Quantum diffusion of microcavity solitons

Coherently-pumped (Kerr) solitons in an ideal optical microcavity are expected to undergo random quantum motion that determines fundamental performance limits in applications of the soli-ton microcombs. Here, this random walk and its impact on Kerr soliton timing jitter is studied experimentally. The quantum limit is discerned by measuring relative position of counter-propagating solitons. Their relative motion features weak interactions and also presents excellent common mode suppression of technical noisetypically hiding the quantum fluctuations. This is in strong contrast to co-propagating solitons which are found to have relative timing jitter well below the quantum limit of a single soliton on account of strong mutual motion correlation. Good agreement is found between theory and experiment. The results establish the fundamental limits to timing jitter in soliton microcombs and provide new insights on multi-soliton physics.