Quantum Information Processing Using Nanocrystal-Microsphere Systems

提出了一种由半导体纳米晶与石英微球高Q回音壁模耦合构成的复合量子点微腔。这种新颖的微腔结合了熔融石英微球的独特性质和高质量量子点中的3D电子限制，并具有超过108的Q因子，比其他现有的半导体微腔大四个数量级。该系统允许制造、选择和组装单个的电子和光子器件，纳米微球系统可用于实现Cirac-Zollertype量子逻辑门。基态和亚稳态的激发态的量子比特被用作。辅助偶极跃迁用于两量子比特操作。量子比特之间的相干相互作用是由高Q WGM中的光子和两个纳米晶体之间的相干光子交换介导的。量子限制斯塔克效应被用来调整一个给定的纳米晶体与WGM的共振和非共振，并控制相干光子交换过程。该计划的研究工作旨在开发实验方法，将纳米晶体阵列共价连接到熔融石英微球的赤道上，并实现纳米晶体与共振WGM之间的相干光子交换。实现量子逻辑门的两个最重要的步骤。新的实验技术也将被开发来研究单个纳米晶体中的退相干过程。为了将纳米晶体阵列附着到熔融二氧化硅微球的赤道上，纳米晶体将通过利用二氧化硅的良好的表面化学和先前的配体交换化学共价连接（化学吸附）到球表面。化学吸附的两种主要方法是形成肽键和与表面结合的硫醇进行配体交换。为了更精确地控制表面的位置，微接触印刷将被用来实现局部表面衍生化。为了实现强偶极耦合和纳米晶体与共振WGM之间的相干光子交换，将使用纳米晶体-微球系统，其中高Q WGM与单个纳米晶体共振耦合。实验研究将利用具有最小g和最大格拉德/ g比值的光学跃迁进行，其中g和格拉德是纳米晶体的总退相干率和辐射退相干率，一种新的光谱技术，利用高灵敏度的高，为了测量单个晶体的吸收和激发光谱，将研制出一种测量单个晶体吸收或发射的Q WGM。受激光子回波也将被用来获得有关纳米晶体的退相干和布居弛豫的信息，特别是与声学声子边带相关的纯退相信息。结合单晶和整体平均光子回波研究的结果，将使我们能够识别具有最小g和最大格拉德/ g比的光学跃迁。这些研究也应该导致急需的理解在nanocrystalline.In的消相干过程完成两个最重要的步骤实现量子点系统的量子逻辑门，拟议的研究应该导致识别的重要问题，技术和基础，在量子点系统的量子信息设备使用。单量子点强耦合机制的实现将开辟半导体量子光学的新领域，并将使单光子水平的光开关和单量子点水平的微激光器等器件成为可能。理解纳米晶体中的退相干过程对于任何使用半导体纳米晶体或更一般的量子点的量子计算方案都是重要的。

A composite quantum-dot (QD) microcavity consisting of semiconductor nanocrystals couplingto a high-Q whispering gallery mode (WGM) of a fused silica microsphere is proposed. This novelmicrocavity combines unique properties of a fused silica microsphere with 3D electronic confinement inhigh quality QDs and features a Q-factor exceeding 108, four orders of magnitude greater than that ofother existing semiconductor microcavities. The proposed system allows fabrication, selection, andassembling of individual electronic and photonic components.The nanocrystal-microsphere system can be used to implement quantum logic gates of Cirac-Zollertype. The ground state and a metastable excited state of the nanocrystal are used as a qubit. Anauxiliary dipole transition is used for two-qubit operations. Coherent interactions between qubits aremediated by a photon in a high-Q WGM and through coherent photon exchange between twonanocrystals. Quantum confined Stark effects are used to tune a given nanocrystal on- and off-resonancewith the WGM and to control the coherent photon exchange process.Research efforts in this program are aimed at developing experimental approaches to covalentlyattach an array of nanocrystals to the equator of a fused silica microsphere and at achieving coherentphoton exchange between a nanocrystal and a resonant WGM, two most important steps towardimplementing the proposed quantum logic gate. New experimental techniques will also be developed toinvestigate decoherence processes in single nanocrystals.To attach an array of nanocrystals to the equator of a fused silica microsphere, the nanocrystalswill be covalently linked (chemisorbed) to the sphere surface by exploiting the well-developed surfacechemistry of silica and precedented ligand exchange chemistry. The two primary means to be used forchemisorption will be peptide bond formation and ligand exchange with surface bound thiols. For a moreprecise control of nanocrystal positions, microcontact printing will be used to achieve localized surfacederivatization. As an alternative, micro-manipulation of nanocrystals by using an atomic forcemicroscope will also be pursued.To achieve strong dipole coupling and the resulting coherent photon exchange between ananocrystal and a resonant WGM, a nanocrystal-microsphere system where a high-Q WGM couplesresonantly to a single nanocrystal will be used. Experimental studies will be carried out by using opticaltransitions with the smallest g and the largest ratio of grad / g where g and grad are the total decoherencerate and the radiative decoherence rate of nanocrystals, respectively.A new spectroscopic technique that takes advantage of the extreme sensitivity of a high-Q WGMto absorption or emission from a single nanocrystal will be developed to measure absorption andexcitation spectra of a single nanocrystal. Stimulated photon echoes will also be used to obtaininformation on both decoherence and population relaxation of nanocrystals and especially on pure-dephasing associated with acoustic phonon side bands. Combining results obtained from the singlenanocrystal and the ensemble-average photon echo investigations will enable us to identify opticaltransitions with the smallest g and the largest ratio of grad / g . These studies should also lead to much-needed understanding of decoherence processes in nanocrystals.In addition to accomplishing two most important steps toward realizing quantum logic gates in aQD system, the proposed research should lead to identification of important issues, both technologicaland fundamental, in quantum information devices using a QD system. The achievement of the strongcoupling regime for a single QD will open up a new frontier of semiconductor quantum optics and willmake possible devices such as optical switching at the level of a single photon and microlasers at the levelof a single QD. The understanding of decoherence processes in nanocrystals should also be important toany quantum computing scheme that uses semiconductor nanocrystals or more generally QDs.