The prevailing model of a network capable of complex quantum information processing consists of solid-state quantum memory and processor bits (qubits) connected via flying qubits in photonic channels. The negatively charged nitrogen-vacancy defect in diamond (NV) has an electronic spin state with long coherence times that can be optically initialized, manipulated, and measured - unique properties that allow the NV to act as an optically accessible solid-state qubit. Unfortunately, the scalability of quantum networks fabricated entirely in diamond is limited due to the stochastic nature of NV placement and the difficulty of large-scale high-quality diamond patterning. In contrast, silicon nitride (SiN) based photonics is a mature field allowing for the fabrication of complex photonic circuits.
Here we present a scalable method for the bottom-up fabrication of a hybrid photonic circuit combining the superior quantum properties of single NVs with the mature fabrication of SiN photonic elements. Using transfer-mask nanolithography developed by our group, we fabricated arrays of single-mode micro-waveguides from diamond membranes with a low density of naturally occurring NVs. These micro-waveguides were optically characterized with a confocal setup. We selected individual diamond micro-waveguides with optimal NV placement and deterministically placed them over air gaps in single-mode SiN waveguides. Simulations indicate that with an adiabatic-like transition between the diamond and SiN waveguide modes, up to 82% of the photons emitted from the NV can be collected into the single mode SiN waveguide. This high collection is crucial for high fidelity spin measurements and network scalability.
We demonstrate efficient coupling of fluorescence from a single NV into a single-mode SiN photonic circuit with a rate of more than 1.4 million photons collected into a single direction of a single-mode SiN waveguide at saturation. The single-photon statistics of signal collected through the waveguide are preserved below saturation with antibunching of photon arrivals below 0.5. Moreover, the NV electron spin coherence time is measured to be comparable to previous bulk measurements, indicating that our nano-fabrication technique and device assembly do not degrade the nuclear and electronic environments around the NV centers. In conclusion, we have demonstrated the high-yield integration of diamond micro-waveguides containing NV centers with high-quality spectral and spin properties into SiN photonic circuitry. Scaling to multiple, high-quality quantum bits integrated into a complex photonic circuit is made possible by pre-screening arrays of diamond micro-waveguides for optimal coupling of fluorescence. This provides a scalable architecture for the creation of a complex photonic circuit for quantum information processing.