Terahertz-induced depletion of the ground-state population of neutral   donors in GaAs measured by resonant elastic light scattering from donor-bound   excitons
  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. 
D. G. Allen, C. R. Stanley, M. S. Sherwin