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Optical atomic clock achieves record accuracy redefining the second

21 Jan 2025

Germany’s PTB develops clock that “ticks” at laser frequency 100,000x faster than microwave cesium clocks.

The next generation of atomic clocks “ticks” at the frequency of a laser. That is around 100,000 times faster than the microwave frequencies of the caesium clocks that currently generate the second. These optical clocks are still in the testing phase, but some of them are already a hundred times more accurate.

This is why they are to become the basis for the global definition of seconds in the International System of Units (SI) in the future. Before that, however, these optical clocks must prove their reliability through repeated tests and global comparisons.

Germany’s Physikalisch-Technische Bundesanstalt (National Metrology Institute), based in Braunschweig, is one of the world’s leading institutions and has so far realized an impressive range of different optical clocks - including optical single ion clocks and optical lattice clocks.

The high level of accuracy has now also been demonstrated in a new type of clock that has the potential to measure time and frequency 1000 times more accurately than the caesium clocks that currently realize the SI second. For this purpose, this new ion crystal clock was compared with other optical clocks and a new accuracy record was achieved. The PTB researchers report their in Physical Review Letters.

An undisturbed quantum system is “very close”

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. All external influences on the atoms must be shielded or measured precisely. This works very well with optical clocks with trapped ions.

The ions can be trapped in a vacuum using electric fields localized to a few nanometers. Thanks to excellent control and isolation, the ideal of an undisturbed quantum system is very close here. Ion clocks have therefore already reached systematic uncertainties beyond the 18th decimal place. If such a clock had been ticking since the Big Bang, it would be at most one second slow today.

Previous ion clocks are operated with a single clock ion. Its small 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 require measurement times of more than three years.

In the newly developed clock, this measurement time is drastically reduced through parallelization: Here, several ions are caught in a trap at the same time, often different ions are combined. Through their interaction, they form a new, crystalline structure.

“This concept also makes it possible to combine the strengths of different ions,” said PTB physicist Jonas Keller. “We use indium ions because of their favorable properties for achieving high levels of accuracy. Ytterbium ions are also added to the crystal for efficient cooling.”

One challenge was to develop an ion trap that could use such a spatially extended crystal as a clock with the same accuracy as individual ions. Another challenge was to develop experimental methods to position the cooling ions within the crystal. The team led by research group leader Tanja Mehlstäubler was able to solve these questions impressively with new ideas: the clock currently achieves an accuracy of close to 18 decimal places.

For the necessary comparisons with other clock systems, two other optical clocks and one microwave clock from PTB were included: an ytterbium single ion clock, a strontium lattice clock and a cesium fountain clock. For the first time, the ratio of the indium to the ytterbium clock reached a total uncertainty below the limit required in the roadmap for redefining 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 types of ions and also opens up the possibility of completely new clock concepts, such as the use of quantum many-body states or the cascaded query of several ensembles. The work was partially funded by the German Research Foundation (DFG) within the framework of the Cluster of Excellence QuantumFrontiers and the Collaborative Research Center DQ-mat.

Optikos Corporation ECOPTIKChangchun Jiu Tian  Optoelectric Co.,Ltd.HÜBNER PhotonicsHamamatsu Photonics Europe GmbHIridian Spectral TechnologiesPhoton Lines Ltd
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