Photon Bunching & Antibunching: g²(τ)

Classical light bunches (g²(0)>1), coherent light is Poissonian (g²(0)=1), quantum light antibunches (g²(0)<1).

Thermal
⟨n⟩ = 0.30
τ_c = 10
g²(0) = —   —
The degree-of-second-order coherence: g²(τ) = ⟨Î(t)Î(t+τ)⟩/⟨Î⟩².
Thermal (chaotic) light: g²(0)=2 — photons arrive in bunches.
Coherent (laser) light: g²(τ)=1 for all τ — Poisson statistics.
Single-photon source: g²(0)=0 — perfect antibunching, one photon at a time.
The Hanbury Brown–Twiss experiment (1956) measured stellar angular diameters using photon correlations.