  Under strong laser fields, electrons in solids radiate high-harmonic fields
by travelling through quantum pathways in Bloch bands in the sub-laser-cycle
timescales. Understanding these pathways in the momentum space through the
high-harmonic radiation can enable an all-optical ultrafast probe to observe
coherent lightwave-driven processes and measure electronic structures as
recently demonstrated for semiconductors. However, such demonstration has been
largely limited for semimetals because the absence of the bandgap hinders an
experimental characterization of the exact pathways. In this study, by
combining electrostatic control of chemical potentials with HHG measurement, we
resolve quantum pathways of massless Dirac fermions in graphene under strong
laser fields. Electrical modulation of HHG reveals quantum interference between
the multi-photon interband excitation channels. As the light-matter interaction
deviates beyond the perturbative regime, elliptically polarized laser fields
efficiently drive massless Dirac fermions via an intricate coupling between the
interband and intraband transitions, which is corroborated by our theoretical
calculations. Our findings pave the way for strong-laser-field tomography of
Dirac electrons in various quantum semimetals and their ultrafast electronics
with a gate control.
