We constrain parity-violating interactions to the surface of last scattering using spectra from the QUaD experiments second and third seasons of observations by searching for a possible systematic rotation of the polarization directions of CMB photon
s. We measure the rotation angle due to such a possible cosmological birefringence to be 0.55 deg. +/- 0.82 deg. (random) +/- 0.5 deg. (systematic) using QUaDs 100 and 150 GHz TB and EB spectra over the multipole range 200 < l < 2000, consistent with null, and constrain Lorentz violating interactions to < 2^-43 GeV (68% confidence limit). This is the best constraint to date on electrodynamic parity violation on cosmological scales.
