In quantum optics, as in classical optics, beam-splitters play an important role in many experimental settings. Electric elds E1 and E2 enter input ports 1 and 2, respectively. Note that jT j2 is the transmitted intensity. The transformation matrix is then given by. A symmetric beam-splitter is a cube of glass which reflects half the light that impinges upon it, while allowing the remaining half to pass through unaffected. For our purposes it can simply be viewed as a device that has two input and two output ports, which we label with $mathrm{mid }0⟩$ and. A beam splitter or beamsplitter is an optical device that splits a beam of light into a transmitted and a reflected beam. It is a crucial part of many optical experimental and measurement systems, such as interferometers, also finding widespread application in fibre optic telecommunications. We use elementary laws of classical and quantum optics to obtain general relations among the magnitudes and phases of these probability amplitudes. Quantum-mechanically, it will not split each photon in two, but it will transmit or reflect each photon with 50% probability (see picture of an actual beamsplitter).
