= Solution
For stationary photodetection, the normalized correlation in <second-order optical coherence> is $g^{(2)}(\tau)=\langle E^-(t)E^-(t+\tau)E^+(t+\tau)E^+(t)\rangle/\langle E^-E^+\rangle^2$. <Normal ordering> expresses the absorption of two <photons>. In a single mode,
$$
g^{(2)}(0)=\frac{\langle N(N-1)\rangle}{\langle N\rangle^2}
=1+\frac{\operatorname{Var}N-\langle N\rangle}{\langle N\rangle^2}.
$$
A <coherent state> has $g^{(2)}(0)=1$. A single-mode thermal field has $g^{(2)}(0)=2$ and bunches, while a <number state> with $n>0$ gives $1-1/n$; a single-photon state has zero ideal coincidence <probability> at zero delay. Antibunching, a zero-delay suppression relative to nearby delays, is distinct from simply reporting an intensity <variance>. Multimode averaging and finite detector response reduce observed contrast.
The <Hanbury Brown and Twiss effect> is measured by splitting the <light> onto two detectors and histogramming coincidences versus delay. Background counts, detector timing, dead time and afterpulsing need calibration. Two detectors avoid confusing the single detector's inability to register arbitrarily close counts with real antibunching. First-order <interference> and second-order correlations contain different information: weak first-order coherence does not imply an absence of intensity bunching.<Correlation between Photons in two Coherent Beams of Light by Robert Hanbury Brown and Richard Quentin Twiss>[1956, two-detector optical correlation experiment]{ref}
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