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 arcminutes and 9 degrees .
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 .