Detection of Quantum State of Electrons Bound to Shallow Donors via Resonance Fluorescence: Ensemble Measurements

Electrons bound to shallow donors in GaAs have a hydrogenic spectrum with principal transitions in the THz range and below the optical phonon energy. The 1S and 2P levels serve as a model solid-state qubit. Bound excitons can be resonantly excited from donors in their 1S state. Since the probability of decay to the 1S state is high, donors in the 1S state scatter light; donors in excited states do not. This is similar to the cycling transition used for readout in ion trap quantum computers. When the bound exciton recombines, some energy may be transferred to the donor, leaving it in an excited state. This Auger process is the limiting factor in making nondemolition measurement of the qubit. We present measurements of the change in resonance fluorescence of ensembles of donor-bound electrons due to THz excitation, corresponding to a decrease in the fraction of electrons in the 1S state. Resonance fluorescence is less destructive to the qubit than photoconductivity-style measurements, and may be used to measure lifetimes of states of bound electrons. Research supported by CNID, DARPA-QUIST, and Sun Microsystems.