Kyungsun Na

Quantum hydrodynamic analysis of decoherence: quantum trajectories and stress tensor

Abstract Quantum trajectories, obtained by integrating equations of motion for elements of the probability fluid, are used to analyze decoherence in a model two-mode system. Analysis of trajectories, flux maps, and the stress tensor for two composite systems, in one of which the system is uncoupled from the environment, leads to a hydrodynamic interpretation of the decoherence process.

Quantum trajectories for resonant scattering

Time-dependent wave packet resonant scattering for the double square barrier has been studied in terms of Bohm quantum trajectories. The high transmission probability for the wave packet with a resonant energy can be explained by the behavior of the quantum trajectories under the influence of the relatively slow formation of a node within the first barrier. This node splits the trajectories into reflected and transmitted components. During this stage, many particle trajectories pass through the double-barrier region and contribute to the transmitted part of the wave packet. Due to the transient nature of the nodes, trajectories in the reflected wave packet bunch together between the nodes for a finite period of time so that temporary structure (localization of particles and accompanying increase in the probability density) develops on small length scales. These calculations also show that the particles gain high momentum near the nodal points, and they reach a uniform momentum distribution after transmitting the barrier region. We have found that the presence of a node between the two barriers influences the different lifetimes of the quasi-bound states.

Electron Conductance and Lifetimes in a Ballistic Electron Waveguide

Ballistic electron waveguides are open quantum systems that can be formed at very low temperatures at a GaAs/AlGaAs interface. Dissipation due to electron–phonon and electron–electron interactions in these systems is negligible. Although the electrons only interact with the walls of the waveguide, they can have a complicated spectrum including both positive energy bound states and quasibound states which appear as complex energy poles of the scattering S-matrix or energy Greens function. The quasibound states can give rise to zeros in the waveguide conductance as the energy of the electrons is varied. The width of the conduction zeros is determined by the lifetimes of the quasibound states. The “complex energy spectrum” associated with the quasibound states also governs the survival probability of electrons placed in the waveguide cavities.

Dynamics of radiation induced isomerization for HCN–CNH

We have analyzed the dynamics underlying the use of sequential radiation pulses to control the isomerization between the HCN and the CNH molecules. The appearance of avoided crossings among Floquet eigenphases as the molecule interacts with the radiation pulses is the key to understanding the isomerization dynamics, both in the adiabatic and nonadiabatic regimes. We find that small detunings of the incident pulses can have a significant effect on the outcome of the isomerization process for the model we consider.

Chaos-assisted adiabatic passage

We study the exact dynamics underlying stimulated Raman adiabatic passage (STIRAP) for a particle in a multilevel anharmonic system (the infinite square well) driven by two sequential laser pulses, each with constant carrier frequency. In phase space regions where the laser pulses create chaos, the particle can be transferred coherently into energy states different from those predicted by traditional STIRAP. It appears that a transition to chaos can provide an additional tool to control the outcome of STIRAP.

Quasibound states in a chaotic molecular system

We show the influence of stable and unstable periodic orbits on the energy eigenstates of the HOCl molecule, both below and above dissociation. The energy range considered is low enough to hold the HO bond fixed, allowing a two-dimensional model of the molecule. Above dissociation unstable periodic orbits, in a chaotic sea, anchor quasibound states of the molecule. Quasibound states are calculated using reaction matrix theory and are found to have lifetimes ranging over five orders of magnitude.

The Green's function and quasibound states for N delta potentials in a Stark field

The energy Greens function for a system with N delta-function potentials (delta potentials) in a constant force field is derived and the condition for finding the poles of the energy Greens function is found from the recurrence relation between the Nth and (N − 1)st energy Greens functions. The energy Greens function is decomposed in terms of the residues and complex energies of the poles (the quasibound states of the system), and this complex spectral decomposition is used to derive the survival probability of an initially bound state. We find that the lifetime of quasibound states can oscillate and poles can appear to merge as the spatial position of the delta potentials is changed. A particle that is initially trapped in the delta-potential system can be significantly stabilized by proper choice of positions of the delta potentials.