= Bouwmeester three-photon GHZ experiment
{c}
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.<Observation of Three-Photon Greenberger-Horne-Zeilinger Entanglement by Dik Bouwmeester and others>[Printed pp. 1346–1347, figure 1 and equations (1)–(10): pump, filters, indistinguishable alternatives and coincidence conditioning]{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.<Observation of Three-Photon Greenberger-Horne-Zeilinger Entanglement by Dik Bouwmeester and others>[Printed p. 1345, paragraph beginning “The main idea”: explicit requirement that both trigger and three output photons be detected]{ref}
Back to article page
