  Mm and sub-mm waves polarimetry is the new frontier of research in Cosmic
Microwave Background and Interstellar Dust studies. Polarimeters working in the
IR to MM range need to be operated at cryogenic temperatures, to limit the
systematic effects related to the emission of the polarization analyzer. In
this paper we study the effect of the temperature of the different components
of a waveplate polarimeter, and describe a system able to rotate, in a
completely automated way, a birefringent crystal at 4K. We simulate the main
systematic effects related to the temperature and non-ideality of the optical
components in a Stokes polarimeter. To limit these effects, a cryogenic
implementation of the polarimeter is mandatory. In our system, the rotation
produced by a step motor, running at room temperature, is transmitted down to
cryogenic temperatures by means of a long shaft and gears running on custom
cryogenic bearings. Our system is able to rotate, in a completely automated
way, a birefringent crystal at 4K, dissipating only a few mW in the cold
environment. A readout system based on optical fibers allows to control the
rotation of the crystal to better than 0.1{\deg}. This device fulfills the
stringent requirements for operation in cryogenic space experiments, like the
forthcoming PILOT, BOOMERanG and LSPE.
