Coupling nitrogen vacancy centre spins in nanodiamonds to external fields

This thesis focuses on the spin physics of nitrogen-vacancy (NV) centres in nanodiamonds (NDs) via their coupling to external fields. In the first part, we theoretically investigate the feasibility of coupling a ND with a high density of NV centres to a gap-tunable flux qubit. We calculate the optimum position including the tolerance for maximizing the coupling and calculate its dependence on the orientation of the ND. We demonstrate that the coherent coupling of a 150-nm ND containing many (10³) NV centres and a flux qubit with dephasing time of ~1 μs is feasible. In the second part, we investigate the spin-coherence properties of NV centres in NDs and discuss them in the context of the NVs-to-flux qubit coupling. In particular, we manipulate the NV spins via an external microwave field using two distinct methods. The first, more conventional method employs a thin wire in direct contact with the sample as the microwave antenna. The second method has been developed by our group. It uses a cavity to produce the microwave field at the position of the NV spins with the advantage over the wire architecture of addressing the NV centre spins, without sample heating and over macroscopic spatial distances (milli-meters versus micro-meters).