Progress on chip-based spontaneous four-wave mixing quantum light sources

In a nonlinear optical crystal, a two-photon state can be produced through SPDC process in a nonlinear optical crystal, in which the annihilation of a pump photon produces a pair of photons with lower frequency. This process conserves both energy and momentum, i.e., ω_p = ω_s +ω_i and κ_p = κ_s +κ_i, where ω and κ denote the angular frequency and momentum of the corresponding pump, signal and idler photons. SFWM is another promising method for obtaining quantum light sources. It is a third-order nonlinear process which annihilates two pump photons of respective frequencies of ω₁ and ω₂ for creation of two new photons of ω3 and ω4 frequencies with energy conservation ω₁ + ω₂ = ω₃ + ω₄, as illustrated. Compared to SPDC sources, photons generated through SFWM have similar frequencies to the pump light making it easier to fulfill the phase matching requirement. For the purpose of miniaturization, generating photon pairs on chip has attracted much attention. On-chip SFWM sources are most commonly based on silicon waveguides due to the sharp refractive index contrast between silicon core and air cladding which leads to tight optical confinement and thus strong nonlinearity. With the help of strong χ(3) coefficient in silicon material, it is possible to achieve efficient four-wave mixing over a waveguide of just a centimeter in length. Moreover, silicon-based waveguide has a narrow spontaneous Raman scattering spectrum, which can be readily rejected through modest optical filtering. With the rapid development of on-chip integration technology, photon pair sources with different materials and structures have emerged like a blowout. To track these progresses, we review the recent development on the generation of photon pairs via on-chip SFWM process and their related applications.

With rapid progress in photonic integration technologies, on-chip SFWM quantum light sources have reached an exciting stage of development. On the one hand, a variety of platforms and structures have been demonstrated to be viable for their realization. On the other hand, significant challenges remain for the widespread use of the light sources as well as their integration to form a complete on-chip system. One challenge is that the pair generation rate of on-chip SFWM sources remains at a relatively low level, which means that high-power pump laser must be used to generate sufficient number of photon pairs. However, beside introducing more noise, it is difficult to integrate high-power lasers on-chip, whose maximum output power is about 100 mW. A long-term solution is likely to be found through further development of integration technology and novel device structures for nonlinear enhancement as well as searching new materials with high χ(3) non-linearity. Another challenge is the collection efficiency of photons, which is the main reason for limiting the counting rate and heralding efficiency of heralded single photon source. It is mainly determined by waveguide transmission loss, coupling loss, channel loss, and detector detection efficiency. With the development of chip preparation technology and superconducting nanowire single photon detectors, it is reasonable to expect that the collection efficiency close to unity can be achieved. In addition, multi-photon emission is also a challenge for all probabilistic quantum light sources. With the improvement of heralding efficiency and the development of photon number resolution detection technology, the problem of multi-photon emission from a heralded single photon source can be resolved, thereby leading to a brighter single photon source or even Fock photon number sources.