Magnetic Noise Sensing and Nanomechanical Transducers for Spin Qubits

Abstract

Nitrogen-Vacancy (NV) centers have shown great promise as magnetometers and as quantum information processors. NV centers have proven to be very well-isolated and extremely localized quantum systems that couple naturally to external magnetic fields. In this thesis, that sensitivity to magnetic fields and otherwise long relaxation time is utilized as a probe near other magnetic-field-generating sources to gain insight into the underlying physics within those sources. Specifically, we show how the NV center can be used to observe a change in the Johnson magnetic field noise outside of conductors due to variations in the electron mean free path within the metal. Furthermore, great progress in engineering two-qubit gates between NV centers is also demonstrated. One avenue towards such a spin-spin coupling is to properly engineer an interaction between each separate NV with a mechanical oscillator; the resulting dynamics give rise to an effective interaction between NV centers. In light of the NV center's natural coupling to magnetic fields, one method to couple NV centers and resonators is through magnetically functionalizing the resonators. In this thesis, we demonstrate the engineering of very well-isolated (high quality factor) magnetically-functionalized resonators ($Q> 4 \times 10^5$), and also reach new highs in magnetic field gradients in such systems integrated with NV centers, both crucial parameters in the final fidelity of two-qubit gates. Separately, initial studies on a new potential quantum platform is presented: that of levitated magnets over superconductors. Such systems have the potential to have very high quality factors, and can be used as novel sensors, a new transducer for NV center entanglement, and to test fundamental gravitational quantum theories. Experimental progress reported here indicates a good understanding of the underlying levitation physics as well as motional spectra, quality factors, and motional nonlinearities. Finally, spin-off work in developing a strong software interface for the lab, pylabcontrol, is also discussed. Adoption of such software can make a big impact within the community, in terms of reducing redundant software development, make experimental work more repeatable, and improving general quality of life within the general experimental physics community.