Project B03: Multi-ion spectroscopy for optical clocks

Results

During the first two funding periods, we have investigated optimal probe times for a given number of atoms and laser noise, developed novel efficient cooling schemes for dual species clocks based on electromagnetically-induced transparency to reduce time-dilation shifts and evaluated systematic shifts of the Al⁺ system to 1 ×10⁻¹⁸. We operated an optical reference using dynamical decoupling to mitigate systematic shifts of up to five ions in a crystals, and operated an optical clock based on two Ca⁺ ions entangled in a decoherence-free sub space

Time dilation shifts due to thermal motion and micromotion are among the major challenges in controlling the dynamics of large ensembles of ions for precision spectroscopy. We therefore investigated micromotion across an ion chain, heating of the motional modes in extended 1D to 3D ion crystals and demonstrated the experimental capability of sorting mixed-species chains for reliable sympathetic cooling conditions. With this the potential of multi-ion clock operation with fractional systematic uncertainties as low as 10^-19 was successfully demonstrated. In succeeding three-cornered-hat clock comparisons a first In⁺/ Yb⁺ Coulomb crystal clock with a total systematic uncertainty of 2.5*10⁻¹⁸ for a single clock ion and first observations of reduced statistical uncertainties in measurements with up to four clock ions were demonstrated.

We have also started frequency comparisons of the Al⁺/Ca⁺ clock, the multi-ion Ca⁺ and entangled Ca⁺ reference, as well as the In⁺ multi-ion clock with the Yb⁺ single ion clocks and the Sr lattice clock in project B02 (optical clocks).