Optically detected coherent spin dynamics of a single electron in a quantum dot

The ability to sequentially initialize, manipulate and read out the state of a qubit, such as an electron spin in a quantum dot (QD), is a requirement in virtually any scheme for quantum information processing(1-3). However, previous optical measurements of a single electron spin have focused on time-averaged detection, with the spin being initialized and read out continuously(4-8). Here, we monitor the coherent evolution of an electron spin in a single QD. We use time-resolved Kerr rotation (KR) spectroscopy, an all-optical, non-destructive technique that enables us to monitor the precession of the spin in a superposition of Zeeman-split sublevels with nanosecond time resolution. The data show an exponential decay of the spin polarizationwith time, and directly reveal the g-factor and spin lifetime of the electron in the QD. Furthermore, the observed spin dynamics provide a sensitive probe of the local nuclear spin environment.