Fig.1. The scheme of installation of Grangier with co-autors. The signal of detector PM0 from the first photon ν1 opens an electronic gate for the time τ during which photomultipliers PM1 and PM2 are ready to accept the second photon ν2. S – radiation source, N1 – counter of the reflected photons, N2 – counter of the passed photons, Nc – coincidence counter of PM1 and PM2