Using only cosmic microwave background polarization data from the polarbear experiment, we measure $B$-mode polarization delensing on subdegree scales at more than $5\sigma$ significance. We achieve a 14% $B$-mode power variance reduction, the highest to date for internal delensing, and improve this result to 22% by applying for the first time an iterative maximum a posteriori delensing method. Our analysis demonstrates the capability of internal delensing as a means of improving constraints on inflationary models, paving the way for the optimal analysis of next-generation primordial $B$-mode experiments.

• Received 1 October 2019
• Revised 20 December 2019
• Accepted 26 February 2020

DOI:https://doi.org/10.1103/PhysRevLett.124.131301