"Spin-based quantum computing" is a proposed method of doing numerical computations by manipulating the spin of particles such as electrons in semiconductors. Spin is an intrinsic property of electrons (and other fundamental particles) like charge or mass, and behaves in many ways similar to the angular momentum of a spinning object-hence the name. The spin of an electron has a direction associated with it, and may interact with its environment by changing its direction. Environmental factors which may affect spins include magnetic fields, the spin of other particles, and collective vibrations of atoms in a solid material. In order to be useful for quantum computing, the spins inside a material must be controllable-that is to say, the spins must not change direction accidentally, or at least the time scale of such accidental changes must be much larger than the time scale of the computing operations. The "spin dephasing" or "spin coherence time" (T2) is an important parameter to describe the rate at which a collection of spins stay pointed in the same direction, and is an important parameter for quantum computation-T2 times at least as long as microseconds are likely necessary to make possible spin-based quantum computing in the important semiconductor gallium arsenide (GaAs). The project proposes to measure T2 spin coherence times in GaAs and related materials through an optically detected spin echo experimental technique.
This project will study electron spin coherence (T2) times in n-GaAs and related materials. Research on electron spin lifetimes is of timely interest due to quantum computing proposals in which the spin of electrons in semiconductors is used as a quantum bit. In order for spin-based quantum computer schemes to work, they must be implemented in materials which have relatively long spin coherence lifetimes. This project will measure T2 times via optically detected spin echoes. The initial experiments will involve measuring time resolved photoluminescence polarization in a novel way where pump and probe optical pulses are applied by modulating a diode laser with an electron pulse sequence generator. That provides a way to measure the spin-flip time, T1-an upper bound for T2. The second set of experiments will be to perform magnetic resonance on the samples, by first optically polarizing the electron spins, and then optically detecting the change in polarization while a magnetic field is swept through resonance (as microwaves are applied at a constant frequency). The third set of experiments will be to combine the first two experiments in this fashion: (1) time-resolved pump and probe optical pulses are applied as in the first experiment, and (2) the microwave frequencies and magnetic field are held fixed at the resonant position and the microwaves are modulated by the electronic pulse generator to get a spin echo sequence of coherent p/4 and p/2 pulses, from which T2 can be deduced by making an optical measurement of the final spin state as a function of microwave pulse delays.
