High-Purity Single Photon Extraction from Diamond Nitrogen-Vacancy Centers in Low-Numerical-Aperture Optical Systems...

Solid-state quantum emitters, such as nitrogen-vacancy (NV) centers in diamond, offer a promising route toward scalable quantum technologies. However, their random spatial distribution and inherently broad dipole emission severely hinder high-purity single-photon extraction-particularly in low-numerical-aperture (NA) optical systems, which are essential for compact and scalable quantum photonic systems. Here, a defect-selective metalens integration approach is presented that allows precise and efficient single photon collection from individual NV centers, even under ultra-low-NA optical systems. Using an in situ transfer-printing process, high-purity silicon dioxide metalenses are stamped precisely and deterministically onto selected NV centers located 25 µm below the diamond surface. This integration reshapes the broad dipolar emission of the targeted NV center into a tightly collimated, low-divergence beam, achieving a 40-fold enhancement in photon collection efficiency with an objective lens of NA = 0.07. Notably, emission is detected exclusively from metalens-coupled NV centers, effectively filtering out background noise from neighboring defects. Second-order correlation measurements yield g⁽²⁾(0) = 0.04, unambiguously confirming the generation of high-purity single photons. This scalable, site-specific approach addresses key limitations in low-NA operation, opening the door to compact and fiber-integrated solid-state quantum photonics.