In quantum Communication we encode information in polarisation state. But this can also be done performing phase modulation. And infact, we can go to more number of states using phase modulation. So, What is the difference between the two?
The phase of a wave is a different property from its polarization state, that’s all.
Stam Nicolis So can we encode information in phase for quantum communication?
I'm not sure what is the quantum communication. But a single quantum (a photon) has well defined polarization state but its phase seems to have no sense at all. Contrary, the (continuous) wave has well defined both phase and polarization, but I don't see why should it be "quantum".
Encoding information, for example using an optical telecommunications system, can use a coherent source for phase modulation. In this case, a burst of coherent photons that share the same phase forms a pulse that carries one or more bits of information depending on the format of the phase system. It takes a few (not that many!) photons to create a signal with adequate signal to noise ratio. Phase modulation for fiber optic telecom systems has been around for 30 years. In these telecom systems, a local oscillator at the receiver decodes the phase of the incoming signal just like in an analogous radio system. As Stam Nicholis said above, the polarization of a lightwave signal is a different property from its phase. In polarization modulation, a device switches the polarization of a signal to one of N states for transmission through a polarization maintaining fiber or free space. At the receiving end, a second device detects the polarization of the signal.
I may add that phase at any instance of a wave is a relative property or changes in the phase are what we detect. On the other hand polarsation state, of a fully
polarised wave, can be measureed in an absolute sense in the laboratory frame.