The Franz-Keldysh effect describes the change in the
absorption spectrum of a semiconductor due to the acceleration of
photo-excited carriers in the presence of a DC electric field. A
recently developed theory predicts that the two-photon absorption
spectrum is affected in a similar way. Quantum interference between
one- and two-photon absorption in a semiconductor results in coherent
control of charge and spin currents. This has found application in
measuring the carrier-envelope phase of broadband optical pulses, and in
studying ultrafast charge and spin transport in semiconductors. I will
present a theory, based on the nonlinear optical Franz-Keldysh effect,
and experimental results in GaAs showing that a DC field causes
qualitative changes in the quantum interference processes.