Signal photon

A signal photon is the photon assigned to a designated signal mode in a photon-pair process such as spontaneous parametric down-conversion. Its partner is called the idler photon. The labels identify modes or experimental roles; they do not universally mean the higher-energy and lower-energy photon.

For a narrowband pump, pair production obeys ${\omega }_p={\omega }_s+{\omega }_i$. Spatial and polarization correlations also depend on the phase matching arrangement. Detecting the idler can herald the presence of the signal, but heralding efficiency depends on collection and loss. At degeneracy, signal and idler can have equal frequencies while remaining distinguishable by propagation direction or another mode label.