  We study the dynamics of the relative phase following the connection of two
independently formed Bose-Einstein condensates. Dissipation is assumed to be
due to the creation of quasiparticles induced by a fluctuating condensate
particle number. The coherence between different values of the phase, which is
characteristic of the initial Fock state, is quickly lost after the net
exchange of a few atoms has taken place. This process effectively measures the
phase and marks the onset of a semiclassical regime in which the system
undergoes Bloch oscillations around the initial particle number. These fast
oscillations excite quasiparticles within each condensate and the system
relaxes at a longer time scale until it displays low-energy, damped Josephson
plasma oscillations, eventually coming to a halt when the equilibrium
configuration is finally reached.

Condensed Matter