  Polarization is a fundamental degree of freedom for light and is widely
leveraged in free space and fiber optics. Non-reciprocal polarization rotation,
enabled via the magneto-optic Faraday effect, has been essentially unbeatable
for broadband isolators and circulators. For integrated photonics foundries,
however, there is still no good path to producing low-loss magneto-optic
components, which has prompted a search for alternatives that do not use
polarization rotation. Moreover, magneto-optic materials tend to be highly
lossy, and while large (10-100 rad/cm) polarization rotation can be achieved,
the key figure of merit (rotation-per-loss) is typically < 1 rad/dB. Here, we
demonstrate that broadband non-reciprocal polarization rotation can be produced
using electro-optics in nanophotonic devices. Our demonstration leverages
electro-optic inter-polarization scattering around 780 nm in lithium niobate,
in which the reciprocity is broken with the help of a radiofrequency stimulus
that carries synthetic momentum. While the demonstrated electro-optic
polarization rotation rate is approximately 1 rad/cm, the exceptionally low
loss of lithium niobate enables non-reciprocal polarization rotators with
figures of merit that are 1-2 orders-of-magnitude better than what is possible
with magneto-optics. This approach can be replicated in III-V platforms paving
the way for high-performance lasers with co-integrated monolithic isolators.
