Dephasing of Josephson oscillations between two coupled Bose-Einstein condensates

We study the dynamics of the relative phase between two atomic Bose-Einstei n condensates coupled via collisions and via a Josephson-like coupling. We derive the equations of the motion for the relative phase and the relative number operators from the second-quantized Hamiltonian of the system using a quantum field theoretical approach. We distinguish the cases in which the two condensates are in the same trap or in two different traps and study t he influence of this difference on the first-order correlation function of atomic fields. In identical traps this function does not undergo dephasing. We calculate the dephasing time for the case of different traps. [S1050-29 47(99)06903-6].