Electron spin manipulation in coupled multiple quantum wells for quantum computing

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We have studied electron spin properties in semiconductor heterostructures in view of applying it for quantum computing. A very long spin life time is required for achieving practical quantum computing using electron spins as qubits. We have experimentally investigated the spin relaxation of electrons which are photogenerated in bulk GaAs region with circularly polarized pump beam, focusing on which part of the sample the dominant spin relaxation takes place. We have separated electron dynamics in the sample into three stages : (1) Rapid energy relaxation immediately after photogeneration, (2)Field-driven transport in the bulk GaAs region, (3)Electron capture process from the bulk GaAs band edge into the bottom of quantum wells, and we have prepared the sample with two InGaAs quantum wells which work as spin detectors. Our experimental results show that the spin relaxation in stage (1) is small, and the spin relaxation during the transport is strongly dependent on the field strength, when the electric field is kept small, the spin relaxation during transport is very small. The spin relaxation in the capture stage is large, dominating the whole spin relaxation process in our sample. This finding would cause problems since in many proposed spintronics and quantum computing schemes the spin injection into the quantum well is assumed. We are preparing samples with different well width to further investigate the effects of energy levels in quantum wells to the spin relaxations.In order to implement a Controlled-Not gate in quantum computing using spins, adjacent electrons should be coupled with exchange interactions. We proposed a CN gate utilizing the exchange interaction of electrons in coupled 2-dimensional layers. However, out basic study shows that the exchange interaction is very small between the electrons in adjacent layers. We project a plan to study the alternative method using electron spins in quantum dots.