All-optical quantum sensing of rotational Brownian motion of magnetic molecules. (arXiv:1907.05396v1 [quant-ph])

Sensing local environment through the motional response of small molecules
lays the foundation of many fundamental technologies. The information of local
viscosity, for example, is contained in the random rotational Brownian motions
of molecules. However, detection of the motions is challenging for molecules
with sub-nanometer scale or high motional rates. Here we propose and
experimentally demonstrate a novel method of detecting fast rotational Brownian
motions of small magnetic molecules. With electronic spins as sensors, we are
able to detect changes in motional rates, which yield different noise spectra
and therefore different relaxation signals of the sensors. As a
proof-of-principle demonstration, we experimentally implemented this method to
detect the motions of gadolinium (Gd) complex molecules with nitrogen-vacancy
(NV) centers in nanodiamonds. With all-optical measurements of the NV centers'
longitudinal relaxation, we distinguished binary solutions with varying
viscosities. Our method paves a new way for detecting fast motions of
sub-nanometer sized magnetic molecules with better spatial resolution than
conventional optical methods. It also provides a new tool in designing better
contrast agents in magnetic resonance imaging.

Article web page: