The difference in the group velocities for the signal and the idler wave, interacting with a sum-frequency wave in a non-linear crystal, is shown to provide a memory mechanism which determines the length of the mode volume occurring in the usual single-mode description of this process in connection with photon antibunching. The consideration also includes the degenerate case (second harmonic generation). The results suggest the use of a c.w. picosecond laser as a primary light source in an experiment designed to demonstrate the antibunching effect. The basic set-up of such an experiment is described.