  Streaking of photoelectrons with optical lasers has been widely used for
temporal characterization of attosecond extreme ultraviolet pulses. Recently,
this technique has been adapted to characterize femtosecond x-ray pulses in
free-electron lasers with the streaking imprinted by farinfrared and Terahertz
(THz) pulses. Here, we report successful implementation of THz streaking for
time-stamping of an ultrashort relativistic electron beam of which the energy
is several orders of magnitude higher than photoelectrons. Such ability is
especially important for MeV ultrafast electron diffraction (UED) applications
where electron beams with a few femtosecond pulse width may be obtained with
longitudinal compression while the arrival time may fluctuate at a much larger
time scale. Using this laser-driven THz streaking technique, the arrival time
of an ultrashort electron beam with 6 fs (rms) pulse width has been determined
with 1.5 fs (rms) accuracy. Furthermore, we have proposed and demonstrated a
non-invasive method for correction of the timing jitter with femtosecond
accuracy through measurement of the compressed beam energy, which may allow one
to advance UED towards sub-10 fs frontier far beyond the ~100 fs (rms) jitter.
