Cond Mat: Alexander Efros - NRL

Fast dephasing of electron spins in an ensemble of quantum dots is
detrimental for applications in quantum-information processing. We show that dephasing can be overcome by using a periodic train of light pulses to
synchronize the phases of the precessing spins, and demonstrate this effect in an ensemble of sinly charged In,Ga)As/GaAs  quantum dots.  We first discuss the physical mechanism of this synchronization in a quantum-dot ensemble.  A periodic train of circularly polarized light pulses from a mode-locked laser  synchronizes the precession of the spins to the laser repetition rate,  transferring the mode-locking into the spin system. The mode-locking technique  allows us to measure the single-spin coherence time to be 3 microseconds,  which is four orders of magnitude longer than the ensemble dephasing dephasing time of 400 picoseconds. The technique also offers the possibility of acheiving all-optical coherent manipulation of spin ensembles, in which electron spins  can be clocked by two trains of pump pulses with a fixed temporal delay. After
this pulse sequence, the quantum-dot ensemble shows multiple bursts of Faraday rotation signals, whose repetition period agrees well with developed theory.   The nuclei in these experiments act constructively, leading to the nuclear- frequency-focusing effect, which moves the electron-spin precession into  dephasing-free subspace. This effect has the potential for focusing all  precession frequencies of the quantum-dot ensemble to a single precession mode.