Non-local quantum gates: a cavity-quantum-electrodynamics implementation

The problems related to the management of large quantum registers could be handled in the context of distributed quantum computation: unitary non-local transformations among spatially separated local processors are realized performing local unitary transformations and exchanging classical communication. In this paper, a scheme is proposed for the implementation of universal non-local quantum gates such as a controlled NOT (CNOT) and a controlled quantum phase gate (CQPG). The system chosen for their physical implementation is a cavity-quantum-electrodynamics (CQED) system formed by two spatially separated microwave cavities and two trapped Rydberg atoms. The procedures to follow for the realization of each step necessary to perform a specific non-local operation are described.

Quantum manifestations of closed orbits in the photoexcitation scaled spectrum of the hydrogen atom in crossed fields

The scaled photoexcitation spectrum of the hydrogen atom in crossed electric and magnetic fields has been obtained by means of accurate quantum mechanical calculation using a new algorithm. Closed orbits in the corresponding classical system have also been obtained, using a new, efficient and practical searching procedure. Two new classes of closed orbit have been identified. Fourier transforming each photoexcitation quantum spectrum to yield a plot against scaled action has allowed direct comparison between peaks in such plots and the scaled action values of closed orbits, Excellent agreement has been found with all peaks assigned.

Path-integral study of a two-dimensional Lennard-Jones glass

The glass transition in a quantum Lennard-Jones mixture is investigated by constant-volume path-integral simulations. Particles are assumed to be distinguishable, and the strength of quantum effects is varied by changing h from zero (the classical case) to one (corresponding to a highly quantum-mechanical regime). Quantum delocalization and zero point energy drastically reduce the sensitivity of structural and thermodynamic properties to the glass transition. Nevertheless, the glass transition temperature T-g can be determined by analyzing the phase space mobility of path-integral centroids. At constant volume, the T-g of the simulated model increases monotonically with increasing h. Low temperature tunneling centers are identified, and the quantum versus thermal character of each center is analyzed. The relation between these centers and soft quasilocalized harmonic vibrations is investigated. Periodic minimizations of the potential energy with respect to the positions of the particles are performed to determine the inherent structure of classical and quantum glassy samples. The geometries corresponding to these energy minima are found to be qualitatively similar in all cases. Systematic comparisons for ordered and disordered structures, harmonic and anharmonic dynamics, classical and quantum systems show that disorder, anharmonicity, and quantum effects are closely interlinked.

Kinetics of phase transformations in a model with metastable fluid-fluid separation: A molecular dynamics study

By molecular dynamics (MD) simulations we study the crystallization process in a model system whose particles interact by a spherical pair potential with a narrow and deep attractive well adjacent to a hard repulsive core. The phase diagram of the model displays a solid-fluid equilibrium, with a metastable fluid-fluid separation. Our computations are restricted to fairly small systems (from 2592 to 10368 particles) and cover long simulation times, with constant energy trajectories extending up to 76x10(6) MD steps. By progressively reducing the system temperature below the solid-fluid line, we first observe the metastable fluid-fluid separation, occurring readily and almost reversibly upon crossing the corresponding line in the phase diagram. The nucleation of the crystal phase takes place when the system is in the two-fluid metastable region. Analysis of the temperature dependence of the nucleation time allows us to estimate directly the nucleation free energy barrier. The results are compared with the predictions of classical nucleation theory. The critical nucleus is identified, and its structure is found to be predominantly fcc. Following nucleation, the solid phase grows steadily across the system, incorporating a large number of localized and extended defects. We discuss the relaxation processes taking place both during and after the crystallization stage. The relevance of our simulation for the kinetics of protein crystallization under normal experimental conditions is discussed. (C) 2002 American Institute of Physics.

Thin Ta films: growth, stability, and diffusion studied by molecular-dynamics simulations

We report results of classical molecular-dynamics simulations of bcc and beta-Ta thin films. Thermal PVD film growth, surface roughness, argon ion bombardment, phase stability and transformation, vacancy and adatom diffusion, and thermal relaxation kinetics are discussed. Distinct differences between the two structures are observed, including a complex vacancy diffusion mechanism in beta-Ta. Embedded atom method potentials, which were fitted to bcc properties, have been used to model the Ta-Ta interactions. In order to verify the application of these potentials to the more complex beta-Ta structure, we have also performed density functional theory calculations. Results and implications of these calculations are discussed.

