The Working Group is concerned with research and development for optical clocks based on trapped ions. A single ion, trapped in ultrahigh vacuum in the oscillating electric field of a Paul trap and laser cooled to near zero temperature, is a nearly perfect object for spectroscopy. External influences are strongly suppressed so that atomic transition frequencies are essentially unperturbed, thus facilitating atomic clocks of the highest accuracy. We investigate the ion 171Yb+ that offers two reference transitions for optical clocks with rather different physical characteristics. The highly forbidden electric octupole reference transition of Yb+ allows a particularly high accuracy: our investigations on an optical clock based on this transition presently indicate a systematic uncertainty of 3·10-18. This approximately corresponds to an uncertainty of 200 optical cycles (260 femtoseconds) per day or of 1 s over the age of the universe.
For comparisons between different optical clocks and frequency measurements relative to caesium fountain clocks, we develop optical frequency comb generators based on femtosecond fiber lasers.
We expect that transitions in atomic nuclei can be even better shielded from interactions with the environment than transitions in the electron shell. In 2003, we proposed a concept for a nuclear optical clock based on a transition in the nucleus of 229Th that lies in the vacuum-ultraviolet spectral range. This proposal has initiated worldwide experimental research activities on the 229Th system.
The high accuracy of optical clocks enables new applications in fundamental research, in particular in tests of postulates and predictions of the theory of relativity, and in searches for "New Physics" phenomena that might point to a unified model of the fundamental forces which manifest in the quantum regime and on the cosmological scale.
Research and Development for Optical Frequency Standards
The Working Group is active in a number of international and national collaborations. In the framework of the European Metrology Research Programme (EMRP) we coordinated the research project "High-accuracy optical clocks with trapped ions" and we are involved in the project "International time scales with optical clocks". Beginning in 2016 we participate in the European Metrology Programme for Innovation and Research (EMPIR) through the projects "Optical clocks with 1E-18 uncertainty" and "Optical frequency transfer – a European network". Our collaboration with nuclear physics and quantum optics research groups on the 229Th nuclear optical clock is funded through the Horizon 2020 Programme of the European Union in the project nuClock.