  Carrier-envelope phase stabilization of optical pulses enables exquisitely
precise measurements by way of direct optical-frequency synthesis, absolute
optical-to-microwave phase conversion, and control of ultrafast waveforms. We
report such phase stabilization for Kerr-microresonator frequency combs
integrated on silicon chips, and verify their fractional-frequency inaccuracy
at <3x10-16. Our work introduces an interlocked Kerr-comb configuration
comprised of one silicon-nitride and one silica microresonator, which feature
nearly harmonic repetition frequencies and can be generated with one laser.
These frequency combs support an ultrafast-laser regime with few-optical-cycle,
1-picosecond-period soliton pulses and a total dispersive-wave-enhanced
bandwidth of 170 THz, while providing a stable phase-link between the optical
and microwave domains. To accommodate low-power and mobile application
platforms, our phase-locked frequency-comb system operates with <250 mW of
chip-coupled power. Our work establishes Kerr-microresonator combs as a viable
technology for applications like optical-atomic timekeeping, optical synthesis,
and related directions.
