  Strong-field quantum electronics is emerging as a potential candidate in
information processing but still coherence vs decoherence is a primary concern
of the concept. Strong-field coherent processes in band gap materials have led
during the last decade to the emergence of high harmonic generation in
semiconductors, petahertz electronics, or strong-field quantum states. However,
the coherent behavior of the sub-optical cycle-driven electrons has never been
directly observed. We report here on the experimental evidence of coherent
ultrashort emission of hot electrons from a nanostructured semiconductor. Our
method uses sub-wavelength electric field enhancement to localize the electron
emission within a nanometer-scale spot. We found similarities with the electron
emission from metallic nanotips in the strong-field regime, a topic that has
opened a vast domain of applications during the last decade. The electron
spectra display both odd and even harmonic orders of the driving femtosecond
laser frequency, a signature of the coherent nature of the electron emission
and their attosecond timing. Our findings complete our knowledge of phenomena
governing coherent strong-field processes in semiconductors and open
perspectives for the generation of future quantum devices operating in the
strong-field regime.
