Optical clocks have reached a proven accuracy of the order of 10⁻¹⁸ in relative frequency, surpassing the present Cs fountains by two orders of magnitude and driving the Consultative Committee for Time and Frequency (CCTF) to initiate work towards a redefinition of the second. However, comparing these clocks with a similar uncertainty is only possible using optical fibres and is presently limited to a few links at distances not exceeding 1000 km. Radio techniques have world-wide coverage and are currently regularly used in UTC with GNSS-based techniques and two-way time transfer using geostationary satellites, but they have limited accuracy.

Now an international collaboration between the NICT, INRIM, INAF and BIPM has compared the Yb lattice clock at INRIM and the Sr lattice clock at NICT using two new and independent radio techniques. One is Very Long Baseline Interferometry (VLBI), a technique used in astronomy for observing extra-galactic objects, and which has been used for the first time for accurate frequency transfer thanks to the development of mobile antennas and ad hoc hardware and processing facilities. The second technique is Integer ambiguity Precise Point Positioning (IPPP) using GNSS signals, the development of which has been driven by the BIPM over the past few years.

The results obtained with the two techniques agree within the uncertainty of the frequency comparisons, below 3 x 10⁻¹⁶ in relative frequency for each technique. In this case, it is limited by the intermittent operation of the optical clocks and the noise contributed by the "flywheel" clocks used to bridge the gaps. Both VLBI and IPPP are capable of frequency transfer accuracy below 1 x 10⁻¹⁶ when averaging several days and they can be easily combined thanks to IPPP using ubiquitous GNSS receivers that can be operated with the heavier VLBI equipment.

Contact Gérard Petit for more information.

Press release:
Signals from distant stars connect optical atomic clocks across Earth for the first time

Mobile VLBI antenna image courtesy of NICT