Quantum mechanics was first applied to optics, and interference in particular, by Paul Dirac. Richard Feynman, in his Lectures on Physics, uses Dirac's notation to describe thought experiments on double-slit interference of electrons. Feynman's approach was extended to N -slit interferometers for either single-photon illumination, or narrow- linewidth laser illumination, that is, illumination by indistinguishable photons, by Frank Duarte. The N -slit interferometer was first applied in the generation and measurement of complex interference patterns.