Phenomenological Theory of Mode Collapse-revival in Confined Bose Gas

A phenomenological theory of mode collapse-revival for a system with a weak nonlinearity is presented. The theory takes into account fluctuations of the number n of quanta of oscillations. The collapse time $\tau _c ^{-1}$ for a mode of frequency $\omega _0$ turns out to be $\omega _0(\sqrt{n/2}\hbar \omega _0 \mid \nu \mid)$, where \nu is a coefficient in a nonlinear correction to the mode frequency with respect to the oscillation energy E. For a Bose gas in a harmonic trap $\tau _c$ is order of $\omega _0^{-1}\sqrt{\mu N/E\hbar \omega _0}$. This value is order of 250 ms. for typical experimental conditions. The coefficient \nu is calculated for the breathing mode in an isotropic trap.