  Fast radio bursts (FRBs) are bright millisecond radio bursts at cosmological
distances. Only a small fraction of FRBs apparently repeat. Polarization, a
fundamental property of electromagnetic signals, often carries critical
information about the radiation processes, the environment, and the intervening
medium of FRBs. Here we report circular polarization detections of two active
repeating FRBs, namely FRBs 20121102A and 20190520B, with the
Five-hundred-meter Aperture Spherical radio Telescope. We detect circular
polarization in both active repeating FRBs, which increases the number of
repeating FRBs with circular polarization to three. In one of the bursts of FRB
20121102A, we detect 64% degree of circular polarization. The observed circular
polarization is unlikely induced by multipath propagation. Our observations
favor circular polarization induced by Faraday conversion or radiation
mechanism intrinsic to the FRB source. The conditions to generate circular
polarization have to be rare in either case.
