= 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.
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