  A detection scheme for characterizing high-energy $\gamma$-ray pulses down to
the zeptosecond timescale is proposed. In contrast to existing attosecond
metrology techniques, our method is not limited by atomic shell physics and
therefore capable of breaking the MeV photon energy and attosecond time-scale
barriers. It is inspired by attosecond streak imaging, but builds upon the
high-energy process of electron-positron pair production in vacuum through the
collision of a test pulse with an intense laser pulse. We discuss necessary
conditions to render the scheme feasible in the upcoming Extreme Light
Infrastructure laser facility.
