HanburyBrownTwissSetup

The HanburyBrownTwissSetup is a measurement arrangement in quantum optics used to probe second-order coherence of light by measuring intensity correlations between two detectors that receive light split from a common source.

The setup is named after Robert Hanbury Brown and Richard Q. Twiss, who introduced it in the 1950s to determine stellar angular diameters. Their intensity interferometry method relies on correlating fluctuations in detected light rather than preserving phase information, revealing spatial coherence properties of light and enabling measurements that are robust to optical path fluctuations.

A typical implementation uses a beam splitter to divide the optical field between two photodetectors. The arrival times of photons at the detectors are recorded, and a correlator computes the coincidence rate as a function of time delay τ. From these coincidences one constructs the second-order coherence function g2(τ) = <I1(t)I2(t+τ)> / <I1><I2>. For stationary light, g2(τ) provides information about photon statistics and coherence. The visibility and width of g2(τ) relate to the source’s coherence time and spatial coherence.

The HanburyBrownTwissSetup has been fundamental in distinguishing light statistics: chaotic or thermal light shows photon bunching with g2(0) > 1 (g2(0) ≈ 2 for ideal thermal light), coherent light yields g2(0) = 1, and single-photon sources can exhibit antibunching with g2(0) < 1. The method underpins intensity interferometry and is widely used in quantum optics to characterize light sources, including laser beams, astronomical light, and quantum emitters, with various detector technologies and data-analysis approaches.