Redefining the second: Optical atomic clock achieves record precision in comparative measurements

A new In+/Yb+ crystal clock ion trap in a vacuum chamber. Credit: PTB
The next generation of atomic clocks “keep time” at laser frequencies. This is about 100,000 times faster than the microwave frequency of cesium clocks, which currently generate seconds. These optical clocks are still being evaluated, but some are already 100 times more accurate than cesium clocks. They will therefore form the basis of the future worldwide definition of the second in the International System of Units (SI).
However, these optical clocks must first prove their reliability by repeatedly testing and participating in global comparisons. PTB is one of the world’s leading institutions and has developed an impressive series of different optical clocks, including single ion clocks and optical lattice clocks.
Such high precision is now being demonstrated in a new type of clock, which has the potential to measure time and frequency 1,000 times more accurately than the cesium clocks that currently deliver SI seconds.
To this end, the new ion crystal watch was compared with other optical watches and achieved a new accuracy record. The results of the measurement campaign were published in Physical Review Letters.
In an optical atomic clock, atoms are irradiated with laser light. If the laser frequency is correct, the quantum mechanical state of the atom changes. For this purpose, the atoms must be protected from external influences and the remaining influences must be precisely measured.
This is very suitable for optical clocks where ions are trapped. Ions are trapped by an electric field and held within a few nanometers in a vacuum. This superior control and separation allows us to get very close to an ideal, undisturbed quantum system.
Therefore, the ion clock already reaches a relative systematic uncertainty beyond 18 decimal places. Even if such a clock had been ticking since the Big Bang, it would have been delayed by up to one second.
Until now, these clocks operated with one separate clock ion. To measure frequency with such low uncertainty, the weak signal must be measured over a long period of time (up to two weeks). Three or more years of measurement time may be required to reach the full potential.


A crystal consisting of indium (pink) ions and ytterbium (blue) ions. Credit: PTB
Newly developed clocks significantly reduce this measurement time by simultaneously trapping multiple ions (often of different types) into a single trap. By interacting, they form new crystal structures.
“Furthermore, this concept allows us to combine the intensities of different types of ions,” explains PTB physicist Jonas Keller. “We use indium ions, which have favorable properties to achieve high precision. Ytterbium ions are added to the crystal for efficient cooling.”
One of the challenges was developing an ion trap that would provide highly accurate conditions for such spatially spread crystals rather than single ions. Another challenge was to develop an experimental method to place cooled ions within the crystal.
Research group leader Tanja Mehlstäubler and her team were able to solve these problems with an impressive new idea. The clock now has an accuracy close to 18 decimal places.
Two more optical clock systems and one microwave clock system from PTB participated in the comparison: a single-ion ytterbium clock, a strontium lattice clock, and a cesium fountain clock. The ratio of indium and ytterbium clocks was the first to reach an overall uncertainty lower than the limit required for such a comparison by the second redefinition roadmap.
This concept promises a new generation of photoionic watches with high stability and precision. This can be applied to other types of ions, opening new opportunities for entirely new clock concepts, such as the use of quantum many-body states and the cascaded investigation of multiple ensembles.
Further information: HN Hausser et al, 115In+−172Yb+ Coulomb Crystal Clock with 2.5×10−18 systematic uncertainty, Physical Review Letters (2025). DOI: 10.1103/PhysRevLett.134.023201
Provided by Federal University of Physics and Technology
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