Photon Statistics — Hanbury Brown–Twiss
The g²(τ) second-order correlation function distinguishes photon sources. Coherent (laser) light gives g²(0)=1; thermal light g²(0)=2 (bunching); single-photon sources give g²(0)=0 (antibunching) — a purely quantum signature.
The HBT experiment splits a beam and measures coincidences at two detectors as a function of time delay τ. g²(τ) = ⟨I(t)I(t+τ)⟩/⟨I(t)⟩²