Abstract：Topological defects, in particular, cosmic strings can be abundant in the universe. Such defects can lead to a Berry phase observable in the Cosmic Microwave Background (CMB). In this background of cosmic strings, this new Berry phase appears as a frequency-independent topological polarization rotation of CMB photons. I will discuss how to look for such polarization rotation in the CMB. I will also discuss how to look for these strings in strongly lensed quasar systems.