An example of the discrete case is a quantum system that can be in two possible states, e.g. the polarization of a photon. When the polarization is measured, it could be the horizontal state | H ⟩ ⟩ or the vertical state | V ⟩ ⟩ . Until its polarization is measured the photon can be in a superposition of both these states, so its state | ψ ψ ⟩ ⟩ could be written as