Atomic clocks operating in the optical domain have rapidly evolved into cutting-edge frequency standards, reaching systematic uncertainties as low as few parts in 1018. Their unrivalled spectral resolution of atomic transitions with sub-Hertz linewidth enables various applications such as test of fundamental physics and relativistic geodesy, where the atoms serve as highly precise sensors to the redshift caused by Earth’s gravitational potential.
I will report the ongoing activities and plans concerning our two strontium lattice clocks at PTB. In our stationary laboratory clock system, we now can achieve clock instabilities of below 1×10-16/(t/s)1/2. This outstanding stability is achieved by the use of a spectrally extremely pure interrogation laser that is pre-stabilized to a cryogenic single-crystal silicon reference resonator. I will discuss the possibility to use this combination of highly stable atomic and opto-mechanical reference for tests of the Einstein equivalence principle. I will also present comparisons of this Sr lattice with the Yb+ single ion clock at PTB. The frequency ratio has been measured repeatedly with uncertainties below 3×10-17. As this frequency ratio is very sensitive to variations of the fine structure constant alpha, we can give stringent limits on temporal variations of this fundamental constant.
Our second Sr lattice clock is transportable. The apparatus was successfully used in a measurement campaign in Italy where we determined in a proof-of-concept experiment the gravity potential change corresponding to 1000 m height difference with our transportable clock. In this experiment, it was first operated in the underground laboratory in the Fréjus car tunnel in the Alps at the French-Italian border and compared via an optical fiber link with clocks at the Italian metrology institute INRIM in Torino. In a second step, the clocks where operated side-by-side at INRIM to eliminate potential clock errors.