  We investigate a dual-channel arrangement for electromagnetically-induced
transparency in a vacuum Rb vapor cell, and report the observation of a
transient spectral feature due to the atoms traversing both beams while
preserving their ground-state spin coherence. Despite a relatively small
fraction of atoms participating in this process, their contribution to the
overall lineshape is not negligible. By adjusting the path difference between
the two optical beams, the differential intensity measurement can produce an
error signal for the microwave frequency stabilization as strong as a
single-channel measurement, but it provides a much higher signal-to-noise ratio
due to the cancellation of intensity noise, dominating the signal channel
detection.
