  In this work we demonstrate the generation of two intense, ultrafast laser
pulses that allow a controlled interferometric measurement of higher harmonic
generation pulses with 12.8 attoseconds in resolution (half the atomic unit of
time) and a precision as low as 680 zeptoseconds ($10^{-21}$ seconds). We
create two replicas of a driving femtosecond pulse which share the same optical
path except at the focus where they converge to two foci. An attosecond pulse
train emerges from each focus through the process of HHG. The two attosecond
pulse trains from each focus interfere in the far field producing a clear
interference pattern in the XUV region. By controlling the relative optical
phase between the two driving laser pulses we are able to actively influence
the delay between the pulses and are able to perform very stable and precise
pump-probe experiments. Because of the phase operation occurs across the entire
spatial profile we effectively create two indistinguishable intense laser
pulses or a common path interferometer for attosecond pulses. Commonality
across the two beams means that they are extremely stable to environmental and
mechanical fluctuations up to a Rayleigh range from the focus. In our opinion
this represents an ideal source for homodyne and heterodyne spectroscopic
measurements with sub-attosecond precision.
