Quantum Information Processing Using Nanocrystal-Microsphere Systems

University Of Oregon Eugene, Eugene OR

Investigators

Abstract

A composite quantum-dot (QD) microcavity consisting of semiconductor nanocrystals coupling to a high-Q whispering gallery mode (WGM) of a fused silica microsphere is proposed. This novel microcavity combines unique properties of a fused silica microsphere with 3D electronic confinement in high quality QDs and features a Q-factor exceeding 108, four orders of magnitude greater than that of other existing semiconductor microcavities. The proposed system allows fabrication, selection, and assembling of individual electronic and photonic components. The nanocrystal-microsphere system can be used to implement quantum logic gates of Cirac-Zoller type. The ground state and a metastable excited state of the nanocrystal are used as a qubit. An auxiliary dipole transition is used for two-qubit operations. Coherent interactions between qubits are mediated by a photon in a high-Q WGM and through coherent photon exchange between two nanocrystals. Quantum confined Stark effects are used to tune a given nanocrystal on- and off-resonance with the WGM and to control the coherent photon exchange process. Research efforts in this program are aimed at developing experimental approaches to covalently attach an array of nanocrystals to the equator of a fused silica microsphere and at achieving coherent photon exchange between a nanocrystal and a resonant WGM, two most important steps toward implementing the proposed quantum logic gate. New experimental techniques will also be developed to investigate decoherence processes in single nanocrystals. To attach an array of nanocrystals to the equator of a fused silica microsphere, the nanocrystals will be covalently linked (chemisorbed) to the sphere surface by exploiting the well-developed surface chemistry of silica and precedented ligand exchange chemistry. The two primary means to be used for chemisorption will be peptide bond formation and ligand exchange with surface bound thiols. For a more precise control of nanocrystal positions, microcontact printing will be used to achieve localized surface derivatization. As an alternative, micro-manipulation of nanocrystals by using an atomic force microscope will also be pursued. To achieve strong dipole coupling and the resulting coherent photon exchange between a nanocrystal and a resonant WGM, a nanocrystal-microsphere system where a high-Q WGM couples resonantly to a single nanocrystal will be used. Experimental studies will be carried out by using optical transitions with the smallest g and the largest ratio of grad / g where g and grad are the total decoherence rate and the radiative decoherence rate of nanocrystals, respectively. A new spectroscopic technique that takes advantage of the extreme sensitivity of a high-Q WGM to absorption or emission from a single nanocrystal will be developed to measure absorption and excitation spectra of a single nanocrystal. Stimulated photon echoes will also be used to obtain information on both decoherence and population relaxation of nanocrystals and especially on pure-dephasing associated with acoustic phonon side bands. Combining results obtained from the single nanocrystal and the ensemble-average photon echo investigations will enable us to identify optical transitions 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 a QD system, the proposed research should lead to identification of important issues, both technological and fundamental, in quantum information devices using a QD system. The achievement of the strong coupling regime for a single QD will open up a new frontier of semiconductor quantum optics and will make possible devices such as optical switching at the level of a single photon and microlasers at the level of a single QD. The understanding of decoherence processes in nanocrystals should also be important to any quantum computing scheme that uses semiconductor nanocrystals or more generally QDs.