Logo of the Physikalisch-Technische Bundesanstalt
symbolic picture: "magazines"

Most accurate and most stable transportable optical clock

Flexible applications possible thanks to operation in a car trailer

PTB-News 2.2017
Especially interesting for


the new system of units

fundamental research in physics

At PTB, a transportable optical strontium lattice clock was developed and tested for the first time in the world. This clock is suitable for international comparisons, geodesic applications and fundamental investigations in physics.

An air-conditioned car trailer full of high-tech physics: PTB's transportable optical clock.

The SI base unit of time, the second, might be realized by optical clocks in the future. That the redefinition of the SI, which is planned for the autumn of 2018, is still based on cesium atomic clocks, is due to the fact that several optical clocks are competing with one another; none of them has managed to clearly emerge at the forefront in the long run – neither with regard to accuracy, nor with regard to stability.

PTB's transportable optical strontium lattice clock is more accurate and more stable than any other transportable clock to date. With a relative uncertainty of 7.4 · 10-17, it has reached a level which is so close to the best stationary optical clocks that they can seriously be compared with each other. This was previously possible only with clocks which are located in the same laboratory or which are connected via a fiberglass link such as the one available between Braunschweig and Paris (see PTB News 3/2016). This has paved the way for a global pooling of optical clocks – which is a precondition for enabling the redefinition of the unit of time, the second.

Due to its high stability, the new clock is interesting for geodesy. Height differences of approx. 10 centimeters can be resolved after as little as an hour – even between two locations that are very far from each other. PTB has therefore been collaborating for several years with geodesists from Leibniz University Hannover within the scope of the DFG's geo-Q Collaborative Research Centre 1128.

During two measurement campaigns outside PTB's site, the clock already turned out to be ten times as precise and a hundred times more stable than the best transportable cesium fountain clocks. Thus, PTB's transportable optical clock is ready for geodetic height measurements, international clock comparisons and precision measurements of fundamental constants – and has taken a step towards future applications in space.


Christian Lisdat
Department 4.3 Quantum Optics and Unit of Length
Phone: +49 (0)531 592-4320,
Opens window for sending emailchristian.lisdat(at)ptb.de

Scientific publication

S. B. Koller, J. Grotti, S. Vogt, A. Al- Masoudi, S. Dörscher, S. Häfner, U. Sterr, C. Lisdat: Transportable optical lattice clock with 7 · 10−17 uncertainty. Phys. Rev. Lett. 118, 073601 (2017)