  New physics in the early Universe could lead to parity-violation in the late
Universe, sourcing statistics whose sign changes under point reflection. The
best constraints on such phenomena have come from the Planck temperature
fluctuations; however, this is already cosmic-variance-limited down to
relatively small scales, thus only small improvements are expected in the
future. Here, we search for signatures of parity-violation in the polarized
CMB, using the Planck PR4 $T$- and $E$-mode data. We perform both a
simulation-based blind test for any parity-violating signal at $\ell<518$, and
a targeted search for primordial $U(1)$ gauge fields (and the amplitudes of a
generic collapsed model) at $\ell<2000$. In all cases, we find no evidence for
new physics, with the model-independent test finding consistency with the
FFP10/NPIPE simulation suite at $(-)0.4\sigma$, and the gauge field test
constraining the fractional amplitude of gauge fields during inflation to be
below $6\times 10^{-19}$ at $95\%$ confidence level for a fiducial model. The
addition of polarization data can significantly improve the constraints,
depending on the particular model of primordial physics, and the bounds will
tighten significantly with the inclusion of smaller-scale information.
