Optically detected quantum dynamics of hydrogenic donor qubits

Orbital states of electrons bound to shallow donors in GaAs provide many of the advantages of trapped atoms for quantum information studies, including optical readout and long lived excited levels. Shallow donors (e.g. S, Si) have a scaled hydrogenic potential with a bound electron 1S-2P transition at 1 THz (4 meV). In a 5 T magnetic field the 1S state and lowest 2P state (2P$^-$) serve as qubit levels. A cycling transition exists for detecting neutral donors in the ground state via the donor bound exciton resonance; excited bound states are dark. Using this optical quantum nondemolition measurement, the relaxation (T$_1$) of donors after THz excitation of the 1S- 2P$^-$ transition is observed to be $>1\mu$s. High resolution spectroscopy indicates dephasing (T$_2^*$) of an ensemble of neutral donors is limited by inhomogeneous broadening to 50 ps. In order to measure the decoherence time (T$_2$), which is expected to be much longer, a rephasing technique is required. For Hahn echo measurements of T$_2$ a 0-24ns, diffraction- compensating free space THz delay line has been constructed.