Attosecond-level synchronisation of chip-integrated oscillators

Synchronised laser oscillators are essential for probing the fastest processes in chemistry, materials science, and biology down to atto-second timescales. Tight synchronisation is also crucial at scientific facilities such as free-electron lasers or radio-telescopes, and increasingly relevant to communication and information technologies in multi-node networks. Current synchronisation approaches based on mode-locked lasers achieve the required performance, but their complexity, cost, and size hinder deployment in multi-node networks. Here, we demonstrate attosecond-level synchronisation between chip-integrated microresonator soliton oscillators operating at either 25 or 300 GHz pulse repetition rate. For synchronisation, each oscillator receives over fibre a pair of continuous-wave lasers as a two-tone timing reference. The lasers power the microcombs and Kerr-nonlinear synchronisation results in integrated relative timing jitter below 400 as (1 kHz to 1 MHz), without any active stabilisation. This approach enables scalable precision timing for large facilities, data centres, disaggregated computing, navigation, and quantum networks; ultimately, it may lead to chip-integrated attosecond photonics.