Study on optical quantum information processing

与所有线性光学量子信息处理相比，基于弱光学非线性的量子信息处理是一种很有前途和实用的方法，因为这样可以大大减少所需的物理资源。在这个项目中，我们研究了半导体中施主杂质提供的三能级Lamda系统。半导体中的施主杂质在低温下捕获一个未配对的电子，并成为电中性(D^0)。在直流磁场作用下，束缚电子的自旋态被塞曼频率分裂，形成亚稳态(量子位)。在激子的光激发下，中性施主捕获一个激子，形成一个新的态，称为施主束缚激子(DOX)。我们之所以把这个系统作为微弱光学非线性的来源，主要是因为它的非均匀线宽很小(~1 GHz)，比自组装的量子点系统小3-4个数量级。我们主要研究了密度低至10~(-13)cm~(-3)的GaAs中的施主束缚电子/激子系统。我们可以演示相干布居捕获(暗态)实验、核场诱导的退相效应(T^*2~少量纳秒)、长的T_1时间常数(~4毫秒)以及单个拉曼脉冲诱导的电子自旋态的拉比振荡。我们还研究了锌硒基质中的施主杂质，得到了类似的结果。这些实验结果表明，施主束缚的电子/激子系统是量子信息处理中一种很有前途的弱光学非线性光源。

Quantum information processing based on weak optical nonlinearity is a promising and practical approach compared to all linear optics quantum information processing, because a required physical resource can be dramatically reduced in this way. In this project we have studied three-level Lamda system provided by donor impurities in semiconductors. A donor impurity in semiconductor captures one un-paired electron at low temperatures and becomes electrically neutral (D^0). Under a dc magnetic field, the bound electron spin states are split by a Zeeman frequency, which constitute a metastable ground states (qubit). Under optical excitation of excitons, the neutral donor captures one exciton and forms a new state, called a donor bound exciton (DoX). The reason why we focused on this system as a source of weak optical nonlinearity is primarily its small inhomogeneous linewidth (~1GHz), which is three-four orders of magnitude smaller than self-assembled quantum dot systems. We mainly studied a donor bound electron/exciton system in GaAs with a density as low as 10^<13> (cm^<-3>). We could demonstrate the coherent population trapping (dark state) experiment, nuclear field induced dephasing effect (T^*_2 ~ few nsec), long T_1 time constant (~4msec), and single Raman pulse induced Rabi oscillation of electron spin states. We have also studied ^<19>F donor impurities in ZnSe matrix and obtained a similar result. These experimental results show that a donor bound electron/ exciton system is a promising candidate as a source of weak optical nonlinearity for quantum information processing.