New Ion Crystal Clock Achieves Unprecedented Accuracy

Recent research published in Physical Review Letters by PTB (Physikalisch-Technische Bundesanstalt) details the development of a new indium-ytterbium crystal optical clock, which achieves an accuracy 1000 times better than current cesium clocks.

The next generation of atomic clocks "ticks" at the frequency of a laser, which is approximately 100,000 times faster than the microwave frequencies used by current cesium clocks that define the second.

Although evaluations of these optical clocks are ongoing, some of them are already 100 times more accurate than cesium clocks. As a result, they will serve as the foundation for the global definition of the second in the International System of Units (SI) in the future.

However, these optical clocks must first demonstrate their dependability through numerous tests and participation in international comparisons. One of the top organizations in the world, PTB, has created an amazing array of optical clocks to date, including single ion clocks and optical lattice clocks.

A new kind of clock, able to measure time and frequency 1000 times more accurately than the cesium clocks that currently define the SI second, has also shown such high accuracy. This was accomplished by comparing the new ion crystal clock to other optical clocks and setting a new accuracy record.

Atoms in an optical atomic clock are exposed to laser light. The atoms alter their quantum-mechanical state if the laser is set to the proper frequency. For this reason, the atoms must be protected from all outside influences, and any residual effects must be precisely measured. For optical clocks with trapped ions, this is highly effective.

The ions can be confined using electric fields and maintained within a few nanometers in a vacuum. This exceptional isolation allows for an ideal, undisturbed quantum system.

Therefore, relative systematic uncertainties for ion clocks have already surpassed the 18th decimal place. If such a clock had been in operation since the Big Bang, it would have lost no more than one second.

These clocks have only been used with a single clock ion thus far. To measure the frequency with such low uncertainty, its weak signal must be measured over extended periods, up to two weeks. It would even be necessary to measure times longer than three years to fully utilize the potential.

The newly developed clock will significantly reduce measuring time by parallelizing the process: multiple ions, often of different types, will be simultaneously trapped in a single trap. Through interaction, they form a new crystalline structure.

In addition, this concept allows the strengths of different types of ions to be combined. We use indium ions as they have favorable properties to achieve high accuracy. For efficient cooling, ytterbium ions are added to the crystal.

Jonas Keller, Physicist, Physikalisch-Technische Bundesanstalt

The creation of an ion trap that offers high-accuracy conditions for a spatially extended crystal as opposed to a single ion was one of the difficulties, while a second challenge was to create experimental techniques to place the cooling ions inside the crystal.

Tanja Mehlstäubler, the Leader of the research group, and her colleagues came up with some amazing new solutions to these problems. At the moment, the clock is accurate to about the 18th decimal place.

A cesium fountain clock, a strontium lattice clock, and a single-ion ytterbium clock were among the other optical and microwave clock systems that took part in the comparisons. According to the roadmap for the redefinition of the second clock, the ratio of the indium clock to the ytterbium clock is the first to achieve an overall uncertainty below the threshold needed for such comparisons.

This recent progress paves the way to a new generation of extremely accurate, stable optical ion clocks. It can also be applied to other kinds of ions and creates new possibilities for completely novel clock concepts, such as the cascaded interrogation of multiple ensembles or the use of quantum many-body states.

The German Research Foundation (DFG) provided partial funding for this work as part of the Quantum Frontiers Cluster of Excellence and the DQ-mat Collaborative Research Center.

Journal Reference:

Hausser, H., et al. (2025) ¹¹⁵In⁺−¹⁷²Yb⁺ Coulomb Crystal Clock with 2.5 × 10⁻¹⁸ Systematic Uncertainty. Physical Review Letters. doi.org/10.1103/physrevlett.134.023201.