Single Optically Detectable Tumbling Spin in Silicon.

We demonstrate single-spin spectroscopy of a fluorescent tumbling defect in silicon called the "G center," behaving as a pseudomolecule randomly reorienting itself in the crystalline matrix. Using high-resolution spin spectroscopy, we reveal a fine magnetic structure resulting from the spin principal axes jumping between discrete orientations in the crystal. Modeling the atomic reorientation of the defect shows that spin tumbling induces variations in the coupling to the microwave magnetic field, enabling position-dependent Rabi frequencies to be detected in coherent spin control experiments. By virtue of its pseudomolecule configuration, the G center in silicon is a unique quantum system to investigate the mutual interaction between optical, spin, and rotation properties in a highly versatile material.