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PTB‘s strontium lattice clock is ticking

Especially interesting for:
  • high-precision time and frequency measurement
  • fundamental physics in space
  • metrology institutes

The frequency of the optical lattice clock, which is based on neutral strontium-87 atoms, has been determined by comparing it to a caesium fountain clock of PTB with a relative uncertainty of 1 ⋅ 10–15. Frequency shifts of the strontium reference transition at 429 THz are under control at a level of better than 2 ⋅ 10–16.

Strontium-Apparatur mit einer blau fluoreszierenden Wolke kalter Atome im Vakuumsystem. Oben rechts: publizierte Frequenzen des 87Sr-Uhrenübergangs 1S0 – 3P0 bei 429 THz (entspricht 698 nm) und das Ergebnis der PTB-Messung. Die senkrechte Linie gibt die Empfehlung des CIPM wieder, die gestrichelten Linien ihre Unsicherheit. (Abbildung: Steinmetz/VISUM)

At many metrology and university institutes, optical frequency standards are being intensively developed to be able to realise the base unit “the second” with higher accuracy in future. For this purpose, a group of these experiments uses the electronic transition 1S03P0 of 87Sr. Contrary to Cs clocks with their microwave transitions, very narrow absorption lines in the visible spectral range are interrogated in the case of these optical frequency standards, which brings about a considerably higher stability. Also the
systematic uncertainties are – in some cases – considerably smaller than those of primary Cs clocks.

In optical clocks, either trapped individual ions or clouds of cold neutral atoms are used as absorbers. In the case of lattice clocks with  strontium, neutral atoms are trapped in a strong laser field – the optical lattice – for which they have to be cooled down to a few millionths of a kelvin by means of lasers. The movement of the atoms is then reduced to the fraction of an optical wavelength, whereby
the Doppler effect becomes negligible. Worldwide, strontium lattice clocks are being developed in at least eight laboratories. Frequency measurements of three experiments are available which show a very good agreement. For the fi rst time now, the frequency of the clock transition of 87Sr has been measured at PTB in comparison with one of PTB‘s Cs fountain clocks. In this frequency measurement, the strontium standard reached a relative uncertainty of below 2⋅10–16 so that these systematic contributions infl uence the total uncertainty of 1⋅10–15 only slightly.

Further optimisations are necessary to reduce the measurement uncertainty still further. This includes controlling the temperature of the black-body radiation of the environment (which infl uences the clock transition) better than is currently the case, and determining the atomic shift coeffi cient more exactly. To be able to carry out the respective measurements, the atoms must be transported into a very well controlled environment for the interrogation of the reference transition. A corresponding set-up is currently in the test phase. With this set-up, it will be possible – in clock operation – to reduce the uncertainty of this frequency shift to just a few 10–18 of the optical frequency. This makes strontium a promising candidate for a redefinition of the second.


Christian Lisdat
Department 4.3
Quantum Optics and Unit of Length
Phone: +49 (0) 531 592-4326
E-mail: christian.lisdat(at)ptb.de

Scientific publication:

Bieler, M:, Spitzer, M.; Pierz, K.; Siegner, U.: Improved Optoelectronic Technique for the Time-Domain Characterization of Sampling  Oscilloscopes. IEEE Transactions on Instrumentation and Measurement 58 (2009), 1065–1071