Kerr rotation studies of single electron spin dynamics in a quantum dot*

Kerr rotation measurements are used to directly and non-destructively probe the dynamics of a single electron spin in a charge-tunable quantum dot \footnote{ J. Berezovsky, M. H. Mikkelsen, O. Gywat, N. G. Stoltz, L. A. Coldren, and D. D. Awschalom,{\em Science Express}, 9 November 2006, (10.1126/science.1133862)}. The dot is formed by interface fluctuations of a GaAs quantum well and embedded in a vertical optical cavity. Using Hanle techniques, we perform single electron Kerr rotation measurements at $T=10\mathrm{K}$ in order to monitor the depolarization of an optically pumped electron spin within an applied transverse magnetic field. This reveals information about the time averaged transverse spin lifetime, $T_2^*$. At gate voltages for which the charging rate of the dot is relatively low, the results yield a $T_2^*$ in agreement with values expected from the hyperfine interaction in these materials. In contrast, at larger charging rates, we find that $T_2^*$ is strongly reduced, indicating the importance of additional decoherence mechanisms in that regime.