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Intercontinental comparison of optical clocks – do all clocks tick the same?

For the first time, optical atomic clocks have been compared over a distance of 9000 km – directly and in real time

PTB-News 1.2015
Especially interesting for
  • developers of optical clocks
  • geodesy
  • fundamental research

Intercontinental comparisons of atomic clocks can, at present, only be realized with the aid of satellites. The measurement noise occurring hereby determines the measurement time required. To compare the optical strontium lattice clocks of PTB with those of the Japanese Institute of Information and Communications Technology (NICT) in Tokyo, a new method with improved stability, developed by NICT, has now been used.

Scheme of the comparison of the optical strontium lattice clocks of PTB in Braunschweig and NICT in Tokyo via satellite. The arrows represent the up-(red) and the down-link of the two-way technique which is very well suited to suppress disturbances.

Optical frequency standards are currently being investigated at research institutes worldwide, for example in view of their use for a new definition of the second. One of the issues hereby is to which extent atomic clocks of the same type really provide identical frequencies. Clocks of the strontium lattice type are relatively often investigated, thus, for example, at NICT and at PTB. In order to compare these two clocks, a measurement campaign was started in the spring of 2013 where a method developed by NICT was used to exploit the carrier phase in two-way frequency comparisons via satellite. Compared to previous satellitebased methods with modulated signals, this leads to lower noise and improved stability. Also, the transmission band width required is smaller. The 9000 km distance between the two institutes was bridged by means of a geostationary TV satellite which was operated continuously for a few months. The main result obtained was the proof of a considerably lower instability for frequency comparisons at short averaging times of 2 · 10−13 in 1 s.

In the summer of 2013, the optical strontium lattice clocks of NICT and PTB were compared directly with each other, which was the first comparison of this kind worldwide. It was shown that the two clocks were in agreement within the total measurement uncertainty of 1.6 · 10–15. Thereby, the measurement uncertainty of the strontium lattice clocks involved is so low that the total measurement uncertainty is mainly due to the comparison. This real-time method is thus an alternative to comparisons via primary caesium atomic clocks or Coordinated Universal Time and is, in principle, available also to institutes without a direct connection to the metrological infrastructure.

Further improvements by one order of magnitude seem possible if an optimized control of the ambient conditions of the emitting and receiving electronics as well as longer measuring times are used. This is particularly interesting for the further development of optical clocks, but also for fundamental experiments on the theory of relativity or for applications in geodesy where the difference in the rate of the clocks could indicate height differences.


Dirk Piester
Department 4.4 Time and Frequency
Phone: +49 (0)531 592-4421
E-mail: dirk.piester(at)ptb.de

Scientific publication

H. Hachisu, M. Fujieda, S. Nagano, T. Gotoh, A. Nogami, T. Ido, S. Falke, N. Huntemann, C. Grebing, B. Lipphardt, C. Lisdat, D. Piester: Direct comparison of optical lattice clocks with an intercontinental baseline of 9000 km. Optics Letters 39, 4072-5 (2014)