Spintronic quantum gates

5. Spintronic quantum gates
The idea of using a single electron or nuclear spin to encode a qubit, and then utilizing this to realize a universal quantum gate, has taken hold. The motivation for this is the realization that spin coherence times in solids are much larger than charge coherence times. Charge coherence times in semiconductors tend to saturate to about 1 ns as the temperature is lowered. This is presumably due to coupling to zero point motion of phonons, which cannot be eliminated by lowering the temperature. On the other hand, electron spin coherence times of 100 ns in GaAs at 5 K have already been reported and much higher coherence times are expected for nuclear spins in
silicon. Therefore, spin is obviously the preferred vehicle to encode qubits in solids.
Using spin to carry out all optical quantum computing has also appeared as a viable and intriguing idea. The advantage of the all-optical scheme over the electronic scheme is that we do not have to read single electron spins electrically to read a qubit. Electrical read out is extremely difficult, although some schemes have been proposed for this purpose. Recently, some experimental progress has been made in this direction, but reading a single qubit in the solid state still remains elusive. The difficult part is that electrical read out requires making contacts to individual quantum dots, which is an engineering challenge. In contrast, optical read out does not require contacts. The qubit is read out using a quantum jump technique, which requires monitoring the fluorescence from a quantum dot. Recently, it has been verified experimentally that the spin state of an electron in a quantum dot can be read by circularly polarized light. Therefore, optical read out appears to be a more practical approach.

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