  We constrain parity-violating interactions to the surface of last scattering
using spectra from the QUaD experiment's second and third seasons of
observations by searching for a possible systematic rotation of the
polarization directions of CMB photons. 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 QUaD's 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.
