superPoissonian

Super-Poissonian describes a statistical property of a counting process where the variance of the count distribution exceeds its mean. In photon statistics, a light source is said to be super-Poissonian if the photon-number variance Var(n) is greater than the mean ⟨n⟩, equivalently if the Fano factor F = Var(n)/⟨n⟩ is greater than 1. This is in contrast to Poissonian statistics, where Var(n) = ⟨n⟩ and F = 1, and sub-Poissonian statistics, where Var(n) < ⟨n⟩ and F < 1.

Super-Poissonian is commonly associated with photon bunching, often observed in thermal or chaotic light such as blackbody radiation or light from filament lamps. The second-order coherence g2(0) for such light is greater than 1, indicating a tendency for photons to arrive in bursts. In contrast, coherent light described by a laser can be Poissonian (F≈1) and certain quantum states such as single-photon Fock states can be sub-Poissonian (F<1).

In experiments, measuring photon counting distributions or time-resolved intensity fluctuations reveals super-Poissonian behavior in multi-mode or classically noisy sources. Excess fluctuations can arise from classical intensity modulation, mode competition, or thermal statistics.

The term is primarily used in quantum optics and photon statistics to describe the dispersion of photon counts, and is not a statement about all processes that happen to produce counts.