A photon is absorbed by a quantum dot memory device thus generating an exciton trapped by the quantum dot. One of the carriers of the exciton is removed from the quantum dot by applying a suitable bias whereas the other carrier of the exciton is prevented from leaving the quantum dot by a barrier. Removing one of the carriers from the quantum well prevents recombination and allows the spin state of the carrier remaining in the quantum dot, which is related to the polarisation state of the photon, to be preserved. The quantum dot memory device can be read by transferring a carrier back into the quantum dot for recombination to emit a photon. The polarisation state of the absorbed photon is recreated in the emitted photon. An optical cavity in the form of a pair of mirrors or a photonic crystal is used to couple photons into and out of the quantum dot memory device.