  We show that for collisions of electrons with a high-intensity laser,
discrete photon emissions introduce a transverse beam spread which is distinct
from that due to classical (or beam shape) effects. Via numerical simulations,
we show that this quantum induced transverse momentum gain of the electron is
manifest in collisions with a realistic laser pulse of intensity within reach
of current technology, and we propose it as a measurable signature of
strong-field quantum electrodynamics.
