  The phenomenon of synchronization occurs universally across the natural
sciences and provides critical insight into the behavior of coupled nonlinear
dynamical systems. It also offers a powerful approach to robust frequency or
temporal locking in diverse applications including communications,
superconductors, and photonics. Here we report the experimental synchronization
of two coupled soliton modelocked chip-based frequency combs separated over
distances of 20 m. We show that such a system obeys the universal Kuramoto
model for synchronization and that the cavity solitons from the microresonators
can be coherently combined which overcomes the fundamental power limit of
microresonator-based combs. This study could significantly expand applications
of microresonator combs, and with its capability for massive integration,
offers a chip-based photonic platform for exploring complex nonlinear systems.
