Gaussian wavepacket dynamics in an anharmonic system

Gaussian wavepacket dynamics and quantum tunneling in asymmetric double-well systems

We find a relationship between the dynamics of the Gaussian wave packet and the dynamics of the corresponding Gaussian Wigner function from the Hamiltonian/symplectic point of view. The main result states that the momentum map corresponding to the natural action of the symplectic group on the Siegel upper half space yields the covariance matrix of the corresponding Gaussian Wigner function. This fact, combined with Kostant’s coadjoint orbit covering theorem, establishes a symplectic/Poisson-geometric connection between the two dynamics. The Hamiltonian formulation naturally gives rise to corrections to the potential terms in the dynamics of both the wave packet and the Wigner function, thereby resulting in slightly different sets of equations from the conventional classical ones. We numerically investigate the effect of the correction term and demonstrate that it improves the accuracy of the dynamics as an approximation to the dynamics of expectation values of observables.

Summary form only given. Wave packets (WPs) has recently emerged as a very active research field in quantum optics. We show here that the evolution of WPs may result in new class of large-scale interference effects: the formation of highly regular spatio-temporal patterns in the QM probability, |/spl Psi/|/sup 2/, with the key mechanism for this being pair interference between the eigen-modes of the system and the degeneracy of this interference. The new patterns have been discovered in the probability density |/spl Psi/|/sup 2/(x,t) of a particle in an infinitely deep square well, by numerical simulations of the evolution of a Gaussian WP; later work by a few groups used other initial conditions and media (e.g. Bose-Einstein condensate and angular WPs). In all these cases, the QM distribution shows a net of canals (minima), and ridges (maxima), running as straight lines in space-time (x,t). Similar patterns in the near field of a diffraction grating, and in matter waves in atom optics reveals a new aspects of the Talbot effect. So far the numerical exploration of these patterns has relied on the specific choice (i) of the initial conditions and (ii) of the square well; no physical explanation of the patterns has been suggested. Recently, we observed a quantum carpet under more general conditions, offered its universal explanation, and predicted distinct curved patterns in general case of an anharmonic potential.

Spreading and recurrence in anharmonic quantum-mechanical systems

Resonant Raman response of impurity molecules in nonmetallic solid matrix is solved exactly. The model embodies the nonradiative interaction between impurity molecule and the nearest-neighbour molecules in the solid. The interatomic vibration of the excited electronic level of the molecule is modelled by harmonic and anharmonic potential. Frank-Condon and anharmonic effects are discussed by selected numerical examples.

The dynamics of collinear exchange reactions is treated within a semiclassical Gaussian wave packet (GWP) description. Two extensions to the previous methodology of GWP dynamics are presented. The first involves the evaluation of the interaction picture wave function propagators directly via the GWP solution to the time-dependent Schrödinger equation. The second involves use of a sum of GWPs to represent the initial translational plane wave on a finite interval. As usual, the vibrational wave function is also represented as a sum of GWPs, where the expansion coefficients are chosen to yield the desired initial vibrational state. The limitations of the previous GWP dynamical methods as applied to the reactive scattering problem are pointed out and the solutions provided by the present extensions are discussed. Results for the H+H2 collinear exchange reaction using the Porter–Karplus II potential energy surface are shown.

We investigated the transmission probability of a single electron transmission through a quantum ring device based on the single-band effective mass approximation method and transfer matrix theory. The time-dependent Schrödinger equation is applied on a Gaussian wave packet passing through the quantum ring system. The electron tunneling resonance peaks split when the electron transmits through a double quantum ring. The splitting energy increases as the distance between the two quantum rings decreases. We studied the tunneling time through the single electron transmission quantum ring from the temporal evolution of the Gaussian wave packet. The electron probability density is sensitive to the thickness of the barrier between the two quantum rings.

We analyze the semiclassical evolution of Gaussian wave packets in chaotic systems. We show that after some short time a Gaussian wave packet becomes a primitive WKB state. From then on, the state can be propagated using the standard time-dependent WKB scheme. Complex trajectories are not necessary to account for the long-time propagation. The Wigner function of the evolving state develops the structure of a classical filament plus quantum oscillations, with phase and amplitude being determined by geometric properties of a classical manifold.

The free particle non-relativistic Schrödinger evolution of a sech wave packet, and the corresponding Wigner function and Bohmian trajectories, are studied. The overall translation and dispersion dynamics are qualitatively similar to corresponding Gaussian wave packets, as is to be expected. On the other hand, certain bizarre and completely unexpected features are also observed, such as wave packet pinching and the instantaneous formation of infinitely many nodes, despite the apparent lack of an interference source.

