  Dynamics of fluctuations in unstable Bose-Einstein condensates is analyzed by
the solution of approximate operator equations. In the case of a condensate
with a negative scattering length the present treatment describes a delay of
collapse, in agreement with recent experiments. In the case of a collision of
two condensate wavepackets it is shown that quantum effects lead to a Bose
enhancement of elastic-scattering losses. In both cases the noncondensate atoms
are formed as entangled pairs in squeezed states.
