Electron spin manipulation in coupled multiple quantum wells for quantum computing

我们研究了半导体异质结构中电子自旋的性质，以期将其应用于量子计算。使用电子自旋作为量子比特来实现实际的量子计算需要非常长的自旋寿命。实验研究了圆偏振泵浦光在GaAs体材料中光生电子的自旋弛豫，着重研究了自旋弛豫主要发生在样品的哪一部分。我们将样品中的电子动力学分为三个阶段：（1）光生后的快速能量弛豫，（2）体GaAs区的场驱动输运，（3）电子从体GaAs带边进入量子威尔斯底部的俘获过程，并制备了两个InGaAs量子威尔斯阱作为自旋探测器的样品。我们的实验结果表明，第（1）阶段的自旋弛豫很小，输运过程中的自旋弛豫强烈地依赖于电场强度，当电场保持较小时，输运过程中的自旋弛豫很小。在捕获阶段的自旋弛豫是大的，在我们的样品中占主导地位的整个自旋弛豫过程。这一发现会引起问题，因为在许多提出的自旋电子学和量子计算方案中，假设自旋注入量子阱。为了进一步研究量子威尔斯能级对自旋弛豫的影响，我们制备了不同阱宽的样品。我们提出了一种利用耦合二维层中电子的交换相互作用的CN门。然而，我们的基础研究表明，相邻层中的电子之间的交换相互作用非常小。我们计划研究在量子点中使用电子自旋的替代方法。

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.