## The quantum information of quantum interference.

We show how one can understand the Hong-Ou-Mandel (HOM) effect from a first quantised perspective, connecting it with the quantum information concept of Schmidt decompositions that are core to entanglement theory. This gives a general approach to distinguishability between particles’ “System” and “Label” degrees of freedom, corresponding to what has been called unitary-unitary duality in many-body physics.