Magnetic field at a jet base: extreme Faraday rotation in 3C 273 revealed by ALMA T. Hovatta, S. O’Sullivan, I. Martí-Vidal, T. Savolainen and A. Tchekhovskoy A&A, 623 (2019) A111 Published online: 12 March 2019 DOI: 10.1051/0004-6361/201832587