  Quantum diffusion of soliton microcombs has long been recognized as their
fundamental noise limit. Here we surpass such limit by utilizing dispersive
wave dynamics in multimode microresonators. Through the recoil force provided
by these dispersive waves, the quantum diffusion can be suppressed to a much
lower level that forms the ultimate fundamental noise limit of soliton
microcombs. Our findings enable coherence engineering of soliton microcombs in
the quantum-limited regime, providing critical guidelines for using soliton
microcombs to synthesize ultralow-noise microwave and optical signals.
