= Idler photon
An <idler photon> is the partner of a <signal photon> in a specified nonlinear optical process. In <spontaneous parametric down-conversion>, one pump <photon> supplies the <energy> of the pair, $\hbar\omega_p=\hbar\omega_s+\hbar\omega_i$.
The idler can be measured to herald a signal <photon> or to condition a correlation measurement. It is not a physically expendable <photon>: losses in its channel reduce heralding rates and can bias an uncorrected coincidence sample. When the two output modes have the same <frequency>, the signal/idler distinction comes from their assigned spatial, polarization or experimental roles rather than from <energy> ordering.
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