  We apply a recently emerging "virtual detector" method to general
free-particle wave packets. We discuss what is actually detected by a virtual
detector and give analytical expressions in both the real space and the
momentum space. We prove that in the asymptotic limit when the time difference
between wave packet creation and detection is large, resembling real detection
processes, the obtained differential momentum distribution is equivalent to
Fourier transforming the free-particle wave packets. We further consider the
situation with the presence of laser fields.
