  From generation of backscatter-free transmission lines, to optical isolators,
to chiral Hamiltonian dynamics, breaking time-reversal symmetry is a key tool
for development of next-generation photonic devices and materials. Of
particular importance is the development of time-reversal-broken devices in the
low-loss regime, where they can be harnessed for quantum materials and
information processors. In this work, we experimentally demonstrate the
isolation of a single, time-reversal broken running-wave mode of a
moderate-finesse optical resonator. Non-planarity of the optical path produces
a round-trip geometrical (Pancharatnam) polarization rotation, breaking the
inversion symmetry of the photonic modes. The residual time-reversal symmetry
between forward-$\sigma^+$/ backwards-$\sigma^-$ modes is broken through an
atomic Faraday rotation induced by an optically pumped ensemble of $^{87}$Rb
atoms residing in the resonator. We observe a splitting of 6.3 linewidths
between time-reversal partners and a corresponding optical isolation of $\sim$
20.1(4) dB, with 83(1)% relative forward cavity transmission. Finally, we
explore the impact of twisted resonators on T-breaking of intra-cavity Rydberg
polaritons, a crucial ingredient of photonic materials and specifically
topological optical matter. As a highly coherent approach to time-reversal
breaking, this work will find immediate application in creation of photonic
materials and also in switchable narrow-band optical isolators.
