The nitrogen-vacancy center in diamond has been explored extensively as a light-matter interface for quantum information applications, however, it is limited by low coherent photon emission and spectral instability. Here, we present a promising interface based on an alternative defect with superior optical properties (the germanium-vacancy) coupled to a finesse-11 000 fiber cavity, resulting in a ${31}_{-15}^{+11}$-fold increase in the spectral density of zero-phonon-line emission. This work sets the stage for cryogenic experiments, where we predict a measurable increase in the spontaneous emission rate.

• Received 18 December 2019
• Revised 16 March 2020
• Accepted 30 March 2020

DOI:https://doi.org/10.1103/PhysRevApplied.13.064016