  Active galactic nuclei (AGN) host some of the most energetic phenomena in the
Universe. AGN are thought to be powered by accretion of matter onto a rotating
disk that surrounds a supermassive black hole. Jet streams can be boosted in
energy near the event horizon of the black hole and then flow outward along the
rotation axis of the disk. The mechanism that forms such a jet and guides it
over scales from a few light-days up to millions of light-years remains
uncertain, but magnetic fields are thought to play a critical role. Using the
Atacama large mm/submm array (ALMA), we have detected a polarization signal
(Faraday rotation) related to the strong magnetic field at the jet base of a
distant AGN, PKS1830-211. The amount of Faraday rotation (rotation measure) is
proportional to the magnetic field strength along the line of sight times the
density of electrons. Although it is impossible to precisely infer the magnetic
fields in the region of Faraday rotation, the high rotation measures derived
suggest magnetic fields of at least tens of Gauss (and possibly considerably
higher) on scales of the order of light days (0.01 pc) from the black hole.
