  Electron-positron pair creation occurs throughout the universe in the
environments of extreme astrophysical objects, such as pulsar magnetospheres
and black hole accretion disks. The difficulty of emulating these environments
in the laboratory has motivated the use of ultrahigh-intensity laser pulses for
pair creation. Here we show that the phase offset between a laser pulse and its
second harmonic can be used to control the relative transverse motion of
electrons and positrons created in the nonlinear Breit-Wheeler process.
Analytic theory and particle-in-cell simulations of a head-on collision between
a two-color laser pulse and electron beam predict that with an appropriate
phase offset, the electrons will drift in one direction and the positrons in
the other. The resulting current may provide a collective signature of
nonlinear Breit-Wheeler, while the spatial separation resulting from the
relative motion may facilitate isolation of positrons for subsequent
applications or detection.
