= Resonance fluorescence antibunching experiment
In 1977, <H. Jeff Kimble>, <Mario Dagenais>, and <Leonard Mandel> excited a dilute <sodium> atomic beam with a resonant <dye laser>. Apertures restricted the observed atoms; a <microscope objective> collected <fluorescence> perpendicular to both beams. A <beam splitter> sent the collected <light> to two <photomultipliers>. Shaped pulses drove the start and stop inputs of a time-to-digital converter; its delay histogram rose away from zero delay, indicating <photon antibunching>. Turning the atomic beam off measured scattered-laser backgrounds. More than one contributing atom and scattered <light> limited the comparison with an ideal isolated <two-level system>; the paper explicitly treated corrections.<Photon Antibunching in Resonance Fluorescence by Harry Jeffrey Kimble and others>[1977, p. 693, figure 2, equation (6), apparatus and atomic-beam-off/on control; MIT mirror]{ref}
A simple reason for suppressed immediate re-emission is visible in an ideal <two-level system>. After <spontaneous emission>, the atom is in $|g\rangle$. Its emission operator $\sigma_-=|g\rangle\langle e|$ obeys $\sigma_-^2=0$, so two instantaneous emissions are impossible. Excitation must rebuild the excited-state population before another emission. This argument explains the zero-delay idealization; the actual <photon counting> record additionally depends on collection, background, timing, and the number of contributing atoms.
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