Meng, K.; Nie, L.; Berger, J.; von Grafenstein, N. R.; Einholz, C.; Rizzato, R.; Schleicher, E.; Bucher, D. B.

Optically addressable spin systems, such as nitrogen-vacancy centers in diamond, have been widely studied for quantum sensing applications. In this work, we demonstrate that flavin-based cryptochrome proteins, which generate radical pairs upon optical excitation, also exhibit optically detected magnetic resonance. We further show that this optical spin interface is tunable by the protein structure. These findings establish radical pairs in proteins as a novel platform for optically addressable spin systems and magnetic field sensors. Additionally, the ability to control spin transitions introduces a new class of biophysical tools that hold promise for enabling multiplexed fluorescence microscopy. Importantly, due to the spin-selective nature of radical pair chemistry, the results lay the groundwork for radiofrequency-based manipulation of biological systems.