Coherent Control of Dark Excitons in Semiconductor Quantum Dots

We review studies of the quantum dot confined dark exciton and demonstrate its use as a matter qubit. The dark exciton is an optically forbidden semiconductor electronic excitation, in which an electron-hole pair is generated with parallel spin projections. This optcally inactive excitation lives orders of magnitude longer than the corresponding optically active excitation, the bright exciton, in which the pair has anti-parallel spins. We show that despite its optical inactivity, the dark exciton can be deterministically generated in any desired coherent superposition of its two eigenstates using a single picosecond optical pulse. We provide lower bounds for the dark exciton life and coherence times and show that its coherent state can be fully controlled using short optical pulses. We also study its behavior in an externally applied magnetic field and present a method for its optical depletion from the quantum dot. Our results demonstrate that the dark exciton is an excellent matter spin qubit.