Collisions between electrons and various molecular targets (${\mathrm{H}}_2\mathrm{O}$, $\mathrm{C}{\mathrm{H}}_4$, ${\mathrm{C}}_3{\mathrm{H}}_4$, ${\mathrm{N}}_2$) at projectile energies above the $K$-shell ionization threshold of the molecule have been investigated experimentally. From electron emission spectra, relative total ionization cross section ${\sigma }_t$ and $K$-shell ionization cross section ${\sigma }_K$ are determined. The ratio ${\sigma }_K∕{\sigma }_t$ is then deduced for each target as a function of the projectile energy and compared with those evaluated in the case of atomic targets. Strong differences between atomic and molecular targets are observed in the slope of the ratio at the highest projectile energies. These differences are explained using the well-known Kim-Rudd formula developed for atomic targets. In the projectile energy range we explored, we develop a simple empirical scaling law for the ratio ${\sigma }_K∕{\sigma }_t$ as a function of the projectile energy.

We report experimental demonstration of population transfer via stimulated Raman adiabatic passage with a rare-earth-ion-doped crystal $({\mathrm{Pr}}^{3+}:{\mathrm{Y}}_2\mathrm{Si}{\mathrm{O}}_5)$. About 90% of population of a ground level is transferred to another ground level with two pulses whose width is shorter than the lifetime of the excited state. The dependence of transferred population on the delay time of the pulse sequence is also experimentally investigated. The delay-time dependence observed in our experiment is similar to that obtained by numerical simulation.

We investigate Landau damping of Bogoliubov excitations in a dilute Bose gas moving in an optical lattice at finite temperatures. Using a one-dimensional tight-binding model, we explicitly obtain the Landau damping rate, the sign of which determines the stability of the condensate. We find that the sign changes at a certain condensate velocity, which is exactly the same as the critical velocity determined by the Landau criterion of superfluidity. This coincidence of the critical velocities reveals the microscopic mechanism of the Landau instability. This instability mechanism shows that a thermal cloud plays a crucial role in the breakdown of superfluids, since the thermal cloud is a vital source of Landau damping. We also examine the possibility of simultaneous disappearance of all damping processes.

We investigate the transition of a quasi-one-dimensional few-boson system from a weakly correlated to a fragmented and finally a fermionized ground state. Our numerically exact analysis, based on a multiconfigurational method, explores the interplay between different shapes of external and interparticle forces. Specifically, we demonstrate that the addition of a central barrier to an otherwise harmonic trap may support the system’s fragmentation, with a symmetry-induced distinction between even and odd atom numbers. Moreover, the impact of inhomogeneous interactions is studied, where the effective coupling strength is spatially modulated. It is laid out how the ground state can be displaced in a controlled way depending on the trap and the degree of modulation. We present the one- and two-body densities and, beyond that, highlight the role of correlations on the basis of the natural occupations.

Utilizing a two-photon absorption scheme in an alkali-metal vapor cell, we suggest a technique where a strong laser beam switches off another laser beam of different wavelength in femtosecond time scales.

In this paper we have extended our previous work [de Oliveira et al., Phys. Rev. A 65, 031401 (2002)] by measuring the $S$ and $D$ Rydberg lifetimes for ${}^{85}\mathrm{Rb}$ for principal quantum number between $n=26$ and 31 using a sample of cold Rydberg atoms. This range is limited by the amplitude of the electric field that we can generate in our present setup.

We investigate how tailoring the phase of the optical field allows us to select the different routes induced by time-delayed femtosecond pulse pairs exciting a two-photon transition in rubidium atoms. It is shown that by introducing an external phase in one of the pulses, the pathways leading to optical interferences can be eliminated, leaving only the quantum interferences.

We point out that quantum defect theory is the appropriate framework to explain and understand the behavior of the solutions of the Newton-Schrödinger equation. We find that, beyond ordinary quantum defect theory, the nonlinearity of the equation induces novel features, in particular a strong state dependence of the quantum defects. We show how this can be compensated by a rescaling of the energy unit.

We consider various dissipative atom-cavity systems and show that their collective dynamics can be used to maximize entanglement for intermediate values of the cavity leakage parameter $\kappa$. We first consider the interaction of a single two-level atom with one of two coupled microwave cavities and show analytically that the atom-cavity entanglement increases with cavity leakage. We next consider a system of two atoms passing successively through a cavity and derive the expression for the maximum value of $\kappa$ in terms of the Rabi angle $gt$, for which the two-atom entanglement can be increased. Finally, numerical investigation of micromaser dynamics also reveals the increase of two-atom entanglement with stronger cavity-environment coupling for experimentally attainable values of the micromaser parameters.

We present multiparty entanglement purification protocols that are capable of purifying arbitrary graph states directly. We develop recurrence and breeding protocols and compare our methods with strategies based on bipartite entanglement purification in static and communication scenarios. We find that direct multiparty purification is of advantage with respect to achievable yields and minimal required fidelity in static scenarios, and with respect to obtainable fidelity in the case of noisy operations in both scenarios.

We present a control procedure to reduce the stochastic ionization of hydrogen atom in a strong microwave field by adding to the original Hamiltonian a comparatively small control term which might consist of an additional set of microwave fields. This modification restores select invariant tori in the dynamics and prevents ionization. We demonstrate the procedure on the one-dimensional model of microwave ionization.

