Researchers report the first demonstration of a frequency-degenerate twin-photon source in an integrated silicon nitride (Si3N4) microring resonator. By using two lasers to pump different resonances of the same tiny ring, they convert pairs of pump photons into pairs of identical photons at telecommunication wavelengths. The same device can also be run as a conventional heralded single-photon source by changing the pumping scheme.

The photon-pair generation relies on a nonlinear process known as spontaneous four-wave mixing (SFWM). In the dual-pump configuration used here — called pump non-degenerate SFWM — two photons from spectrally different pumps are absorbed and two identical photons are produced at a central resonance. The team aligned the two pump lasers to resonances on either side of the target mode (labelled Ch33 and Ch35) so that the twin photons appear in the central channel (Ch34). They kept the pump power low so that the main non-vacuum output is the two-photon component of a single-mode squeezed state.

To show they really produced twin photons, the authors measured photon correlations. They report a maximum coincidence-to-accidental ratio (CAR) of 5.4 ± 0.6. CAR is the ratio of true coincident photon detections to random coincidences; higher values indicate stronger non-classical correlations. The experiment also compared the dual-pump case with single-pump backgrounds and found a clear bunching peak at zero delay only when both pumps were present, supporting the interpretation that the twin photons come from the pump non-degenerate SFWM process. Detection was done with efficient superconducting nanowire single-photon detectors (SNSPDs).

The microring also served as a heralded single-photon source in the pump-degenerate SFWM regime, where a single pump resonance is driven and frequency-distinct signal and idler photons are produced. In that mode the device showed a measured spectral purity of P = 0.67 ± 0.05 and a heralded anti-bunching value g_h^(2)(0) = 0.0042 ± 0.0015. Spectral purity near 1 would mean the photons occupy a single spectral mode; here the value indicates some multimode character. The very low heralded g^(2) value indicates strong suppression of multi-photon events and therefore good single-photon quality.