  Laser sources power extreme data transmission as well as computing
acceleration, access to ultrahigh-speed signaling, and sensing for chemicals,
distance, and pattern recognition. The ever-growing scale of these applications
drives innovation in multi-wavelength lasers for massively parallel processing.
We report a nanophotonic Kerr-resonator circuit that consumes the power of an
input laser and generates a soliton frequency comb at approaching unit
efficiency. By coupling forward and backward propagation, we realize a
bidirectional Kerr resonator that supports universal phase matching but also
opens excess loss by double-sided emission. Therefore, we induce reflection of
the resonator's forward, external-coupling port to favor backward propagation,
resulting in efficient, one-sided soliton formation. Coherent backscattering
with nanophotonics provides the control to put arbitrary phase-matching and
efficient laser-power consumption on equal footing in Kerr resonators. In the
overcoupled-resonator regime, we measure 65% conversion efficiency of a 40 mW
input pump laser, and the nonlinear circuit consumes 97% of the pump,
generating the maximum possible comb power. Our work opens up high-efficiency
soliton formation in integrated photonics, exploring how energy flows in
nonlinear circuits and enabling laser sources for advanced transmission,
computing, quantum sensing, and artificial-intelligence applications.
