Exciton-spin memory with a semiconductor quantum dot molecule

We report on a single photon and spin storage device based on a semiconductor quantum dot molecule. Optically excited single electron-hole pairs are trapped within the molecule, and their recombination rate is electrically controlled over 3 orders of magnitude. Single photons are stored up to 1 μs and read out on a subnanosecond time scale. By using resonant excitation, the circular polarization of individual photons is transferred into the spin state of electron-hole pairs with a fidelity above 80%, which does not degrade for storage times up to the 12.5 ns repetition period of the experiment.