Optical (also known as photonic or bosonic) quantum computing uses photons, so-called “flying qubits”, to encode |0〉 or |1〉. Several different methods exist for encoding |0〉 and |1〉, including the photon polarization or the spin angular momentum state (polarization encoding), different optical paths in an optical system (path encoding), the spatial distribution of light such as different optical modes in an optical waveguide (orbital angular momentum encoding), and time or frequency encoding. We have already examined the case of polarization encoding for quantum communication in Chap. 6 . Let us consider the key optical elements used for optical quantum computing, using path encoding as an example.