A single atom is excited by laser light and scatters one photon after another (IMAGE)
Caption
A single atom is excited by laser light and scatters one photon after another. An optical filter removes certain colour components from this stream of single photons. This causes the remaining photons to become pairs that leave the filter simultaneously. (Image: Department of Physics, Humboldt-Universität zu Berlin)
Credit
Department of Physics, Humboldt-Universität zu Berlin
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CC BY-NC