The Task Group “Transportable Ultra-stable Clocks” is focussed on the develop- ment and application of transportable highly accurate clocks. The best present optical clocks are so accurate that only clocks at the same location can be compared. However, the most interesting applications of these clocks occur outside the laboratory, for example in space, for navigation, geodesy and tracking spacecrafts in deep space missions.

The Task Group investigates various methods to make such clocks available for these applications. Ultra-stable optical resonators are developed that can be transported without any loss of performance. For applications requiring high absolute accuracy, a complete optical lattice clock is miniaturized, making it suitable for field measurements such as relativistic geodesy. Further investigations quantify the improvements that are possible in satellite-based geodesy, when highly stable clocks are used in the receivers. To investigate this, a transportable hydrogen maser of the Physikalisch-Technische Bundesanstalt (PTB) was employed.

Instead of transporting clocks, optical frequencies can also be transmitted through optical fibers that are widely used for telecommunication. In a first successfully test, a stabilized link between the PTB in Braunschweig and the Institute for Quantum Optics in Hannover was established. Over a distance of 73 kilometres an optical frequency could be transmitted with a residual uncertainty well below that of the participating optical clocks.