Mandel Q parameter
The Mandel Q parameter tells you how much a light source’s photon numbers deviate from Poisson (random) statistics. It was introduced by Leonard Mandel. Negative Q means sub-Poissonian statistics, where the photon-number fluctuations are smaller than the mean and cannot be explained by classical light.
Q is the normalized difference between the variance and the mean of the photon number: Q = [Var(n) − ⟨n⟩] / ⟨n⟩, where n is the photon-number operator.
- The smallest value is Q = −1, which happens for exact photon-number (Fock) states that have zero number fluctuations.
- For thermal (black-body) light, the statistics are Bose-Einstein and Q equals the mean ⟨n⟩.
- For ideal coherent light, the statistics are Poissonian and Q = 0.
This page was last edited on 2 February 2026, at 23:55 (CET).