E-Mail Since the very beginning of quantum physics, a hundred years ago, it has been known that all particles in the universe fall into two categories: fermions and bosons. For instance, the protons found in atomic nuclei are fermions, while bosons include photons--which are particles of light- as well as the BroutEnglert-Higgs boson, for which François Englert, a professor at ULB, was awarded a Nobel Prize in Physics in 2013. Bosons--especially photons--have a natural tendency to clump together. One of the most remarkable experiments that demonstrated photons' tendency to coalesce was conducted in 1987, when three physicists identified an effect that was since named after them: the Hong-Ou-Mandel effect. If two photons are sent simultaneously, each towards a different side of a beam splitter--a sort of semitransparent mirror--, one could expect that each photon will be either reflected or transmitted.