In situ accurate determination of the zero time delay between two   independent ultrashort laser pulses by observing the oscillation of an atomic   excited wave packet
  We propose a novel method that uses the oscillation of an atomic excited wave packet observed through a pump-probe technique to accurately determine the zero time delay between a pair of ultrashort laser pulses. This physically-based approach provides an easy fix for the intractable problem of synchronizing two different femtosecond laser pulses in a practical experimental environment, especially where an in situ time zero measurement with high accuracy is required. 
Qun Zhang and John W. Hepburn