We present the strongest constraints to date on anisotropies of CMB polarization rotation derived from 150 GHz data taken by the BICEP2 & Keck Array CMB experiments up to and including the 2014 observing season ( BK14 ) .
The definition of polarization angle in BK14 maps has gone through self-calibration in which the overall angle is adjusted to minimize the observed TB and EB power spectra .
After this procedure , the QU maps lose sensitivity to a uniform polarization rotation but are still sensitive to anisotropies of polarization rotation .
This analysis places constraints on the anisotropies of polarization rotation , which could be generated by CMB photons interacting with axion-like pseudoscalar fields or Faraday rotation induced by primordial magnetic fields .
The sensitivity of BK14 maps ( \sim 3 \mu K-arcmin ) makes it possible to reconstruct anisotropies of polarization rotation angle and measure their angular power spectrum much more precisely than previous attempts .
Our data are found to be consistent with no polarization rotation anisotropies , improving the upper bound on the amplitude of the rotation angle spectrum by roughly an order of magnitude compared to the previous best constraints .
Our results lead to an order of magnitude better constraint on the coupling constant of the Chern-Simons electromagnetic term f _ { a } \geq 1.7 \times 10 ^ { 2 } \times ( H _ { I } / 2 \pi ) ( 2 \sigma ) than the constraint derived from uniform rotation , where H _ { I } is the inflationary Hubble scale .
The upper bound on the amplitude of the primordial magnetic fields is 30 nG ( 2 \sigma ) from the polarization rotation anisotropies .