  We investigate the dynamics of bosonic atoms in elongated Josephson
junctions. We find that these systems are characterized by an intrinsic
coupling between the Josephson mode of macroscopic quantum tunneling and the
sound modes. This coupling of Josephson and sound modes gives rise to a damped
and stochastic Langevin dynamics for the Josephson degree of freedom. From a
microscopic Lagrangian, we deduce and investigate the damping coefficient and
the stochastic noise, which includes thermal and quantum fluctuations. Finally,
we study the time evolution of relative-phase and population-imbalance
fluctuations of the Josephson mode and their oscillating thermalization to
equilibrium.
