  Low-loss optical isolators and circulators are critical nonreciprocal
components for signal routing and protection, but their chip-scale integration
is not yet practical using standard photonics foundry processes. The
significant challenges that confront integration of magneto-optic nonreciprocal
systems on chip have made imperative the exploration of magnet free
alternatives. However, none of these approaches have yet demonstrated linear
optical isolation with ideal characteristics over a microscale footprint -
simultaneously incorporating large contrast with ultralow forward loss - having
fundamental compatibility with photonic integration in standard waveguide
materials. Here we demonstrate that complete linear optical isolation can be
obtained within any dielectric waveguide using only a whispering-gallery
microresonator pumped by a single-frequency laser. The isolation originates
from a nonreciprocal induced transparency based on a coherent light-sound
interaction, with the coupling originating from the traveling-wave Brillouin
scattering interaction, that breaks time-reversal symmetry within the
waveguide-resonator system. Our result demonstrates that material-agnostic and
wavelength-agnostic optical isolation is far more accessible for chip-scale
photonics than previously thought.
