Typically, linear optical components such as directional couplers (which act as beamsplitters between waveguide modes), and phase shifters to form nested Mach–Zehnder interferometers[17][18][19] are used to encode a qubit in the spatial degree of freedom.
[47] Photons are particularly attractive carriers of quantum information due to their low decoherence properties, light-speed transmission and ease of manipulation.
Single die photonic circuits offer the following advantages over bulk optics: Being based on well-developed fabrication techniques, the elements employed in Integrated Quantum Photonics are more readily miniaturisable, and products based on this approach can be manufactured using existing production processes and methods.
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.
Large silicon-on-insulator (SOI) wafers up to 300 mm in diameter can be obtained commercially, making the technology both available and reproducible.
Lithium niobate offers a large second-order optical nonlinearity, enabling generation of photon pairs via spontaneous parametric down-conversion.
This can also be leveraged to manipulate phase and perform mode conversion at high speeds, and offers a promising route to feed-forward for quantum computation, multiplexed (deterministic) single photons sources).
[51] Photonic waveguides made from group III-V materials on insulator, such as (Al)GaAs and InP, provide some of the largest second and third order nonlinearities, large refractive index contrast providing large modal confinement, and wide optical bandgaps resulting in negligible two-photon absorption at telecommunications wavelengths.
[58][59][60][61] 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.