A Controversial Experiment
When Hanbury Brown and Twiss announced in 1956 that photons from a thermal source tend to arrive in pairs (bunching), many physicists were skeptical — how could independent photons know about each other? The controversy spurred the development of quantum coherence theory by Roy Glauber and launched quantum optics as a discipline. This simulation recreates the HBT measurement for three fundamentally different light sources.
Measuring g⁽²⁾(τ)
The experiment splits light onto two single-photon detectors and counts coincidences as a function of the time delay τ between detection events. For thermal light, excess coincidences at τ = 0 reveal bunching — photons prefer to arrive together. The characteristic timescale of bunching equals the coherence time τ_c of the source, typically nanoseconds for filtered thermal light.
Three Quantum Signatures
The value of g⁽²⁾(0) cleanly separates three regimes: thermal light gives g⁽²⁾(0) = 2 (bunching from Bose-Einstein statistics), coherent laser light gives g⁽²⁾(0) = 1 (random Poisson arrivals), and a single quantum emitter gives g⁽²⁾(0) → 0 (antibunching — one photon cannot be split). The antibunching dip below 1 has no classical wave explanation and is the definitive signature of the particle nature of light.
Modern Applications
HBT measurements are now routine in quantum optics labs worldwide. They certify single-photon sources for quantum cryptography, characterize quantum dots and nitrogen-vacancy centers, and probe photon statistics in cavity QED and circuit QED systems. In astronomy, revived intensity interferometry using modern detectors promises sub-milliarcsecond stellar imaging with arrays of optical telescopes.