A new type of atomic clocks has been intensively investigated for more than 30 years – optical clocks. They also rely on an atomic transition as a reference, however, not in the microwave range, but in the optical spectral range. The oscillator used in these clocks is – correspondingly – a frequency-stabilized laser. By increasing the clock frequency by five orders of magnitude (from approx. 1010 Hz in a caesium clock to approx. 1015 Hz in an optical clock), considerable frequency stability and accuracy gains have been demonstrated. The short-term stability is improved due to the considerably higher number of oscillations per time unit. Enhanced accuracy is obtained because some of the disturbing external influences on the atom cause a characteristic shift of fixed magnitude which has, however, less influence at the higher frequency of the optical clock than for a clock operating in the microwave range. On short time scales, optical clocks are now more stable and accurate by about a factor of 100 than caesium fountain clocks. Based on a single trapped Ytterbium (171Yb+) ion, one of our groups has developed such an optical frequency standard which today is among the most accurate in the world.
Gamma spectroscopy has shown that the nucleus of Thorium (229Th) possesses an isomeric state at the unusually low excitation energy of 7.8 ± 0.5 eV. Light of the corresponding transition wavelength in the range of 160 nm can be produced by frequency upconversion of tunable lasers so that laser spectroscopy methods can be applied to a nuclear system for the first time. In one of our groups, high-resolution laser spectroscopy of the nuclear transition of 229Th using ions stored in a trap is thus pursued with a two-fold motivation: searches for “new physics” beyond the standard model and – not unexpected – even a new kind of ultra-precise atomic clock could be developed.