  We study theoretically an atomic Bose-Einstein condensate in a double-well
trap both quantum mechanically and classically under conditions such that in
the classical model an unstable equilibrium dissolves into large-scale
oscillations of the atoms between the potential wells. Quantum mechanics alone
does not exhibit such nonlinear dynamics, but measurements of the atom numbers
in the potential wells may nevertheless cause the condensate to behave
essentially classically.
