A beamsplitter routes photons to detectors D1 and D2. Classical fields always show some simultaneous hits (g²(0)≥1). A single quantum emitter can only emit one photon at a time — so D1 and D2 cannot click simultaneously: g²(0)→0.
The dip width equals the photon coherence time τ_c. This experiment (Kimble 1977) was the first direct proof that light is quantized into discrete photons.