Type of Document	Dissertation
Author	Stepanenko, Dimitrije
URN	etd-06272005-222752
Title	Symmetry and Control in Spin-Based Quantum Computing
Degree	Doctor of Philosophy
Department	Physics, Department of
Advisory Committee	Advisor Name Title Nicholas E. Bonesteel Committee Chair Mark Riley Committee Member Stephan von Molnar Committee Member Vladimir Dobrosavljevic Committee Member Washington Mio Committee Member
Keywords	spin-orbit coupling quantum dots quantum computing
Date of Defense	2005-06-10
Availability	unrestricted
Abstract A promising proposal for quantum computation, due to Loss and DiVincenzo, is based on using electron spins in quantum dots as qubits --- two-level systems which are the quantum analogues of classical bits. Two-qubit operations (quantum gates) are then carried out by switching on and off the exchange interaction between neighboring spins (i.e. ``pulsing" the interaction). This thesis presents a study of the effects of anisotropic corrections to the exchange interaction due to spin-orbit coupling on this scheme. It is shown that {it time-symmetric pulsing} automatically eliminates some undesirable terms in the resulting quantum gates, and well-chosen pulse shapes can produce an effectively isotropic exchange gate which can be used for universal quantum computation. Deviations from perfect time-symmetric pulsing are then studied in the context of a microscopic model of GaAs quantum dots. A new proposal for universal quantum computation which uses control of anisotropic corrections is then presented. In this proposal, the number of pulses required to carry out quantum gates scales as the inverse of a dimensionless measure of the degree of control. The size of this dimensionless ``figure-of-merit" depends on (i) variation of anisotropy with interdot distance, and (ii) restrictions on the pulse duration due to decoherence for slow pulses and nonadiabatic transitions for fast pulses. Taking these constraints into account, the figure-of-merit is estimated for GaAs quantum dots and shown to be large enough to be useful for quantum computation.
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