We present new constraints on anisotropic birefringence of the cosmic microwave background polarization using two seasons of data from the Atacama Cosmology Telescope covering 456 square degrees of sky. The birefringence power spectrum, measured using a curved-sky quadratic estimator, is consistent with zero. Our results provide the tightest current constraint on birefringence over a range of angular scales between 5 arc minutes and 9°. We improve previous upper limits on the amplitude of a scale-invariant birefringence power spectrum by a factor of between 2 and 3. Assuming a nearly massless axion field during inflation, our result is equivalent to a $2\sigma$ upper limit on the Chern-Simons coupling constant between axions and photons of $g_{\alpha \gamma }<4.0\times {10}^{-2}/H_I$, where $H_I$ is the inflationary Hubble scale.

• Received 1 February 2020
• Accepted 2 April 2020

DOI:https://doi.org/10.1103/PhysRevD.101.083527