  The quantum fluctuation effects in the time-of-flight (TOF) experiment for a
condensate released from an optical lattice potential is studied within the
truncated Wigner approximation. By investigating both the spatial and momentum
density distributions, we find that the condensate fraction decreases
monotonically in time and hence cannot be measured in the standard TOF image.
We then propose a semi-quantitative analysis for such dynamical quantum
depletion process. Our study shows a universal algebraic decay of the true
condensate fraction, and have a very good agreement with numerical results. We
also discuss possible methods to determine the condensate fraction inside the
optical lattice, and its implication to the TOF experiments in higher
dimensional systems.
