  A commonly held tenet is that lasers well above threshold emit photons in a
coherent state, which follow a Poissonian statistics when measured in photon
number. This feature is often exploited to build quantum-based random number
generators or to derive the secure key rate of quantum key distribution
systems. Hence the photon number distribution of the light source can directly
impact the randomness and the security distilled from such devices. Here, we
propose a method based on measuring correlation functions to experimentally
characterise a light source's photon statistics and use it in the estimation of
a quantum key distribution system's key rate. This promises to be a useful tool
for the certification of quantum-related technologies.
