Measurement of the Subcycle Timing of Attosecond XUV Bursts in High-Harmonic Generation

The absolute timing of the high-harmonic attosecond pulse train with respect to the generating IR pump cycle has been measured for the first time. The attosecond pulses occur $190\ifmmode\pm\else\textpm\fi{}20\text{ }\mathrm{a}\mathrm{s}$ after each pump field maxima (twice per optical cycle), in agreement with the ``short'' quantum path of the quasiclassical model of harmonic generation.