Dark matter and dark energy could be ultralight pseudoscalar fields that couple to electromagnetism through a Chern-Simons term. This would cause a parity-breaking rotation of the plane of linearly polarized light from the cosmic microwave background, known as cosmic birefringence, which is not predicted by $\mathrm{\Lambda }$ cold dark matter. Recent hints of a nonzero cosmic birefringence angle have been found in both the Planck Data Release 4 and Atacama Cosmology Telescope Data Release 6 datasets independently. This work jointly analyzes the polarized maps of both, leveraging the cross-power spectra between the two telescopes. Taking the reported instrumental priors at face value, the joint analysis yields $\beta =0.277°\pm 0.057°$, which is nonzero with a statistical significance of $4.8\sigma$. A robustness test mitigating contamination from dust emission excludes $\beta =0°$ at $3.5\sigma$, and retrieving $\beta =0°$ would require both the dust $EB$ modeling and instrumental priors to fail simultaneously. However, unresolved systematics in the data must be understood before we can draw strong cosmological conclusions.

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• Cosmic microwave background
• Cosmology