Multiphoton states related via linear optics
Piotr Migdał, Javier Rodríguez-Laguna, Michał Oszmaniec, Maciej Lewenstein · Physical Review A · 2014
The problem of transforming n-photon pure states into each other by linear optics without postselection is investigated by finding functions on the Hilbert space that are invariant under linear optical transformations.