An optical clock based on ytterbium ions is so accurate that, if such a timepiece were in use since the universe began, it would now only be off by about 30 seconds. Scientists at Physikalisch-Technische Bundesanstalt (PTB) have excited a quantum-mechanically strongly “forbidden” transition of a single ytterbium ion, measuring it with extreme accuracy down to 17 digits after the decimal point. Optical transitions are the modern counterpart of the pendulum of a mechanical clock. In atomic clocks, the “pendulum” is the radiation that excites the transition between two atomic states of different energy. In the case of cesium atomic clocks — which currently determine time — it lies in the microwave range. For optical clocks, it lies in the range of laser light so that their “pendulum” oscillates with higher velocity; consequently, optical clocks are regarded as the atomic clocks of the future. The ion trap of the ytterbium clock at PTB. (Image: PTB) A special “forbidden” transition of ytterbium ion was considered for the initial research at PTB. In quantum mechanics, “forbidden” means that the jump between the two energy states of an atom is almost impossible because of the conservation of symmetry and angular momentum. In the ytterbium ion, Yb+, the F-state can be active up to six years. This extended lifetime allows the observation of an extremely narrow resonance during the laser excitation in this state. A narrow resonance line is mandatory for an accurate optical clock. In 1997, the British National Physical Laboratory, the sister institute of PTB, successfully achieved laser excitation of the Yb+ F-state from the ground state. The excitation needs relatively high laser intensity because the transition is strongly forbidden. The electron structure of the ion as a whole is disturbed, and a shift occurs at the resonance frequency. Because of this, the related atomic clock exhibits a rate based on the laser intensity. Now, the PTB scientists have demonstrated that the seamless resonance frequency can be precisely resolved by alternating the excitation of the ion with two different laser intensities. From this, they also can investigate other frequency shifts in atomic clocks. The Yb+ F-state shows insignificant shifts resulting from the special electronic structure of the state, contributing to further advancements of the atomic clock. The relative uncertainty of the Yb+ frequency was determined with 7 • 10-17, using cesium clocks. The research opens up the possibility of investigating the accuracy of the optical clock by frequency comparisons of the two transitions in one ion, without having to refer to a cesium clock. The institutes plan to team with other European partners to further develop this optical clock. Their work appeared in Physical Review Letters. For more information, visit: www.ptb.de