy loss spectrum (EELS) was also recorded in the nonelectric dipolar interaction mode ($100\mathrm{eV}$ incident energy, 10° scattering angle) over the $8–14\mathrm{eV}$ energy-loss range and the excited states in the $11–14\mathrm{eV}$ spectral region have been observed. An $\mathrm{He}\mathrm{I}$ photoelectron spectrum recorded between 11.0 and $19.8\mathrm{eV}$ is compared with earlier lower resolution results. This has allowed us to derive a more precise value of $12.291\pm 0.002\mathrm{eV}$ for the ground neutral state vertical ionization energy. All spectra presented in this paper represent the highest resolution data yet reported for perfluorocyclobutane, to the best of our knowledge. Ab initio calculations have been performed for helping in the assignment of the spectral bands for both neutral excited states and ionic states.

We propose a method of manipulating selectively the symmetric Dicke subspace in the internal degrees of freedom of $N$ trapped ions. We show that the direct access to ionic-motional subspaces, based on a suitable tuning of motion-dependent ac Stark shifts, induces a two-level dynamics involving previously selected ionic Dicke states. In this manner, it is possible to produce, sequentially and unitarily, ionic Dicke states with increasing excitation number. Moreover, we propose a probabilistic technique to produce directly any ionic Dicke state assuming suitable initial conditions.

We present a simple quantum mechanical model to describe Coulomb explosion of $\mathrm{H}_2{}^{+}$ and $\mathrm{D}_2{}^{+}$ by short, intense infrared laser pulses. The model is based on the length gauge version of the molecular strong-field approximation and is valid when the process of dissociation prior to ionization is negligible. The results are compared with recent experimental data for the proton kinetic energy spectrum [Th. Ergler et al., Phys. Rev. Lett. 95, 093001 (2005); D. S. Murphy et al., J. Phys. B 40, S359 (2007)]. Using a Franck-Condon distribution over initial vibrational states, the theory reproduces the overall shape of the spectrum with only a small overestimation of slow protons. The agreement between theory and experiment can be made perfect by using a non-Frank-Condon initial distribution characteristic for $\mathrm{H}_2{}^{+}\left(\mathrm{D}_2{}^{+}\right)$ targets produced by strong-field ionization of ${\mathrm{H}}_2\left({\mathrm{D}}_2\right)$. For comparison, we also present results obtained by two different tunneling models for this process.

We study the rotation of atoms in one-dimensional lattice rings. In particular, the “fast mode,” where the ground-state atoms rotate faster than the stirring rotating the atoms, is studied both analytically and numerically. The conditions for the transition to the fast mode are found to be very different from those in continuum rings. We argue that these transition frequencies remain unchanged for bosonic condensates described in a mean field. We show that fermionic interactions and filling factors have a significant effect on the transition to the fast mode and the Pauli principle may suppress it altogether.

Superconductivity and superfluidity of fermions require, within the BCS theory, matching of the Fermi energies of the two interacting Fermion species. Difference in the number densities of the two species leads either to a normal state, to phase separation, or—potentially—to exotic forms of superfluidity such as Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, Sarma state, or breached pair state. We consider ultracold Fermi gases with polarization, i.e., spin-density imbalance. We show that, due to the gases being trapped and isolated from the environment in terms of particle exchange, exotic forms of superfluidity appear as a shell around the Bardeen-Cooper-Schrieffer (BCS) superfluid core of the gas and, for large density imbalance, in the core as well. We obtain these results by describing the effect of the trapping potential by using the Bogoliubov-de Gennes equations. For comparison to experiments, we calculate also the condensate fraction, and show that, in the center of the trap, a polarized superfluid leads to a small dip in the central density difference. We compare the results to those given by local density approximation and find qualitatively different behavior.

A unidirectional optical oscillator consists of an optical amplifier whose outgoing field is reinjected into the incoming one by means of a ring cavity. We used recently a liquid crystal light valve as optical amplifier and reported experimental evidence of cavity field oscillations. The light-valve provides a high gain and a wide transverse size, activating a large number of cavity modes, both transverse and longitudinal. A mean-field approximation along the cavity axis is not suitable for this system, thus we introduce a model which accounts for the activation of different transverse and longitudinal modes and shows numerically that their interaction generates three-dimensional spatiotemporal pulses localized along the cavity axis. The generation of these pulses is experimentally verified.

The propagation of an arbitrary polarized few-cycle ultrashort laser pulse in a degenerate three-level medium is investigated by using an iterative predictor-corrector finite-difference time-domain method. It is found that the polarization evolution of the ultrashort laser pulse is dependent not only on the initial atomic coherence of the medium but also on the polarization condition of the incident laser pulse. When the initial effective area is equal to $2\pi$, complete linear-to-circular and circular-to-linear polarization conversion of few-cycle ultrashort laser pulses can be achieved due to the quantum interference effects between the two different transition paths.

The potential of controlling the locations of the cutoff and higher opaque ranges (ORs) in transmission of $s$-polarized waves through metallic photonic crystals (PCs) by varying direction of incidence is demonstrated. The emphasis is put on the case of rather thick rods while the plasma frequency of the rod material is comparable with the frequency of the incident wave. It is shown that both weak and strong sensitivity to the angle variation can be realized by a proper choice of parameters of the rods and host medium. In particular, the difference in cutoff values can be varied from a few percent in the case of a high-$\epsilon$ host medium to several tens percent in the case of an air medium, while the angle is varied from 0 to $\pi ∕3$. Approximate formulas, which have been derived for cases rather far from the considered ones, can still be used for obtaining majorant estimates. The effects exerted by the plasma frequency and rod shape on the observed features are discussed. In addition, situations are considered where the cutoffs disappear.

When molecules interact with light sources of femtosecond or shorter duration the rotational degrees of freedom are frozen during the response to the strong nonperturbative interaction. We analytically derive how the frozen degrees of freedom affect the measurable signals in high-order harmonic generation and above-threshold ionization. High-order harmonic generation exhibits optical coherence in the signal from different orientations of the molecule. For ionization, the contributions from different orientations are added incoherently. The consequences of these findings are illustrated by numerical results.

We present an analytic example of a two-dimensional quantum-mechanical system, where the exponential suppression of the probability of overbarrier reflection changes nonmonotonically with energy. The suppression is minimal at certain “optimal” energies where reflection occurs with exponentially larger probability than at other energies.

The linear optical creation of Gaussian cluster states, a potential resource for universal quantum computation, is investigated. First, using Bloch-Messiah reduction, we show how to achieve canonical cluster-state generation, otherwise based on pairwise acting quantum nondemolition gates, by off-line squeezers and beam splitters. Moreover, we find that, in terms of squeezing resources, the canonical states are rather wasteful. Hence we propose a systematic way to create a whole family of cluster-type states, including potentially cheaper states. Any given cluster (or graph) state can be realized this way. As an example, we consider a protocol in which a single-mode quantum state propagates through a multiple-rail cluster. Such a redundant encoding may reduce errors due to finite squeezing.