= Photon bunching
<Photon bunching> is enhanced near-coincident detection relative to an appropriate independent-count reference. It need not require nonclassical <light>. In a classical intensity model with nonnegative instantaneous <light intensity> $I$, ideal detection with a conditional <Poisson distribution> gives
$$
g^{(2)}(0)=\frac{\langle I^2\rangle}{\langle I\rangle^2}
=1+\frac{\operatorname{Var}I}{\langle I\rangle^2}\geq1.
$$
A fluctuating intensity therefore bunches counts. For a stationary classical intensity with finite second moment, the <Cauchy-Schwarz inequality> further gives $\langle I(t)I(t+\tau)\rangle\leq\langle I^2\rangle$. A zero-delay correlation smaller than one at a later delay cannot be explained by this model.
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