modulated by u03c6 a and u03c6 b may be outputted with a probability of 1 16 and a single photon detection distribution of a single photon interfering based on a phase difference of u03c6 a u2212 u03c6 b may be outputted with a probability of u215b a single photon distribution of a single photon interfering based on a phase difference of u03c6 a u2212 u03c6 b u2212 u03c0 it may be outputted to a center of an output 2 772 table 2 illustrates an example of a phase modulation based quantum key distribution method table 2 organizes an amount of phase modulation allowed by an optical phase modulator of each of a transmitter and the receiver after a quantum key distribution protocol being referred to as bennett brassard 84 bb84 is applied referring to fig2 the transmitting end optical phase modulator 230 may perform four levels of phase modulations by combining bit information corresponding to a quantum key and basis information enabling a secure communication each bit may have two orthogonal basis information the basis information may be randomly generated therefore a single photon having a phase value such as 0 u03c0 2 u03c0 3 u03c0 2 and the like may be generated a.