  We introduce a classical stochastic field method that accounts for the
quantum fluctuations responsible for spontaneous initiation of various atom
optics processes. We assume a delta-correlated Gaussian noise in all initially
empty modes of atomic field. Its strength is determined by comparison with the
analytical results for two colliding condensates in the low loss limit. Our
method is applied to the atomic four wave mixing experiment performed at MIT
[Vogels {\it et. al.}, Phys. Rev. Lett. {\bf 89}, 020401, (2002)], for the
first time reproducing experimental data.
