Quantum Information Processing Using Nanocrystal-Microsphere Systems

提出了一种复合量子点（QD）微腔，该微腔由半导体纳米晶体耦合到熔融二氧化硅微球的高q低语通道模式（WGM）。这种新型微腔结合了熔融二氧化硅微球的独特特性和高质量量子点的三维电子约束，其q因子超过108，比其他现有的半导体微腔高4个数量级。所提出的系统允许制造、选择和组装单个电子和光子元件。纳米晶微球系统可用于实现Cirac-Zollertype的量子逻辑门。利用纳米晶体的基态和亚稳态激发态作为量子比特。辅助偶极跃迁用于双量子位运算。量子比特之间的相干相互作用由高q WGM中的光子介导，并通过两个晶体之间的相干光子交换。量子限制斯塔克效应用于调节给定的纳米晶体与WGM的开启和关闭共振，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，将一组纳米晶体共价附着在熔融二氧化硅微球的赤道上，并在纳米晶体和谐振WGM之间实现相干光子交换，这是实现所提出的量子逻辑门的两个最重要的步骤。新的实验技术也将用于研究单纳米晶体的退相干过程。为了将一组纳米晶体附着在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表面化学和先前的配体交换化学被共价连接（化学吸附）到球体表面。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交换配体。为了更精确地控制纳米晶体的位置，微接触印刷将用于实现局部表面衍生化。作为一种替代方法，利用原子力显微镜对纳米晶体进行微观操作也将被研究。为了实现强偶极子耦合以及由此产生的纳米晶体和共振WGM之间的相干光子交换，将使用一种纳米晶体-微球系统，其中高q WGM与单个纳米晶体共振耦合。实验研究将通过最小g和最大grad / g比的光学跃迁进行，其中g和grad分别是纳米晶体的总退相干率和辐射退相干率。利用高q wgmo对单纳米晶体吸收或发射的极高灵敏度，将开发一种新的光谱技术来测量单纳米晶体的吸收和激发光谱。受激光子回波也将用于获得纳米晶体的退相干和种群松弛的信息，特别是与声学声子边带相关的纯去相。结合从单晶和集成平均光子回波研究中获得的结果，将使我们能够识别具有最小g和最大grad / g比的光学跃迁。这些研究也将导致对纳米晶体退相干过程急需的理解。除了完成在QD系统中实现量子逻辑门的两个最重要的步骤外，所提出的研究应导致识别使用QD系统的量子信息设备中的重要问题，包括技术和基础问题。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普遍的量子点的量子计算方案也很重要。

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.