Gravitational lensing effect on cosmic birefringence

We calculate the effect of gravitational lensing on the parity-odd power spectrum of the cosmic microwave background (CMB) polarization induced by axionlike particles (ALPs). Several recent works have reported a tantalizing hint of cosmic birefringence, a rotation of the linear polarization plane of CMB, which ALPs can explain. In future CMB observations, we can measure cosmic birefringence more precisely to get insight into ALPs. We find that the lensing effect is necessary to fit the observed EB power spectrum induced by cosmic birefringence in future CMB observations, including Simons Observatory and CMB-S4. We also show that the estimated ALPs parameters are biased if we ignore the lensing effect. Therefore, the lensing correction to the parity-odd power spectra must be included in future high-resolution CMB experiments.