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, .
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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