  Kerr soliton microcombs have recently emerged as a prominent topic in
integrated photonics and enabled new horizons for optical frequency metrology.
Kerr soliton microcombs, as its name suggests, are based on the high-order
cubic optical nonlinearity. It is desirable to exploit quadratic photonic
materials, namely Pockels materials, for soliton generation and on-chip
implementation of 1f-2f comb self-referencing. Such quadratically-driven
solitons have been theoretically proposed, but have not yet been observed in a
nanophotonic platform despite of recent progresses in quadratic comb generation
in free-space and crystalline resonators. Here we report photonic chip-based
Pockels microcomb solitons driven by three-wave mixing in an aluminum nitride
microring resonator. In contrast to typical Kerr solitons, our Pockels soliton
features unity soliton generation fidelity, two-by-two annihilation of
multi-soliton states, favorable tuning dynamics, and high pump-to-soliton
conversion efficiency.
