Ningyuan Jia and colleagues from the University of Chicago, USA, have developed a device to break time-reversal symmetry using a four-mirror running-wave resonator. They twist the resonator out of plane by 6° to break inversion symmetry. An optically active ensemble with 87Rb atoms is placed in the resonator to provide a resonator-enhanced atomic Faraday effect that breaks the time-reversal symmetry. These two broken symmetries result in a frequency shift of 9.4 MHz between the forward- and backward-propagating modes. Choosing the incident beam’s polarization isolates one of the propagating modes. The team obtained an isolation of 20 dB for the backward mode, with 83% relative forward cavity transmission. Cavity Rydberg electromagnetically induced transparency (EIT) spectra were measured for forward- (σ+) and backward-propagating polarized (σ–) light. When the cavity was in resonance with the σ+ light, probing in the forward direction revealed a standard EIT spectrum, while probing in the backward direction revealed only a weak Fano feature for cavity tuning. These findings may be useful for creating new photonics materials and switchable narrow-band optical isolators.