Ultrafast optical spin echo of a single electron spin in a quantum dot

We report on the ultrafast optical implementation of a Hahn Echo sequence on a single electron spin in an InGaAs quantum dot. With this technique, we were able to overcome the shot-to-shot variations of the electron spin's magnetic environment in our multi-shot, time-averaged read-out scheme. We measured the electron spin coherence time T$_{2}$, both as a function of applied magnetic field, and for different types of sample surface treatment. Measured T$_{2}$-times of 3 $\mu $s, together with our experiment all-optical single spin rotation times of 30 ps, would allow 10$^{5}$ single qubit gate operations. Furthermore, we observe pronounced non-linear, hysteretic effects in a 2-pulse Ramsey interference experiment, which we attribute to an electron-spin dependent polarization of the nuclear spins. .