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The 1999 experiment used an approximately $200\mathrm{fs}$, $394\mathrm{nm}$ pump pulse and a beta barium borate crystal. Two polarization-entangled photon pairs supplied four photons. A trigger and three other photodetectors selected a GHZ state through fourfold coincidence counting. Narrow interference filters lengthened the photon coherence time to approximately $500\mathrm{fs}$, exceeding the pump duration and suppressing timing information about which pair supplied which photon. The optical mode matching was essential to coherence, not merely to routing photons.^[ref]

The observed state was selected by all four detections, which also measured the photons. A trigger alone did not demonstrate a guaranteed surviving three-photon output. This distinction connects postselection to what an apparatus actually supplies, rather than identifying a fourfold coincidence with an unconditional source.^[ref]

An interference filter uses wave interference in a layered optical structure to transmit or reject selected wavelengths. The accumulated optical path length makes reflected contributions reinforce or cancel differently across the optical spectrum.

A narrow-band example can isolate emission before a photodetector or reject an excitation beam in fluorescence microscopy. Tilting the filter changes the propagation angles and optical paths inside its layers, shifting the transmitted band. A filter characterized in a collimated beam may therefore behave differently in a strongly converging beam; temperature and polarization can also affect transmission.