Decoherence, Manipulation And Controllable Coupling Of Spin Qubits

This PhD thesis is mainly focused on the quantum computing with spins. The background, the development, and the potential application of quantum computing are outlined.A NV center in diamond is a good candidate of qubit, owing to its long qubit coherence time, even at a room temperature. We study the dephasing of a central spin-1model which applies to NV centers in diamond at certain circumstances. By investigating this model, we find an interesting dephasing mechanism for the electron spin in a nuclear spin environment, named as the "hyperfine mediated spectral diffusion". In this mechanism, the quantum fluctuations of the Overhauser field come from the "hyperfine mediated interaction". It should be noted that for the traditional central spin-1/2model, the "hyperfine mediated interaction" does not introduce fluctuations to the Overhauser field.In order to implement universal quantum computing, it is necessary to achieve inter-actions between qubits. The singlet-triplet (ST0) qubit, due to its controllability by an external electric field, is of great current interest. We propose a completely controllable scheme to couple two STo qubits via an exchange-coupling mechanism. By designing a particular magnetic field, we can avoid the leakage from the Hilbert space of the singlet-triplet qubits. Our scheme is experimentally realizable.A nanowire quantum dot with strong spin-orbit coupling can be used to achieve a spin-orbit qubit. Different from the conventional spin qubit, the spin-orbit qubit contains both orbital and spin degrees of freedom of an electron. Thus, the spin-orbit qubit can be manipulated by an external a.c. electric field, an effect called electric-dipole spin resonance. We analytically study the electric-dipole spin resonance in a nanowire quantum dot with strong spin-orbit coupling and find that there is an optimal spin-orbit coupling, where the Rabi frequency induced by the external a.c. electric field becomes maximal. Both the level spacing and the Rabi frequency have periodic responses to the direction of the applied external magnetic field. Thus, we propose to determine the Rashba and Dresselhaus spin-orbit couplings in the nanowire by monitoring these responses.