Study on optical quantum information processing

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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.