Photodetection probabilities are calculated far from a nonlinear medium or atom in parametric down-conversion. These probabilities may be heuristically interpreted as due to single-photon states with photodetection amplitudes having a power law tail of exponent 5/4 in configuration space.
A new theory for multimode parametric downconversion is given that yields differen, t predictions from existing treatments for photodetection tails. The theory is rigorous and is derived from first principles. The cases of both a single-atom and a nonlinear-crystal source are treated. The detection is taken to be far from the source. For a single atom the photodetection amplitude has a tail in the reduced time that falls off asymptotically as the inverse second power. The corresponding tail for the case of a crystal has an inverse quartic falloff.
Extreme events appear in many physics phenomena, whenever the probability distribution has a ''heavy tail'', differing very much from the equilibrium one. Most unusual are the cases of power-law (Pareto) probability distributions. Among their many manifestations in physics, from ''rogue waves'' in the ocean to Levy flights in random walks, Pareto dependences can follow very different power laws. For some outstanding cases the power exponents are less than 2, leading to indefinite mean values, let alone higher moments. Here we present the first evidence of indefinite-mean Pareto distribution of photon numbers at the output of nonlinear effects pumped by parametrically amplified vacuum noise, known as bright squeezed vacuum (BSV). We observe a Pareto distribution with power exponent 1.31 when BSV is used as a pump for supercontinuum generation, and other heavy-tailed distributions (however with definite moments) when it pumps optical harmonics generation. Unlike in other fields, we can flexibly control the Pareto exponent by changing the experimental parameters. This extremely fluctuating light is interesting for ghost imaging and quantum thermodynamics as a resource to produce more efficiently non-equilibrium states by single-photon subtraction, the latter we demonstrate experimentally.