Attosecond-level synchronisation of chip-integrated oscillators

Attosecond science provides a window to the fastest processes in chemistry, materials science, and biology. Accessing this time scale requires precisely synchronised oscillators. In free-electron X-ray lasers, which also provide sub-atomic resolution, synchronisation must be achieved across hundreds of meters. Current approaches to synchronisation based on mode-locked lasers deliver this level of performance but complexity, cost and size hinder their deployment in facility-wide multi-node networks. Here, we demonstrate attosecond-level synchronisation of two chip-integrated photonic oscillators (microcombs) separated by 100 m of fibre. A pair of continuous-wave lasers establishes a time reference that is delivered over fibre, and on-chip Kerr-nonlinear synchronisation results in an integrated relative timing jitter of the microcombs below 400 as (1 kHz to 1 MHz), without any active stabilisation. These results unlock precision timing at scale for large facilities and next-generation technologies such as disaggregated computing and quantum networks, and ultimately may lead to chip-integrated attosecond photonics.