  Stokes parameters (${\bf S}$) in Poincar\'e sphere are very useful values to
describe the polarisation state of photons. However, the fundamental principle
of the nature of polarisation is not completely understood, yet, because we
have no concrete consensus how to describe spin of photons,
quantum-mechanically. Here, we have considered a monochromatic coherent ray of
photons, described by a many-body coherent state, and tried to establish a
fundamental basis to describe the spin state of photons, in connection with a
classical description based on Stokes parameters. We show that a spinor
description of the coherent state is equivalent to Jones vector for
polarisation states, and obtain the spin operators (${\bf \hat{S}}$) of all
components based on rotators in a $SU(2)$ group theory. Polarisation
controllers such as phase-shifters and rotators are also obtained as
quantum-mechanical operators to change the phase of the wavefunction for
polarisation states. We show that the Stokes parameters are quantum-mechanical
average of the obtained spin operators, ${\bf S} = \langle {\bf \hat{S}}
\rangle $.
