A CMB Millikan Experiment with Cosmic Axiverse Strings

We study axion strings of hyperlight axions coupled to photons. These axion strings produce a distinct quantized polarization rotation of CMB photons which is O(1%).  As the CMB light passes many strings, this polarization rotation converts E-modes to B-modes and adds up like a random walk. Using numerical simulations we show that the expected size of the final result is well within the reach of current and future CMB experiments through the measurement of correlations of CMB B-modes with E- and T-modes. The quantized polarization rotation angle is topological in nature and its value depends only on the anomaly coefficient, independent of other details such as the axion decay constant. Measurement of the anomaly coefficient provides information about the UV theory, such as the quantization of electric charge and the value of the fundamental unit of charge.