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We construct a generally applicable short-time perturbative expansion for coherence loss. Successive terms of this expansion yield characteristic times for decorrelation processes involving successive powers of the Hamiltonian. The second order results are sufficient to precisely reproduce expressions for "decoherence times" obtained in the literature by much more involved and indirect methods. Examples illustrating the influence of initial conditions and the need to evaluate higher order terms are given in the context of the Jaynes-Cummings model. It is shown that, in this case, the short-time decoherence behavior can probe the importance of antiresonant contributions. [S0031-9007(96)00590-X] PACS numbers: 03.65.Db, 05.45.+bThe study of open quantum systems and/or subsystems has recently attracted the attention of physicists from very different areas: cosmology [1], condensed matter [2], quantum optics [3], particle physics [4], as well as of theorists working on the fundamentals of the quantum measurement process [5]. The problem can be stated very generally by considering several interacting subsystems and asking for the looks of the effective dynamics of one such subsystem. Generic, exact answers within the standard framework of quantum mechanics have been given before [6]. Recent experimental developments [7] as well as the analysis of models related to them [8] now indicate, however, that the specific knowledge of the (often very short) time scale for the onset of decoherence processes may be of considerable value. In order to meet such demand we develop here a short-time perturbative scheme to extract decorrelation time scales from the in general highly nonlinear effective dynamics of open quantum subsystems. Our results are generally applicable to situations in which the subsystem of interest appears as part of a larger, closed Hamiltonian system. They are based on a hierarchical analysis of the short-time dynamics of intersubsystem correlation processes which bears a strong resemblance in spirit to ordinary timedependent perturbation expansions.We consider the general case of a dynamically closed (i.e., autonomous) quantum system which is described as being composed of two interacting subsystems, so that the full Hamiltonian is written as a sum of three termsthe last of which represents the interaction between the subsystems, while H 0 describes their bare dynamics. Note that no a priori limitation is being imposed on the nature or complexity of the subsystems. In particular, all current models involving quantum systems coupled to dynamically implemented reservoirs (e.g., [9]) fit into the above characterization, the same being true all the way to 0031-9007͞96͞77(2)͞207(4)$10.00

We study the effects of channel coupling in the excitation dynamics of giant resonances in relativistic heavy ions collisions. For this purpose, we use a semiclassical approximation to the coupled-channels problem and separate the Coulomb and the nuclear parts of the coupling into their main multipole components. In order to assess the importance of multistep processes, we neglect the resonance widths and solve the set of coupled equations exactly. Finite widths are then considered. In this case, we handle the coupling of the ground state with the dominant giant dipole resonance exactly and study the excitation of the remaining resonances within the coupled-channels Born approximation. A comparison with recent experimental data is made.

The dynamics of the population imbalance of bosons in a double-well potential is investigated from the point of view of many-body quantum mechanics in the framework of the two-mode model. For small initial population imbalances, coherent superpositions of almost equally spaced energy eigenstates lead to Josephson oscillations. The suppression of tunneling at population imbalance beyond a critical value is related to a high concentration of initial state population in the region of the energy spectrum with quasi-degenerate doublets resulting in imbalance oscillations with a very small amplitude. For unaccessible long times, however, the system recovers the regime of Josephson oscillations. The understanding of many-body quantum systems from the theoretical and experimental points of view has undergone a considerable development during the past decade. Unifying concepts of several branches of physics are under development, creating an interdisciplinary scenario for the understanding of quantum mechanical paradigms. One of the simplest many-body systems to be realized experimentally and studied theoretically are ultracold bosons in a double-well potential. This system is very rich exhibiting a great variety of quantum phenomena such as interference [1], tunneling/selftrapping [2,3,4,5,6,7], entanglement of macroscopic superpositions [8]. Lately this system has been extensively studied, especially after the implementation of several experiments in the area. The usual theoretical approach to weakly interacting Bose-Einstein condensates (BECs) is the mean-field approximation, a nonlinear Gross-Pitaevski equation [3,9,10,11,12,13,14,15,16], which has proven very adequate in explaining a wide variety of experiments.More recently, the dynamics of population distribution between two or more wells of an optical lattice have been experimentally investigated. In particular, Josephson oscillations have been observed in a 1D optical lattice [17,18] and recently the density distribution of tunneling 87 Rb particles is directly observed [2]. In this experiment, initial population differences between the left and right well components are controlled by loading the BEC into an asymmetric double-well potential. The Josephson dynamics is initiated at t = 0 by non-adiabatically changing the potential to a symmetric double-well. When the initial population imbalance is below a critical value, the system presents Josephson oscillations between the two sides of the well. However, above this critical value tunneling is not observed. Based on a mean field treatment, this is usually attributed to macroscopic self-trapping. In the present work, we discuss an alternative approach to this system based on exact numerical solutions of the two-mode Bose-Hubbard Hamiltonian [19]: