For many years, caesium atomic clocks have reliably kept time around the world. However, the future belongs to even more accurate optical atomic clocks. In a few years, these could change the definition of the base unit of time, the second, in the International System of Units (SI). Which of the various optical clocks will serve as the basis for this is still completely open. Another type could join the multitude of optical clocks that the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig, the leading institute in this field, has developed: an optical multi-ion clock with ytterbium-173 ions. It could combine the high accuracy of individual ions with the improved stability of multiple ions. This is the result of a collaboration between the PTB and the Thai metrology institute NIMT. The results are also interesting for quantum computer and basic research, offering a new insight into the interior of the atom.

Optical atomic clocks with single ions (such as ytterbium-171) are particularly accurate, while clocks with multiple particles (such as strontium atoms) are characterised by their high stability. Tanja Mehlstäubler, professor of quantum optics and metrology and known, among other things, for her work on topological defects in ion Coulomb crystals, is researching a combination of the two properties and has already realised a multi-ion clock with indium. Now she also has her sights set on ytterbium for the multi-ion idea, but a new isotope: ytterbium-173. ‘This isotope has a particularly interesting transition,’ explains the physicist.

Transition is the name given to the quantum leap that every atomic clock aims to achieve: the change in quantum state that can only be achieved with a very specific frequency of microwave or laser radiation. Microwave radiation is used for the current caesium atomic clocks. Optical clocks work with laser radiation. Because these oscillations are about a hundred thousand times faster, time can be divided more finely and measured more accurately.

The quantum leap in the new ytterbium isotope leads to an excited state with a very long lifetime. ‘This enables us to take more stable measurements,’ explains Jialiang Yu from NIMT. ‘But such transitions normally require strong laser light, which in turn can have major disadvantages.’ However, this ytterbium isotope has a very specially shaped atomic nucleus and special properties that enabled the team to overcome the problems and even control several ions at the same time.

This clears the way for an optical ytterbium clock with multiple ions that combines the high accuracy of single-ion clocks with the improved stability of multi-ion operation. The new atomic species is also very well suited as a multi-qubit for quantum information, as the quantum states can be manipulated extremely precisely using laser radiation and more quantum information can be encoded simultaneously. This opens up new possibilities for quantum computer research.

The first-ever measurement of the lifetime of the clock state provides valuable information about the structure of the atomic nucleus and enables sensitive tests of nuclear physics, such as possible effects beyond the Standard Model of physics.

The work was supported by the German Research Foundation (DQ-mat), the German Excellence Initiative (QuantumFrontiers-390837967) as part of the EU-wide metrology research programme (EMPIR project 22IEM01 TOCK) and by the Max Planck-RIKEN-PTB Centre for Time, Constants and Fundamental Symmetries.

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