Above-Unity Coherent Cooperativity of Tin-Vacancy Centers in Diamond Photonic Crystal Cavities

The tin-vacancy center in diamond (SnV) has emerged as a compelling building block for realizing next-generation quantum networks thanks to its excellent optical and spin properties. Coupling to photonic crystal cavities (PCCs) promises to further enhance the SnV light-matter interface and unlock a diverse range of entanglement generation protocols. Recent pioneering experiments showing Purcell enhancement of SnV centers in PCCs underscore this potential. However, optical coupling that is coherent—the key ingredient for use in quantum protocols—has so far remained elusive. Here, we demonstrate above-unity coherent cooperativity of SnV centers embedded in photonic crystal cavities. We fabricate free-standing PCCs using a quasi-isotropic undercut. Across two samples, we conduct room-temperature characterizations, measuring resonances for 327 cavities, with an average quality factor exceeding Q = 1.1 ( 4 ) × 10 4 . Two cavity-coupled emitters are examined in detail, exhibiting quality factors up to Q = 25.4 ( 4 ) × 10 3 and Purcell-reduced lifetimes corresponding to cooperativities up to C = 20.3 ( 11 ) . Furthermore, the single SnVs are observed to strongly modulate the cavity transmission with an extinction contrast up to 98.8(4)%% on resonance. Finally, SnV linewidth measurements reveal above-unity coherent cooperativities in both devices, with the highest value being C coh = 8.3 ( 12 ) . These results open the door to using cavity-coupled SnV centers as efficient, coherent light-matter interfaces for future quantum networks.