From Free-Space Quantum Optics Experiments to PEALD SiN Waveguides for Integrated Quantum Photonic Devices

Photonic platforms are strong candidates for scalable quantum information processing because the photonic circuit itself operates at room temperature, its qubits are largely immune to thermal decoherence, and the platform is compatible with existing fiber and CMOS infrastructure. The dominant error channel in these platforms is not decoherence but optical loss, which makes the quality of the guiding material the limiting factor when quantum optics experiments are translated from free-space benches to integrated circuits. A free-space heralded single-photon quantum eraser at 810 nm restores a fringe visibility of 0.900 ± 0.012 from 0.060 ± 0.002 and sets the specification for the thin film. This work reports a plasma-enhanced atomic layer deposition (PEALD) process for silicon nitride (SixNy) from bis(diethylamino)silane (BDEAS) and N2-plasma, developed to supply the core of such waveguides. At 810 nm, the thin film reaches a refractive index of 1.824, and the Urbach term of the ellipsometric fit gives an extinction coefficient of 7.9 × 10−10. Also, hard X-ray photoelectron spectroscopy shows an N:Si ratio of 1.16 with 12.7% carbon. Furthermore, the SixNy waveguide core can be clad with a lower refractive index thin film of SiO2 by the same process.