  Photoproduction of electron-positron pairs by the strong-field Breit-Wheeler
process in an intense laser field is studied. The laser field is assumed to
consist of two consecutive short pulses, with a variable time delay in between.
By numerical calculations within the framework of scalar quantum
electrodynamics, we demonstrate that the time delay exerts a strong impact on
the pair-creation probability. For the case when both pulses are identical, the
effect is traced back to the relative quantum phase of the interfering S-matrix
amplitudes and explained within a simplified analytical model. Conversely, when
the two laser pulses differ from each other, the pair-creation probability
depends not only on the time delay but, in general, also on the temporal order
of the pulses.
