We can model a photon going through the interferometer by considering that at each point it can be in a superposition of only two paths: the "lower" path which starts from the left, goes straight through both beam splitters, and ends at the top, and the "upper" path which starts from the bottom, goes straight through both beam splitters, and ends at the right. The quantum state of the photon is therefore a vector formula_109 that is a superposition of the "lower" path formula_110 and the "upper" path formula_111, that is, formula_112 for complex formula_113. In order to respect the postulate that formula_114 we require that formula_115.