  High harmonic emissions from crystalline solids contain rich information on
the dynamics of electrons driven by intense infrared laser fields and have been
intensively studied owing to their potential use as a probe of microscopic
electronic structures. Especially, the ability to measure the temporal response
of high harmonics may allow us to investigate electron dynamics directly in
quantum materials. However, this most essential aspect of high harmonic
emissions has been challenging to measure. Here, we propose a simple solution
for this problem: a high harmonic interferometer, where high harmonics are
generated in each of the path of a Mach-Zehnder interferometer and an
interferogram of them is captured. The high harmonic interferometer allows us
to achieve a relative time resolution between the target and reference high
harmonics of less than 150 attoseconds, which is fine enough to track sub-cycle
dynamics of electrons in solids. By using high harmonic interferometrer, we
succeeded in capturing the real time dynamics of Floquet states in WSe2, whose
indirect signature had so far been caught only by time-averaged measurement.
Our simple technique will open a door to attosecond electron dynamics in
solids.
