  Sequences of alternating-sign time-dependent electric field pulses lead to
coherent interference effects in Schwinger vacuum pair production, producing a
Ramsey interferometer, an all-optical time-domain realization of the
multiple-slit interference effect, directly from the quantum vacuum. The
interference, obeying fermionic quantum statistics, is manifest in the momentum
dependence of the number of produced electrons and positrons along the linearly
polarized electric field. The central value grows like $N^2$ for $N$ pulses
[i.e., $N$ "slits"], and the functional form is well-described by a coherent
multiple-slit expression. This behavior is generic for many driven quantum
systems.
