The not quite Loudon-Fearn-Rarity-Tapster dip and its impact on the development of photonic quantum information.

This paper presents a short history of the discovery by Rodney Loudon and Heidi Fearn of the counter-intuitive destructive interference effect occurring when two indistinguishable photons meet at a beamsplitter. This effect, commonly known as the Hong Ou Mandel effect, underpins much of present day photonic quantum information processing. Here I try to review its development from inception to present day proposals of million qubit photonic quantum computers.This article is part of the theme issue 'The quantum theory of Light'.