We report Atacama Large Millimeter Array ( ALMA ) Band 6 full-polarization observations of the lensed blazar PKS 1830 - 211 during its record-breaking radio and gamma-ray flare in the spring of 2019 .
The observations were taken close to the peak of the gamma activity and show a clear difference in polarization state between the two time-delayed images .
The leading image has a fractional polarization about three times lower than the trailing image , implying that significant depolarization occurred during the flare .
In addition , we observe clear intra-hour variability of the polarization properties between the two lensed images , with a quasi-linear increase in the differential electric-vector position angle at a rate of about two degrees per hour , associated with changes in the relative fractional polarization of \sim 10 % .
This variability , combined with the lower polarization close to the peak of gamma activity , is in agreement with models of magnetic turbulence to explain polarization variability in blazar jets .
Finally , the comparison of results from the full and differential polarization analysis confirms that the differential polarization technique ( Martí-Vidal et al . 19 ) can provide useful information on the polarization state of sources like gravitationally lensed radio-loud quasars .