Using ALMA to calibrate the polarization angle at CMB frequencies

<p>The CMB polarization has recently been used to test for Cosmic Polarization Rotation, a phenomenon linked the the violation of fundamental physical principles. These tests are at the moment limited by the accuracy in the calibration of the polarization angle. This problem is also affecting the accuracy of the CMB B-mode polarization measurements, since errors in the zero point of the polarization angle would produce a leakage of E-modes into B-modes. Ways to solve the problem will be discussed, in particular those involving the observation with ALMA of polarized radio sources at CMB frequencies.