Ultranarrow ionization resonances in a quantum dot under broadband excitation

Semiconductor quantum dots driven by the broadband radiation fields of nearby quantum point contacts provide an interesting setting for probing dynamics in driven quantum systems at the nanoscale. We report on real-time charge-sensing measurements of the dot occupation, which reveal sharp resonances in the ionization rate as a function of gate voltage and applied magnetic field. Despite the broadband nature of excitation, the resonance widths are much smaller than the scale of thermal broadening. We show that such resonant enhancement of ionization is not accounted for by conventional approaches relying on elastic scattering processes, but can be explained via a mechanism based on a bottleneck process that is relieved near excited state level crossings. The experiment thus reveals a regime of a strongly driven quantum dynamics in few-electron systems. The theoretical results are in good agreement with observations.