Optical Coherent Control of a Single Charged Indium Arsenide Quantum Dot.

[摘要] Optically driven self-assembled quantum dots are leading candidates for use inquantum computers because of their potential for high speed gate operations andrelatively compact design. In this approach, each dot is charged with a single electronwhose spin serves as the quantum bit (qubit). This thesis addresses the need fora universal set of gates to physically implement quantum computing and discussesinitial steps toward spin-photon entanglement. Both are addressed through coherentcontrol of the spin state in a magnetic field with optical pulses.This work experimentally demonstrates coherent rotations of the electron spin.First, the electron spin is rotated at up to 0.5 terahertz about the optical axis by adetuned picosecond optical pulse. The rotation occurs via a nearly resonant stimulatedspin-flip Raman process involving a negatively charged exciton (trion). Second,rotation about an orthogonal axis is demonstrated due to electron spin precessionabout the magnetic field between two detuned optical pulses. The magnitude of theelectron g factor is measured to be 0.4, and the rotation speed is 30 gigahertz in a 6.6 Tesla field. Geometric phases are detected in quantum dots for the first time due tocyclic Rabi oscillations driven by a resonant continuous-wave laser. These geometricphases provide another method of spin rotation and can be used in a gate. Anycombination of these methods that provides spin rotation about two orthogonal axescan form an arbitrary rotation and together with a phase gate can form any unitarysingle qubit gate.Finally, this thesis also presents a detailed experimental procedure for creating apartial entanglement of internal variables within a quantum dot spin-exciton system,as a preliminary step to spin-photon entanglement. It has been proposed that entanglementbetween non-adjacent qubits in a large qubit network could occur via aphoton, fueling research to demonstrate spin-photon entanglement. The experimentproposed here will create the precursor state in 25 picoseconds with a predicted fidelityof 0.985, and the precursor superposition state has a theoretical entropy ofentanglement of 0.92.

[发布机构] University of Michigan

[效力级别] Quantum Computing

[关键词] Quantum Dot;Quantum Computing;Optical Spectroscopy;Entanglement;Coherent Control;Physics;Science;Applied Physics