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           COLLOQUIUM OF THE COMPUTATIONAL MATERIALS SCIENCE CENTER
                              College of Science
                       (CDS Department CSI 898-Sec 001)
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                 To make spins in charged quantum dots identical

                              Alexander Efros

              Center for Computational Materials Science, Naval Research
                              Laboratory, Washington DC


The growing attention to the manipulation of electron spins localized in 
quantum dots is connected with its critical importance for quantum computing 
and quantum information processing. The quantum dots grown today, however, 
are never identical. The small deviation of their shape and size, as well as 
random fluctuations of the nuclear hyperfine magnetic field leads to a 
dispersion of electron spin precession frequencies in a magnetic field. This 
dispersion causes fast dephasing of electron spins in an ensemble of quantum 
dots, which 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 singly charged (In,Ga)As/GaAs quantum dots. We first 
discuss the physical mechanism of this synchronization for pulses of different 
intensity. 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 time of 400 picoseconds. The interference also gives the possibility 
for all-optical coherent manipulation of spin ensembles, in which the electron 
spins can be clocked by two trains of pump pulses with a fixed temporal delay. 

                        Monday , September 17, 2007
                                  4:30 pm
                     Room 301, Research I, Fairfax Campus 

Refreshments will be served at 4:15 PM.
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Find the schedule at www.cmasc.gmu.edu/seminar/schedule.html
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