Quantum Information Processing Using Nanocrystal-Microsphere Systems

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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.