  Diamond nanocrystals can harbor spin qubit sensors capable of probing the
physical properties of biological systems with nanoscale spatial resolution.
These diamond nanosensors can readily be delivered into intact cells and even
living organisms. However, applications beyond current proof-of-principle
experiments require a substantial increase in sensitivity, which is generally
limited by surface-noise-induced spin dephasing and relaxation. In this work,
we significantly reduce magnetic surface noise by engineering core-shell
structures, which in combination with dynamical decoupling result in qubit
coherence times (T2) ranging from 52us to 87us - a drastic improvement over the
1.1us to 35us seen in bare particles. This improvement in spin coherence,
combined with an overall increase in particle fluorescence, corresponds to a
two-order-of-magnitude reduction in integration time. Probing qubit dynamics at
a single particle level, furthermore, reveals that the noise characteristics
fundamentally change from a bath with spins that rearrange their spatial
configuration during the course of an experiment to a more dilute static bath.
The observed results shed light on the underlying mechanisms governing spin
dephasing in diamond nanocrystals and offer an effective noise mitigation
strategy based on engineered core-shell structures.
