Ultrafast optical spin echo in a single quantum dot

Many proposed photonic quantum networks rely on matter qubits to serve as memory elements(1,2). The spin of a single electron confined in a semiconductor quantum dot forms a promising matter qubit that may be interfaced with a photonic network(3). Ultrafast optical spin control allows gate operations to be performed on the spin within a picosecond timescale(4-14), orders of magnitude faster than microwave or electrical control(15,16). One obstacle to storing quantum information in a single quantum dot spin is the apparent nanosecond-timescale dephasing due to slow variations in the background nuclear magnetic field(15-17). Here we use an ultrafast, all-optical spin echo technique to increase the decoherence time of a single quantum dot electron spin from nanoseconds to several microseconds. The ratio of decoherence time to gate time exceeds 10(5), suggesting strong promise for future photonic quantum information processors(18) and repeater networks(1,2).