İnce spektrum ve bose-einstein yoğunlaşıklarında uyarım ömrü

In this thesis we study two distinct mechanisms of phase diffusion in atomic Bose-Einstein condensates (BEC?s). The first one is studied via a so-called toy-model andgives a finite life time that the ground state of a Bose-Einstein condensate can remaincoherent. This mechanism, which was before used for coherent states, is generalizedto squeezed and the thermal-coherent states, which we introduce as states havingboth coherent and thermal character. Then the recently introduced thin spectrumformalism is reviewed and applied to BEC?s in order to obtain a decay rate for ex-citations. Relation of thin spectra with Spontaneous Symmetry Breaking (SSB) isstudied and some expansions on the formalism are made.iv