  It is known that under resonance conditions, a group of strongly interacting
bosonic atoms, trapped in a double-well potential, mimics a single particle,
performing Rabi oscillations between the wells. By implication, all atoms need
to tunnel at roughly the same time, even though the Bose-Hubbard Hamiltonian
accounts only for one-atom-at-a-time transfers. We analyse the mechanism of
this collective behaviour, evaluate the Rabi frequencies in the process, and
discuss the limitation of this simple picture. In particular, is is shown that
the small rapid oscillations superimposed on the slow Rabi cycle, result from
splitting the transferred cluster at the sudden onset of tunnelling, and
disappear if tunnelling is turned on gradually.
