A scalable gallium-phosphide-on-diamond spin-photon interface

TL;DR

This paper demonstrates a scalable, high-cooperativity quantum interface using gallium phosphide nanophotonics integrated with diamond to enhance quantum emitter-photon interactions for quantum networking.

Contribution

It introduces the first high-cooperativity coupling of quantum defects to hybrid nanophotonics on a scalable, planar platform combining gallium phosphide and diamond.

Findings

Over 600 GaP nanophotonic cavities integrated on diamond.

Confirmed above-unity cooperativity with multiple measurements.

Achieved spin-dependent transmission switching and single-shot spin readout.

Abstract

The efficient interfacing of quantum emitters and photons is fundamental to quantum networking. Quantum defects embedded in integrated nanophotonic circuits are promising for such applications due to the deterministic light-matter interactions of high-cooperativity ($C>1$) cavity quantum electrodynamics and potential for scalable integration with active photonic processing. Silicon-vacancy (SiV) centers embedded in diamond nanophotonic cavities are a leading approach due to their excellent optical and spin coherence, however their long-term scalability is limited by the diamond itself, as its suspended geometry and weak nonlinearity necessitates coupling to a second processing chip. Here we realize the first high-cooperativity coupling of quantum defects to hybrid-integrated nanophotonics in a scalable, planar platform. We integrate more than 600 gallium phosphide (GaP) nanophotonic cavities on a diamond substrate with near-surface SiV centers. We examine a particular device with two strongly coupled SiV centers in detail, confirming above-unity cooperativity via multiple independent measurements. Application of an external magnetic field via a permanent magnet enables optical resolution of the SiV spin transitions from which we determine a spin-relaxation time $T_1>0.4$ ms at 4 K. We utilize the high cooperativity coupling to observe spin-dependent transmission switching and the quantum jumps of the SiV spin via single-shot readout. These results, coupled with GaP's strong nonlinear properties, establish GaP-on-diamond as a scalable planar platform for quantum network applications.