Engineering Quantum Sensors for Probing Spin and Magnon Dynamics

Diamond nitrogen-vacancy (NV) centers have emerged as a uniquely versatile platform for quantum sensing across disciplines—from single-molecule biophysics to condensed-matter spin dynamics and magnonics. This thesis presents four complementary advances that to- gether extend the utility of NV centers in challenging environments and lay the groundwork for integrated quantum sensing technologies:

1. Biocompatible surface functionalization architecture for a diamond quan- tum sensor. We develop a sub-2 nm surface chemistry that immobilizes individual proteins and DNA directly on bulk diamond-achieving controlled adsorption density and preserving near-surface NV coherence times near 100 μs under physiological condi- tions for over five days. This architecture enables nanoscale nuclear magnetic resonance spectroscopy of single biomolecules.

2. Engineering Dark Spin-Free Diamond Interfaces. By coating diamond with a thin TiO2 layer, we suppress intrinsic "dark" surface electron spins from ∼2 000 μm−2 to below the ∼ 200 μm−2 detection limit of our NV sensors. This reduction yields a two-fold enhancement in spin-echo coherence times and is corroborated by electronic- structure modeling of the diamond-TiO2 interface.

3. Central Spin Decoherence via Mean-Field in Dissipative Baths. We derive an exact expression for the coherence of a qubit coupled via σ(s)z σ(e)z to a single dissipa- tive electron spin undergoing an N-pulse CPMG sequence, showing that conventional spectral methods arise as the leading-order approximation. Extending to many-spin ensembles, a spatially averaged mean-field approximation renders the problem analyt- ically tractable and informs the design of optimized decoupling protocols.

4. Probing coherent nonlinear magnon dynamics with a qubit sensor. We per- form the first single-NV measurement of parametrically pumped magnon lifetimes- finding decay times on the order of tens of nanoseconds-under unprotected conditions. To move beyond lifetime characterization and access coherent dynamics, we then in- troduce a protection protocol that suppresses magnon-induced NV relaxation while en- hancing magnon population. With this scheme, we observe Rabi-like oscillations of the parametrically pumped magnons, confirming their phase coherence at the nanoscale.

5. Magnetic-field control system design and nuclear-spin control. I developed an automated, closed-loop magnetic-field control system that positions a permanent mag- net on a motorized 3-D translation and rotation stage, delivering fields up to ∼4000 G at the sample. The setup aligns any chosen NV axis within a few minutes and provides a stable, reproducible vector field. This stability enables high-fidelity single-shot readout of the NV electron spin—using the coupled nuclear spin as a quantum memory—and supports coherent nuclear-spin manipulation.

Collectively, these contributions advance diamond NV centers as a powerful, coherent quan- tum sensor platform-spanning biological spectroscopy, surface-spin engineering, rigorous spin-bath modeling, and nanoscale magnonics—and pave the way toward multifunctional quantum devices in sensing and information processing.