A big advantage of using silicon is that the circuits can be tuned actively using integrated thermal microheaters or p-i-n modulators, after the devices have been fabricated. The other big benefit of silicon is its compatibility with CMOS technology, which allows leveraging the mature fabrication infrastructure of the semiconductor electronics industry. The structures are different from modern electronic ones, however, they are readily scalable. Silicon has a really high refractive index of ~3.5 at the 1550 nm wavelength commonly used in optical telecommunications. It also makes a nice interface with glass which has lower refractive index of ~1.44. This allows waveguides made form silicon and glass to be small and have tight bends, which allows you to make denser systems. Large silicon-on-insulator (SOI) wafers up to 300 mm in diameter can be commercially obtained, making the technology reproducible. Conventional fabrication technologies are based on photolithographic processes, which enable strong miniaturization and mass production. In quantum optics applications a relevant role has been played also by the direct inscription of the circuits by femtosecond lasers or UV lasers; these are serial fabrication technologies, which are particularly convenient for research purposes, where novel designs have to be tested with rapid fabrication turnaround. However, laser-written waveguides are not suitable for mass production and miniaturization due to the serial nature of the inscription technique, and due to the very low refractive index contrast allowed by these materials, as opposed to silicon photonic circuits. Femtosecond laser written quantum circuits have proven particularly suited for the manipulation of the polarization degree of freedom and for building circuits with innovative three-dimensional design. Quantum information is encoded on-chip in either the path, polarisation, time bin or frequency state of the photon, and manipulated using active integrated components in a compact and stable manner. Channels  Produces a fixed phase