Dynamics of Coherent and Incoherent Spin Polarizations in Ensembles of Quantum Dots

The temperature dependence of spin coherence in InGaAs quantum dots is obtained from quantum beats observed in polarization-resolved pump-probe experiments. Within the same sample we clearly distinguish between coherent spin dynamics leading to quantum beats and incoherent long-lived spin-memory effects. Analysis of the coherent data using a theoretical model reveals $\approx 10$ times greater stability of the spin coherence at high temperature compared to that found previously for exciton states in four-wave-mixing experiments by Borri et al. [Phys. Rev. Lett. 87, 157401 (2001)]. The data on incoherent polarization reveal a new form of spin memory based on charged quantum dots.