  The rapidly growing field of integrated photonics is enabling a large number
of novel devices for optical data processing, neuromorphic computing and
circuits for quantum photonics. While many photonic devices are based on linear
optics, nonlinear responses at low threshold power are of high interest for
optical switching and computing. In the case of counterpropagating light,
nonlinear interactions can be utilized for chip-based isolators and logic
gates. In our work we find a symmetry breaking of the phases of
counterpropagating light waves in high-Q ring resonators. This abrupt change in
the phases can be used for optical switches and logic gates. In addition to our
experimental results, we provide theoretical models that describe the phase
symmetry breaking of counterpropagating light in ring resonators.
