Optically detected measurement of the ground-state population of an ensemble of neutral donors in GaAs

Strong resonant elastic light scattering (RELS) from the donor-bound exciton transition in GaAs (1.5140.3em0exeV) occurs at neutral donors in the ground $(1S)$ state, but not at neutral donors in excited hydrogenic states. When 1.60.3em0exTHz radiation is incident on an ensemble of neutral donors, we observe up to a 30% decrease in the RELS, corresponding to a decrease in the population of neutral donors in their ground states. This signature allows for optical detection of the ground-state population of donors, and is similar to quantum nondemolition measurement techniques used for readout of ion trap quantum computers. In this scheme, Auger recombination of the bound exciton, which changes the state of the donor during measurement, limits the measurement fidelity and maximum near infrared laser excitation intensity.