# All-optical quantum sensing of rotational Brownian motion of magnetic molecules

## TL;DR

This paper introduces a novel all-optical method using NV centers in nanodiamonds to detect fast rotational Brownian motions of magnetic molecules, enabling high-resolution sensing of local viscosity and molecular dynamics.

## Contribution

The work demonstrates a new all-optical technique for detecting rapid rotational motions of small magnetic molecules using NV centers, surpassing conventional optical methods in spatial resolution.

## Key findings

- Successfully detected motions of Gd molecules with NV centers.
- Differentiated solutions with varying viscosities through relaxation signals.
- Established a new approach for high-resolution molecular motion sensing.

## Abstract

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.