Concertedness and solvent effects in multiple proton transfer reactions: The formic acid dimer in solution

The issue of multiple proton transfer (PT) reactions in solution is addressed by performing molecular dynamics simulations for a formic acid dimer embedded in a water cluster. The reactant species is treated quantum mechanically, within a density functional approach, while the solvent is represented by a classical model. By constraining different distances within the dimer we analyze the PT process in a variety of situations representative of more complex environments. Free energy profiles are presented, and analyzed in terms of typical solvated configurations extracted from the simulations. A decrease in the PT barrier height upon solvation is rationalized in terms of a transition state which is more polarized than the stable states. The dynamics of the double PT process is studied in a low-barrier case and correlated with solvent polarization fluctuations. Cooperative effects in the motion of the two protons are observed in two different situations: when the solvent polarization does not favor the transfer of one of the two protons and when the motion of the two protons is not synchronized. This body of observations is correlated with local structural and dynamical properties of the solvent in the vicinity of the reactant. (C) 2000 American Institute of Physics. [S0021-9606(00)51121-0].

Development of complex classical force fields through force matching to ab initio data: Application to a room-temperature ionic liquid

Recent experimental neutron diffraction data and ab initio molecular dynamics simulation of the ionic liquid dimethylimidazolium chloride ([dmim]Cl) have provided a structural description of the system at the molecular level. However, partial radial distribution functions calculated from the latter, when compared to previous classical simulation results, highlight some limitations in the structural description offered by force fieldbased simulations. With the availability of ab initio data it is possible to improve the classical description of [dmim]Cl by using the force matching approach, and the strategy for fitting complex force fields in their original functional form is discussed. A self-consistent optimization method for the generation of classical potentials of general functional form is presented and applied, and a force field that better reproduces the observed first principles forces is obtained. When used in simulation, it predicts structural data which reproduces more faithfully that observed in the ab initio studies. Some possible refinements to the technique, its application, and the general suitability of common potential energy functions used within many ionic liquid force fields are discussed.

Correlated electron-ion dynamics: the excitation of atomic motion by energetic electrons

Correlated electron-ion dynamics (CEID) is an extension of molecular dynamics that allows us to introduce in a correct manner the exchange of energy between electrons and ions. The formalism is based on a systematic approximation: small amplitude moment expansion. This formalism is extended here to include the explicit quantum spread of the ions and a generalization of the Hartree-Fock approximation for incoherent sums of Slater determinants. We demonstrate that the resultant dynamical equations reproduce analytically the selection rules for inelastic electron-phonon scattering from perturbation theory, which control the mutually driven excitations of the two interacting subsystems. We then use CEID to make direct numerical simulations of inelastic current-voltage spectroscopy in atomic wires, and to exhibit the crossover from ionic cooling to heating as a function of the relative degree of excitation of the electronic and ionic subsystems.

Non-linear conductance of disordered quantum wires

The self-consistent electron potential in a current-carrying disordered quantum wire is spatially inhomogeneous due to the formation of resistivity dipoles across scattering centres. In this paper it is argued that these inhomogeneities in the potential result in a suppression of the differential conductance of such a wire at finite applied voltage. A semi-classical argument allows this suppression, quadratic in the voltage, to be related directly to the amount of intrinsic defect scattering in the wire. This result is then tested against numerical calculations.

Dynamical simulation of inelastic quantum transport

A method for correlated quantum electron-ion dynamics is combined with a method for electronic open boundaries to simulate in real time the heating, and eventual equilibration at an elevated vibrational energy, of a quantum ion under current flow in an atomic wire, together with the response of the current to the ionic heating. The method can also be used to extract inelastic current voltage corrections under steady-state conditions. However, in its present form the open-boundary method contains an approximation that limits the resolution of current-voltage features. The results of the simulations are tested against analytical results from scattering theory. Directions for the improvement of the method are summarized at the end.

