  Solid-state defect qubit systems with spin-photon interfaces show great
promise for quantum information and metrology applications. Photon collection
efficiency, however, presents a major challenge for defect qubits in high
refractive index host materials. Inverse-design optimization of photonic
devices enables unprecedented flexibility in tailoring critical parameters of a
spin-photon interface including spectral response, photon polarization and
collection mode. Further, the design process can incorporate additional
constraints, such as fabrication tolerance and material processing limitations.
Here we design and demonstrate a compact hybrid gallium phosphide on diamond
inverse-design planar dielectric structure coupled to single near-surface
nitrogen-vacancy centers formed by implantation and annealing. We observe
device operation near the theoretical limit and measure up to a 14-fold
broadband enhancement in photon extraction efficiency. We expect that such
inverse-designed devices will enable realization of scalable arrays of
single-photon emitters, rapid characterization of new quantum emitters, sensing
and efficient heralded entanglement schemes.
