  We study electron-positron pair creation by a strong and slowly varying
electric field, assisted by a weaker and more rapidly changing field (e.g., in
the keV regime) plus an additional high-energy (say MeV) photon. It turns out
that this combination can yield a pair creation probability which is
exponentially larger than in the case where one (or more) of the three
ingredients is missing. Apart from a deeper understanding of these
non-perturbative phenomena, this double enhancement may pave the way for an
experimental verification of this fundamental prediction.