State-Selective Photodissociation Dynamics of Formaldehyde: Near Threshold Studies of the H+HCO Product Channel

The laser-induced photodissociation of formaldehyde in the wavelength range 309<λ<330nm 309<λ<330nm has been investigated using H (Rydberg) atom photofragment translational spectroscopy. Photolysis wavelengths corresponding to specific rovibronic transitions in the A ˜ A 2 1 ←X ˜ A 1 1 ÃA21←X̃A11 2 1 0 4 3 0 201403 , 2 2 0 4 1 0 202401 , 2 2 0 4 3 0 202403 , 2 3 0 4 1 0 203401 , and 2 1 0 5 1 0 201501 bands of H 2 CO H2CO were studied. The total kinetic energy release spectra so derived can be used to determine partial rotational state population distributions of the HCO cofragment. HCO product state distributions have been derived following the population of various different N K a NKa levels in the A ˜ A 2 1 ÃA21 2 2 4 3 2243 and 2 3 4 1 2341 states. Two distinct spectral signatures are identified, suggesting competition between dissociation pathways involving the X ˜ A 1 1 X̃A11 and the a ˜ A 2 3 ãA23 potential energy surfaces. Most rovibrational states of H 2 CO(A ˜ A 2 1 ) H2CO(ÃA21) investigated in this work produceH+HCO(X ˜ A ′ 2 ) H+HCO(X̃A′2) photofragments with a broad kinetic energy distribution and significant population in high energy rotational states of HCO. Photodissociation via the A ˜ A 2 1 ÃA21 2 2 4 3 2243 1 1,1 11,1 (and 1 1,0 11,0 ) rovibronic states yields predominantly HCO fragments with low internal energy, a signature that these rovibronic levels are perturbed by the a ˜ A 2 3 ãA23 state. The results also suggest the need for further careful measurements of the H+HCO H+HCO quantum yield from H 2 CO H2CO photolysis at energies approaching, and above, the barrier to C–H bond fission on the a ˜ A 2 3 ãA23 potential energy surface.

Quantum revivals in ultrashort intense field dissociation of molecular ions

Ultrashort (<15 fs) high intensity (1014-1016 W cm-2) laser pulses have provided novel methods for investigation of the dynamics of simple molecular ions such as H2+ and D2+. In this paper we report on simulations carried out for the D2+ molecular ion, within the Born- Oppenheimer and two-state approximations. These simulations allow one to investigate the dissociation dynamics of the D2+ molecular ion when subjected to such ultrashort, intense laser pulses. In particular, these simulations are compared to the results from recent pump-probe experiments, in which, the nuclear vibrational motion of D2+ has been imaged. Simulations suggest that the nature of the dissociation process, be it 1- or 2-photon, may be influenced by the tuning of the pump-probe delay time.

Liquid structure of the ionic liquid, 1-methyl-4-cyanopyridinium bis{(trifluoromethyl)sulfonyl}imide determined from neutron scattering and molecular dynamics simulations

The liquid structure of 1-methyl-4-cyanopyridinium bis {(trifluoromethyl)sulfonyl}imide, a prototypical ionic liquid containing an electron-withdrawing group on the cation, has been investigated at 368 K. Experimental neutron scattering combined with empirical potential structure refinement analysis of the data and classical molecular dynamics simulations have been used to probe the liquid structure in detail. Both techniques generated highly consistent results that provide valuable validation of the force fields and refinement approaches. A significant degree of apparent charge ordering is found in the liquid structure, although the nonspherical shape of the ions results in interpenetration of cations into the first shell of adjacent cations, with much shorter closest contact distances than the averaged center-of-mass cation-cation and cation-anion separations.

Newtonian origin of the spin motive force in ferromagnetic atomic wires

We demonstrate numerically the existence of a spin-motive force acting on spin carriers when moving in a time and space dependent internal ?eld. This is the case for electrons in a one-dimensional wire with a precessing domain wall. The effect can be explained solely by adiabatic dynamics and is shown to exist for both classical and quantum systems.

Reducing quantum control for spin-spin entanglement distribution

We present a protocol that sets maximum stationary entanglement between remote spins through scattering of mobile mediators without initialization, post-selection or feedback of the mediators' state. No time-resolved tuning is needed and, counterintuitively, the protocol generates two-qubit singlet states even when classical mediators are used. The mechanism responsible for this effect is resilient against non-optimal coupling strengths and dephasing affecting the spins. The scheme uses itinerant particles and scattering centres and can be implemented in various settings. When quantum dots and photons are used a striking result is found: injection of classical mediators, rather than quantum ones, improves the scheme efficiency.