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We studied the polarization behavior of the quasar 3C 273 over the 1 mmwavelength band at ALMA with a total bandwidth of 7.5GHz across 223 to 243 GHzat 0.8 arcsec resolution, corresponding to 2.1 kpc at the distance of 3C 273.With these observations we were able to probe the optically thin polarizedemission close to the jet base, and constrain the magnetic field structure. Wecomputed the Faraday rotation measure using simple linear fitting and Faradayrotation measure synthesis. In addition, we modeled the broadband behavior ofthe fractional Stokes Q and U parameters (qu-fitting). The systematicuncertainties in the polarization observations at ALMA were assessed throughMonte Carlo simulations. We find the unresolved core of 3C 273 to be 1.8%linearly polarized. We detect a very high rotation measure (RM) of ~5.0 x10^5rad/m^2 over the 1 mm band when assuming a single polarized component and anexternal RM screen. This results in a rotation of >40 deg of the intrinsicelectric vector position angle, which is significantly higher than typicallyassumed for millimeter wavelengths. The polarization fraction increases as afunction of wavelength, which according to our qu-fitting could be due tomultiple polarized components of different Faraday depth within our beam, orinternal Faraday rotation. With our limited wavelength coverage, we cannotdistinguish between the cases, and additional multifrequency and high angularresolution observations are needed to determine the location and structure ofthe magnetic field of the Faraday active region. Comparing our RM estimate withvalues obtained at lower frequencies, the RM increases as a function ofobserving frequency, following a power law with an index of ~2.0 consistentwith a sheath surrounding a conically expanding jet. We also detect ~0.2%circular polarization, although further observations are needed to confirm thisresult.

Magnetic field at a jet base: extreme Faraday rotation in 3C 273 revealed by ALMA