High visibility on-chip quantum interference among indistinguishable single-photons from multiples sources is a key prerequisite for integrated linear optical quantum computing. Resonant enhancement in micro-ring resonators naturally enables brighter, purer and more indistinguishable single-photon production without any tight spectral filtering. The indistinguisha-bility of heralded single-photons from multiple micro-ring resonators has not been measured in any photonic platform. Here, we report on-chip indistinguishability measurements of heralded single-photons generated from independent micro-ring resonators by using an on-chip Mach-Zehnder interferometer and spectral demultiplexer. We measured the raw heralded two-photon interference fringe visibility as 72 ± 3%. This result agrees with our model, which includes device imperfections, spectral impurity and multi-pair emissions. We identify multi-pair emissions as the main factor limiting the nonclassical interference visibility, and show a route towards achieving near unity visibility in future experiments.
Photonic chips with silicon nitride (
Si3N4) microring resonators are well established as heralded single-photon sources, but their operation as frequency-degenerate twin-photon sources has not previously been demonstrated. Here, we realise a twin-photon source at telecommunication wavelengths in a
Si3N4 ring microresonator via an inverse four-wave mixing (FWM) process, in which two photons from spectrally distinct pumps are converted into a pair of identical twin photons. The measurements show a maximum coincidence-to-accidental ratio (CAR) of
5.4±0.6. In addition, the microresonator functions as a heralded single-photon source through pump-degenerate spontaneous four-wave mixing (SFWM), exhibiting a spectral purity of
P=0.67±0.05 and a heralded anti-bunching of
gh(2)(0)=0.0042±0.0015. Together, these results demonstrate both photon-generation schemes on a single integrated
Si3N4 platform, highlighting its potential for scalable, tailored quantum light generation.
Time correlated photon pairs are used to produce heralded single photon states for quantum integrated circuits. These states are generated by photon sources, which are called heralded single photon sources. They are based on the detection of one photon which heralds the presence of the other. In this way, pure single photon states can be probabilistically generated and, subsequently, manipulated in complex photonic circuits. Heralded single photon sources are the topic of this review. The authors detail the main parameters and the experiments involved in their characterization, with a focus on their use in integrated photonic circuits. Different geometries and technological platforms are compared, and an assessment of their performances is performed. The final result is that nearly perfect single photon sources are possible and feasible in an integrated platform. A few open issues are also underlined.