  We search for evidence of parity-violating physics in the Planck 2018
polarization data, and report on a new measurement of the cosmic birefringence
angle, $\beta$. The previous measurements are limited by the systematic
uncertainty in the absolute polarization angles of the Planck detectors. We
mitigate this systematic uncertainty completely by simultaneously determining
$\beta$ and the angle miscalibration using the observed cross-correlation of
the $E$- and $B$-mode polarization of the cosmic microwave background and the
Galactic foreground emission. We show that the systematic errors are
effectively mitigated and achieve a factor-of-$2$ smaller uncertainty than the
previous measurement, finding $\beta=0.35 \pm 0.14\,\deg$ (68% C.L.), which
excludes $\beta = 0$ at $99.2$% C.L. This corresponds to the statistical
significance of $2.4\sigma$.
