The next generation of atomic clocks will “tick” at the frequency of a laser. This is around 100,000 times faster than the microwave frequencies of the cesium clocks that currently still generate the second. These optical clocks are still in the testing phase, but some of them are already a hundred times more accurate. That is why they are to become the basis for the worldwide definition of the second in the International System of Units (SI) in the future. Before that, however, these optical clocks must prove their reliability through repeated tests and worldwide comparisons. The Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig is one of the world's leading institutions in this field and has so far realized an impressive range of different optical clocks – including optical single-ion clocks and optical lattice clocks. Now, the high accuracy has also been demonstrated in a novel type of clock that has the potential to measure time and frequency 1000 times more accurately than the cesium clocks that currently realize the SI second. To this end, this new ion crystal clock was compared with other optical clocks and a new accuracy record was achieved. The researchers report on the results of the measurement campaign in the current issue of Physical Review Letters.

Approximation to the ideal of an undisturbed quantum system

In an optical atomic clock, atoms are irradiated with laser light. If the laser has exactly the right frequency, the atoms change their quantum mechanical state. To achieve this, all external influences on the atoms must be shielded or precisely measured. This is possible with optical clocks with trapped ions. The ions can be localized in a vacuum to within a few nanometers using electric fields. Thanks to excellent control and isolation, one comes very close to the ideal of an undisturbed quantum system. Ion clocks have therefore already achieved systematic uncertainties beyond the 18th decimal place. Such a clock, if it had been ticking since the Big Bang, would be off by no more than one second today.

Ions in captivity

Existing ion clocks are operated with a single clock ion. Its weak signal must be measured over long periods of time, up to two weeks, to determine a frequency at this level. To exploit the full potential, it would even take measurement times of more than three years. In the newly developed clock, this measurement time is drastically reduced by parallelization: here, several ions are trapped simultaneously in a trap, often combining different ions. Through their interaction, they form a new crystalline structure. “This concept also makes it possible to combine the strengths of different ions,” explains PTB physicist Jonas Keller: “We use indium ions because of their favorable properties for achieving high accuracies. For efficient cooling, ytterbium ions are additionally mixed into the crystal.”

Accuracy close to the 18th decimal place

One challenge was to develop an ion trap that could use such a spatially extended crystal as a clock just as accurately as individual ions. Another challenge was to develop experimental methods to position the cooling ions within the crystal. The team headed by research group leader Tanja Mehlstäubler was able to solve these questions impressively with new approaches: as mentioned above, the clock currently achieves an accuracy close to the 18th decimal place.

Prerequisite for the redefinition of the second given for the first time

For the necessary comparisons with other clock systems, two further optical clocks and a microwave clock from the PTB were included: an ytterbium single-ion clock, a strontium lattice clock and a caesium fountain clock. In this context, the ratio of the indium clock to the ytterbium clock achieved for the first time a total uncertainty below the limit which is required in the roadmap for the redefinition of the second for such measurements. The concept promises a new generation of ion clocks with high stability and accuracy. It is also applicable to other ion species and also opens up the possibility of completely new clock concepts, such as the use of quantum many-particle states or the cascaded interrogation of multiple ensembles.

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