Decoherence And The Effect On The Quantum Coherence In A Dilute Gas

The decoherence is expected to play an importing role in explaining the results in many experiments, and is also expected to have a relation to solve the quantum measure-ment problem. Nowadays decoherence effects have been observed in lots of systems. The ultracold atom system, especially the Bose-Einstein condensation (BEC) provides us a platform for observing the quantum quantities of a many-body system. It’s well known that when the temperature of the cloud drops below the BEC transition temper-ature Tc, the Gross-Pitaevskii equation (GP equation) can well characterize the motion of the cloud. But at a temperature above, but not much above Tc, there is not a good theory to characterize the cloud. In this thesis, we apply decoherence theoryset to an ultracold atom system and try to set up an atom model. We also discuss the decoherence in an open system coupled with a chaotic environment.An experiment is discussed, in which the interference patterns formed by a dilute gas above Tc is observed (PRA71,043615). It’s found that the contrast measured is higher than which is predicted by a thermal atom model, meaning that there is extra quantum coherence. We analyze the experiment result and found that the higher con-trast can’t be explained by a simply modified thermal model. Hence, from the theory of quantum Brownian motion, we set up a hybrid model in which a part of atoms lying in a condensed state so-called coherently condensed state。We give an analytic ex-pression with some undetermined parameters for the proportion of the atoms lying in the coherently condensed state, and find it can explain most of the experimental result semiquantitatively by an appropriate choice of parameters. to check this model, we give some advices for further experiments.We also study an open quantum system coupled with a chaotic large environment. We find that, after decoherence has happened, the environment components of the state vector of the total system can be replaced by some randomly-chosen vectors in the Hilbert space of the environment. This process is called the ’randomization of the envi-ronment’(ROE). The further evolution of the reduced density matrices obtained in this way are quite close to those computed from the exact, continuous Schrodinger evolution of the total system, even the Hamiltonian is changed at the time of ROE. This means the correlations between the components coupled with different pointer states have de-stroyed, and the information of the environment is unimportant for us. When the ROE is applicable, it should be reasonable to expect a ’real’ decoherence or ’branching’ in the many-worlds interpretation may have happened, and the relate decomposition of the total system can be given a mixed-state interpretation. We expect the ROE can provide a criterion for the ’real’ decoherence or ’branching’.