Quantum computation using optical control of electron spins in quantum dots

量子点中的电子自旋是未来大规模量子计算的重要候选者之一。我们已经提出了一套利用光子辅助隧穿及其库仑阻塞的通用量子门。在本项目中，我们对这一方案的可行性进行了各种实验研究。此外，量子计算还需要连接到光子量子信息处理。因此，我们提出了一种转换光子量子比特和电子自旋量子比特的方案，并对这一方案进行了实验评估。在实验之前，我们对我们的万能门进行了计算机模拟，发现如果电子自旋的相干性大于100ps，就有可能实现CNOT门的实验演示。然后，我们开发了一种基于四波混频的测量技术来检测电子自旋相干性。我们将其应用于量子阱中电子的自旋弛豫测量，建立了泵浦-探测技术。自旋绕射四波混频的计算结果与泵浦探测的结果一致，说明了该方法的有效性。然后，我们将其应用于InAlAs/AlGaAs量子点系综，发现这些量子点的最长相干性长达7 ns。这个数值比传统的条纹相机测量方法要长得多，这表明在精确测量自旋相干性时，需要像我们这样的共振激发和检测测量，而不是像扫描相机那样的非共振测量。我们还提出了一种新的量子比特转换方案，利用核场实现量子点中的简并电子能级和能量分裂空穴能级。通过对InAlAs/AlGaAs量子点中电子自旋的光学定向来实现核场，我们发现可以实现光子与十电子自旋之间的量子比特转换的能带结构。

An electron spin in a quantum dot is one of important candidates to be used for future large scale quantum computing. We have already proposed a set of universal quantum gates using photon-assisted tunneling and its coulomb blockade. In this project, we have done various experimental studies on the feasibility of this proposal. Also, the quantum computing is required to be connected to the photonic quantum infromation processing. Therefore we proposed a scheme to cunvert a photon qubit and an electron spin qubit and experimentally evaluated this proposal.Before experiments, we did computer simulation of our universal gates and found that the experimental demonstration of the CNOT gateis possible ifthe coheraice of an electron spin is largo-than 100 ps.Then, we developed a measurem ent technology based on the four-wave-mixing FWM to detarmine the electron spin coherence. We applied this to the spin relaxation measurements of electrons in a quantum well for which the pump -probe technique is established. The result of the new method, i.e. spin-diffracted FWM gave the same result as the pump -probe, indicating the validity of the method. We then, applied this to the ensemble of InAlAs/AlGaAs quantum dots and found that the longest coherence of these dots is as long as 7 ns. This value is much longer than those in the conventional method using streak-camera and indicates that the measurements of resonant excitation and dedection such as ours are necessary for precise determination of spin coherence instead of the non-resonant measurements such as those using streak-camers.We also proposed a new qubit conversion scheme using the nuclear field to realize the degenerate electron levels and energy-split hole levels in a quantum dot. By exerimentally realizing the nuclear field by optically orienting the electron spins in an InAlAs/AlGaAs quantum dot, we found that we can realize the band structure for a qubit conversion between a photon and an dectron spin.