Valley-resolved optical spectroscopy and coherent excitation of quantum Hall edge states in graphene

Current-carrying chiral edge states in quantum Hall (QH) systems exhibit fascinating properties typically studied using electron spectroscopy and interferometry. We demonstrate that electron occupation, current, and coherence in graphene chiral edge states can be selectively probed and controlled by terahertz or infrared radiation with single-quasiparticle sensitivity, without affecting electron states in the bulk. Graphene provides a particularly versatile platform enabling valley-selective optical excitations into a given edge state. The underlying physical mechanism arises from the unavoidable nonadiabaticity and inversion symmetry breaking of electron states near the edge. This leads to Landau-level- and valley-specific absorbance peaks that are distinct from bulk resonances, enabling selective excitation and coherent control of individual edge channels. Moreover, inversion symmetry breaking allows second-order nonlinear optical rectification in the electric-dipole approximation, producing quasi-dc edge currents. The shape of an incident single-photon state controls the excited electron's quantum state, offering new opportunities for optoelectronic control, quantum Hall interferometry, and quantum gates.