We have recently published a new semiclassical method, generalized Gaussian wave packet dynamics, which extends Gaussian wave packet dynamics into complex phase space. Although we were able to give an accurate formulation of the method, we had at the time of writing that paper only an intuitive, heuristic understanding of the deeper causes which make the method work. A more mathematical understanding was needed. To close this gap we show in this paper the equivalence of the new method with a first order expansion of ℏ of the Schrödinger equation. We further prove that the new method is equivalent to the stationary phase approximation, using the usual WKB formula for the propagator. The latter equivalence enables us to show that all the symmetry properties of time-dependent quantum mechanics also hold in the new semiclassical theory. Finally, we provide some elaboration of the method, and clarify several issues that were not discussed before. With this new insight we are able to formulate a simple rule for the calculation of semiclassical wave functions that contain contributions from more than one branch. This corrects for the divergence of semiclassical wave functions near caustics, a problem that we encountered in the preceding paper.

This paper concerns an approximation of the expectation values of the position and momentum of the solution to the semiclassical Schrödinger equation with a Gaussian as the initial condition. Of particular interest is the approximation obtained by our symplectic/Hamiltonian formulation of the Gaussian wave packet dynamics that introduces a correction term to the conventional formulation using the classical Hamiltonian system by Hagedorn and others. The main result is a proof that our formulation gives a higher-order approximation than the classical formulation does to the expectation value dynamics under certain conditions on the potential function. Specifically, as the semiclassical parameter \(\varepsilon \) approaches 0, our dynamics gives an \(O(\varepsilon ^{3/2})\) approximation of the expectation value dynamics, whereas the classical one gives an \(O(\varepsilon )\) approximation.

Gaussian wave packet dynamics and scattering in the interaction picture

A wavepacket spreading under quadratic potentials is shown to have an exact classical analogue in the spreading of a classical ensemble taken from the Wigner distribution. The wavepacket motion under anharmonic potentials is complicated by greater spreading combined with additional interferences. Both are shown to be well handled by Heller’s frozen Gaussian swarm technique. Applications are outlined to spectral quantization and Franck–Condon transitions by Fourier analysis of the time correlation function. The chapter concludes with an introduction to the popular unitary Herman–Kluk coherent state propagation scheme. Advantages over semiclassical Van Vleck propagation are discussed, and a Filinov–based scheme for handling weakly chaotic systems is described.

We present a new interpretation of the decoherence correction in surface hopping by examining the inconsistency of the traditional time-dependent Schrödinger equation and propose an elegant decoherence correction algorithm to deal with wave packet branching. In contrast to the widely used approaches based on decoherence rates, our branching corrected surface hopping (BCSH) resets the wavefunction directly after wave packet branching is identified through prediction of trajectory reflection. The appealing simplicity and reliability of BCSH are demonstrated in a series of widely studied one-dimensional and two-dimensional scattering models using exact quantum solutions and existing surface hopping approaches as references. The BCSH approach exhibits a high performance in all investigated systems, showing good potential for applications in general nonadiabatic dynamics simulations.

An exact analytical approach to study the time evolution of the interaction between a system of N two-level atoms with a single-mode cavity field is proposed. Supposing that at the initial moment the system of radiators is prepared in a superposition of states, the wave functions of the atom–field interaction for three and four atoms are presented exactly. In the case of an odd number of atoms in the cavity, the quantum Rabi frequencies have non-zero values; otherwise one zero value of the quantum Rabi frequency is characteristic of the even number of atoms. It was shown that with an increase of the odd number of atoms the non-zero quantum Rabi frequencies in the system increase too. It was established that a single atom has one quantum Rabi frequency, three atoms have two frequencies and five atoms have three. For the even number of atoms, a zero value of eigenstates of radiators in the interaction with a single cavity mode is observed. In this case the number of collective levels in the system is odd and the trapping states of atoms in the cavity are possible. From analytical solutions it follows that it is possible to realize the micro-maser regimes for three atoms choosing the flying time for which the radiators pass totally in the ground state. The dependence of the flying time on the number of atoms is established as well as the cavity state for which the atomic ensemble comes back to its initial state. The behavior of quantum fluctuations of photon numbers and atomic inversion during collective absorption and radiation is exactly analytically and numerically studied. The exact quantum solutions for these qubits, formed from two-level atoms, can be used in quantum processing of information encapsuled in the initial states of the atomic ensemble and the cavity field.