  We study the imprint of a coupling of scalar dark energy to photon on the
cosmic microwave background (CMB) polarization. The time-evolving field value
as well as the perturbation of the scalar generically induce $B$-mode
polarization. Future CMB data will find either a cosmic parity violation in
temperature-polarization correlation due to the field value, or
perturbation-induced $B$-mode polarization that is almost indistinguishable
from that generated by gravitational lensing or primordial gravitational waves.
