  We show that models of new particle physics containing massless pseudoscalar
fields super-weakly coupled to photons can be very efficiently probed with CMB
polarization anisotropies. The stochastic pseudoscalar fluctuations generated
during inflation provide a mechanism for converting E-mode polarization to
B-mode during photon propagation from the surface of last scattering. The
efficiency of this conversion process is controlled by the dimensionless ratio
H/(2\pi f_a), where H is the Hubble scale during inflation, and f_a^{-1} is the
strength of the pseudoscalar coupling to photons. The current observational
limits on the B-mode constrain this ratio to be less than 0.07, which in many
models of inflation translates to a sensitivity to values of f_a in excess of
10^{14} GeV, surpassing the sensitivity of other tests.
