In previous work, we measured a single electron spin in a quantum dot (QD) using Kerr rotation with continuous wave pump and probe lasers. These measurements revealed the steady-state spin polarization, concealing information about the evolution of the spin state in time. However, using time delayed pump and probe pulses allows for the coherent dynamics of the spin in the QD to be directly observed. The spins are initialized by the pump pulse at time t = 0, and measured at a later time by the probe pulse. The detection of a single electron spin is observed by an odd-Lorentzian Kerr rotation feature centered at the X- (negatively-charged exciton) transition energy, as shown in fig. 1b-f. The amplitude of the odd-Lorentzian feature is found from fits to the data and is proportional to the projection of the spin in the QD along the measurements axis. Plotting the amplitude of the signal as a function of pump-probe delay, the coherent evolution of the spin can be mapped out, as seen in fig. 3a. The data fits well to a single exponential decay convoluted with the shape of the 3 ns long probe pulse. A time-averaged transverse spin-lifetime of ~ 8 ns is found.
In Fig. 2a the precession of a single electron spin is shown at three different magnetic fields. The precession frequency, as found from fits to the data, is shown in Fig. 4b as a function of magnetic field. A linear fit to these data yields an electron g-factor of |g| = 0.17 ± 0.02.
To learn more about our studies, please refer to: "Optically detected coherent spin dynamics of a single electron in a quantum dot", M. H. Mikkelsen, J. Berezovsky, N. G. Stoltz, L. A. Coldren, and D. D. Awschalom, Nature Physics 3, 770 (2007).