The two- and three-body Coulomb explosion of carbonyl sulfide (OCS) by $790\mathrm{nm}$, $50\mathrm{fs}$ laser pulses focused to $\approx {10}^{16}\mathrm{W}{\mathrm{cm}}^{-2}$ has been investigated by the three-dimensional covariance mapping technique. In a triatomic molecule, a single charge state, in this case the trication, has been observed to dissociate into two distinct energy channels. With the aid of a three-dimensional visualization technique to reveal the ionization hierarchy, evidence is presented for the existence of two sets of ionization pathways resulting from these two initial states. While one group of ions can be modeled using a classical enhanced ionization model, the second group, consisting of mainly asymmetric channels, cannot. The results provide clear evidence that an enhanced ionization approach must also be accompanied by an appreciation of the effects of excited ionic states and multielectronic processes.

We derive a system of nonpolynomial Schrödinger equations for one-dimensional wave functions of two components in a binary self-attractive Bose-Einstein condensate loaded in a cigar-shaped trap. The system is obtained by means of the variational approximation, starting from the coupled three-dimensional (3D) Gross-Pitaevskii equations and assuming, as usual, the factorization of 3D wave functions. The system can be obtained in a tractable form under a natural condition of symmetry between the two species. A family of vector (two-component) soliton solutions is constructed. Collisions between orthogonal solitons (ones belonging to the different components) are investigated by means of simulations. The collisions are essentially inelastic. They result in strong excitation of intrinsic vibrations in the solitons, and create a small orthogonal component (“shadow") in each colliding soliton. The collision may initiate collapse, which depends on the mass and velocities of the solitons.

We propose schemes for the deterministic preparation of pure multipartite entangled vibrational states for $N$ ions trapped in a single-mode cavity. Through sequentially sending lasers with appropriate phases and amplitudes to drive the ions, two kinds of pure multipartite entangled continuous variable entangled states respectively suggested by van Loock and Braunstein [Phys. Rev. Lett. 84, 3482 (2000)] and Pfister et al. [Phys. Rev. A 70, 020302(R) (2004)] can be unconditionally prepared with the help of cavity dissipation.

We show how ultracold polar molecules, suggested as a new platform for quantum computation, can be manipulated to switch “on” and “off” their strong dipole-dipole interactions. This can be accomplished through selective excitation of states with considerably different dipole moments. We discuss different schemes for quantum gates using real molecules: CO, LiH, and CaF, as examples of polar molecules which are being experimentally studied at ultracold temperatures. These schemes can be realized in several recently proposed architectures.

A major challenge for quantum computation in ion trap systems is scalable integration of error correction and fault tolerance. We analyze a distributed architecture with rapid high-fidelity local control within nodes and entangled links between nodes alleviating long-distance transport. We demonstrate fault-tolerant operator measurements which are used for error correction and nonlocal gates. This scheme is readily applied to linear ion traps which cannot be scaled up beyond a few ions per individual trap but which have access to a probabilistic entanglement mechanism. A proof-of-concept system is presented which is within the reach of current experiment.

The adiabatic theorem of quantum physics has been, in recent times, utilized in the design of local search quantum algorithms, and has been proven to be equivalent to standard quantum computation, that is, the use of unitary operators [D. Aharonov et al. in Proceedings of the 45th Annual Symposium on the Foundations of Computer Science, 2004, Rome, Italy (IEEE Computer Society Press, New York, 2004), pp. 42–51]. Hence, the study of the time complexity of adiabatic evolution algorithms gives insight into the computational power of quantum algorithms. In this paper, we present two different approaches of evaluating the time complexity for local adiabatic evolution using time-independent parameters, thus providing effective tests (not requiring the evaluation of the entire time-dependent gap function) for the time complexity of newly developed algorithms. We further illustrate our tests by displaying results from the numerical simulation of some problems, viz. specially modified instances of the Hamming weight problem.

In a departure from our previous work modeling antiproton capture on helium, with its relatively straightforward, fixed center target, extending the semiclassical approach to molecular hydrogen, a multicenter target with target masses identical to the projectile, requires that we reconsider the fundamental nature of our model, in particular the momentum-dependent Heisenberg core used to stabilize electrons in the ground state. Here we discuss the main features of our Kirschbaum-Wilets model of molecular hydrogen as it is calibrated against proton collision data, and then used to study antiproton collisions. Details of the collision process are presented, including the energy and angular momentum states of incident antiprotons that result in formation of $p\overline{p}$, and the resulting distribution of protonium atomic states.

We study effects of direct interatomic interaction on cooperative processes in atom-photon dynamics. Using a model of two-level atoms with Ising-type interaction as an example, it is demonstrated that interparticle interaction combined with atom-field coupling can introduce additional interatomic correlations acting as a phase synchronizing factor. For the case of weakly interacting atoms with $J<\hslash {\omega }_0$, where $J$ is the interparticle coupling constant and ${\omega }_0$ is the atomic frequency, dynamical regimes of cooperative relaxation of atoms are analyzed in the Born-Markov approximation both numerically and using the mean field approximation. We show that interparticle correlations induced by the direct interaction result in inhibition of incoherent spontaneous decay leading to the regime of collective pulse relaxation which differs from superradiance in nature. For superradiant transition, the synchronizing effect of interatomic interaction is found to manifest itself in enhancement of superradiance. When the interaction is strong and $J>\hslash {\omega }_0$, one-particle one-photon transitions are excluded and transition to the regime of multiphoton relaxation occurs. Using a simple model of two atoms in a high-$Q$ single mode cavity we show that such transition is accompanied by Rabi oscillations involving many-atom multiphoton states. Dephasing effects of dipole-dipole interaction and solitonic mechanism of relaxation are discussed.