New “pendulum” for the ytterbium clock

Optical transitions are the modern counterpart of the pendulum of a mechanical clock. The faster the pendulum swings, the more precise the clock can be. In the case of atomic clocks, the “pendulum” is the radiation that excites the transition between two atomic states of different energy. In the experiment performed at PTB, the scientists devoted themselves to a special forbidden transition. In quantum mechanics, “forbidden” means that the jump between the two energy states of the atoms is almost impossible due to the conservation of symmetry and angular momentum. The excited state can then be very persistent: In the case investigated here, the lifetime of the F-state in the ytterbium ion Yb+ amounts to approx. six years. Due to this long lifetime, an extremely narrow resonance – whose linewidth only depends on the quality of the laser used – can be observed when this state is excited by means of a laser. A narrow resonance line is an important prerequisite for an exact optical clock. At the British National Physical Laboratory (NPL), the laser excitation of this Yb+ Fstate from the ground state was achieved for the first time in 1997. As the transition is, however, strongly forbidden, a relatively high laser intensity is required for its excitation. This disturbs the electron structure of the ion as a whole and leads to a shift of the resonance frequency, so that an atomic clock based on it would exhibit a rate depending on the laser intensity. At PTB, it has now become possible to show that alternating excitation of the ion with two different laser intensities allows the unperturbed resonance frequency to be determined with high accuracy. This has allowed other frequency shifts often occurring in atomic clocks – e. g. by electric fields or the thermal radiation of the environment – to be investigated. It has turned out that these are unexpectedly small in the case of the Yb+</su> F-state, which can be attributed to the special electronic structure of this state. This is a decisive advantage for the further development of this atomic clock. In the experiments carried out at PTB, the relative uncertainty of the Yb+ frequency was determined with 7 ∙ 10–17. This corresponds to an uncertainty of the atomic clock of only approx. 30 seconds over the age of the universe.

Both groups at NPL and PTB have measured the frequency of the Yb⁺ transition with their caesium clocks, and the results are in agreement within the scope of the uncertainties (1 ∙ 10^–15 and 8 ∙ 10^–16), which are mainly determined by the caesium clocks. In a research project recently approved within the scope of the European Metrology Research Programme, the two institutes will, in future, cooperate with other European partners even more intensively in the development of this optical clock. In the case of the Yb⁺ ion, what is of particular interest is that it has two transitions which are suitable for optical clocks: less strongly forbidden, but also very precise – at a wavelength of 436 nm the excitation of the D-level can be used. This opens up the possibility of investigating the accuracy of the optical clock by comparing the frequency of the two transitions in one ion, without having to refer to a caesium clock.