  The recently reported non-zero isotropic birefringence angle in Planck 2018
polarization data provides a tantalizing hint for new physics of axions. In
this paper, we explain this by a string theory motivated axion with a monodromy
potential that plays the role of dark energy. Upon using the birefringence
measurement and the constraint on the equation of state for dark energy in this
scenario, we find an upper bound on the axion decay constant as $f_a \lesssim
10^{16}$ GeV. This naturally gives an energy scale of order GUT and can resolve
the theoretical issue of super-Planckian field range of the conventional axion
dark energy model. We further study the implications of cosmic birefringence
for the underlying theory and its consequences for the string swampland
conjectures. We finally discuss oscillatory features in the dark energy sector
and the expected cosmic birefringence tomography.
