  Strong resonant elastic light scattering (RELS) from the donor-bound exciton
transition in GaAs (1.514eV) occurs at neutral donors in the ground (1S) state,
but not at neutral donors in excited hydrogenic states. When 1.6 THz 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 optical detection method is similar to quantum
nondemolition measurement techniques used for readout of ion trap quantum
computers and diamond nitrogen-vacancy centers. In this scheme, Auger
recombination of the bound exciton, which changes the state of the donor during
measurement, limits the measurement fidelity and maximum NIR excitation
intensity.
