The comparison of distant atomic clocks is foundational to international timekeeping, global positioning and tests of fundamental physics. Optical-fibre links allow the most precise optical clocks to be compared, without degradation, over intracontinental distances up to thousands of kilometres, but intercontinental comparisons remain limited by the performance of satellite transfer techniques. Here we show that very long baseline interferometry (VLBI), although originally developed for radio astronomy and geodesy, can overcome this limit and compare remote clocks through the observation of extragalactic radio sources. We developed dedicated transportable VLBI stations that use broadband detection and demonstrate the comparison of two optical clocks located in Italy and Japan separated by 9,000 km. This system demonstrates performance beyond satellite techniques and can pave the way for future long-term stable international clock comparisons.

远距离原子钟的比较是国际计时，全球定位和基础物理测试的基础。光纤链路可以在长达数千公里的洲际距离内比较最精确的光钟，而不会降低精度，但是洲际比较仍然受到卫星传输技术性能的限制。在这里，我们显示了很长的基线干涉测量法（VLBI），尽管最初是为射电天文学和大地测量学开发的，但可以克服此限制并通过观测银河外的无线电源来比较远程时钟。我们开发了专用的可移动VLBI站，这些站使用宽带检测，并演示了相距9,000公里的意大利和日本的两个光学时钟的比较。