Quantum mechanical description of linear optics

Linear optical networks with any number of modes are described quantum mechanically. We reformulate the conventional formal description based on input-output operator relations and obtain an intuitive description based on the possible paths taken by the photons. The effect of a linear optical network on localized photons is treated within the same formalism. The potential and limitations of linear optics for application in quantum information processing are briefly discussed. (C) 2004 American Association of Physics Teachers.