Most accurate optical atomic clock ever made will redefine the meaning of a 'second'
01-21-2025

Most accurate optical atomic clock ever made will redefine the meaning of a 'second'

Timekeeping has always been fundamental to human civilization. From ancient sundials to mechanical clocks and modern atomic optical timekeepers, each innovation has brought greater precision.

Now, a revolutionary breakthrough in atomic clocks is set to redefine how we measure time. The next generation of optical clocks uses laser frequencies that “tick” 100,000 times faster than the microwave frequencies of current cesium clocks.

This dramatic increase in frequency allows for a level of precision never before achieved. Some optical clocks already outperform cesium clocks in accuracy by a factor of 100.

Optical clocks: Future of time measurement

As these clocks continue to improve, they will soon define the worldwide standard for the second in the International System of Units (SI).

However, before that happens, they must prove their reliability through repeated testing and global comparisons.

The Physikalisch-Technische Bundesanstalt (PTB), Germany’s national metrology institute, is at the forefront of this research.

PTB scientists have developed various optical clocks, including single-ion and optical lattice clocks, that push the boundaries of precision.

A recent breakthrough at PTB demonstrated record-breaking accuracy in a new type of ion crystal clock.

This clock could measure time and frequency 1,000 times more accurately than today’s cesium clocks. The results were published in Physical Review Letters.

How optical clocks work

Optical atomic clocks use laser light to excite atoms. If the laser is tuned to the right frequency, the atoms transition between quantum states.

To ensure accuracy, these atoms must be shielded from external influences, and any remaining disturbances must be precisely measured.

Ion clocks are particularly well-suited for this purpose. Their ions are held in place by electrical fields, and isolated in a vacuum to within a few nanometers.

Thanks to this level of control, ion clocks have achieved systematic uncertainties only beyond the 18th decimal place.

“Such a clock, if it had been ticking since the Big Bang, would have lost one second at most,” researchers noted.

Faster approach with ion crystal clocks

Until now, these clocks operated with a single ion, requiring long measurement periods – up to two weeks – to achieve their precision. To fully exploit their potential, they would need measurement times exceeding three years.

The newly developed optical ion crystal clock reduces this time significantly by introducing parallelization.

Instead of a single ion, multiple ions of different types are trapped together to form a structured crystal. This approach enhances measurement efficiency and combines the strengths of different ions.

“In addition, this concept allows the strengths of different types of ions to be combined,” explained PTB physicist Jonas Keller.

“We use indium ions, as they have favorable properties, to achieve high accuracy. For efficient cooling, ytterbium ions are added to the crystal.”

One of the biggest challenges was designing an ion trap that could provide high-accuracy conditions for a larger crystal rather than a single ion. Another hurdle was developing methods to position cooling ions precisely within the crystal.

Research group leader Tanja Mehlstäubler and her team overcame these obstacles with innovative solutions, allowing the new clock to reach accuracy close to the 18th decimal place.

Optical clocks in global comparisons

To ensure that the new ion crystal clock works as accurately as claimed, researchers at PTB compared its performance with other highly precise clocks.

These included two optical clocks – a single-ion ytterbium clock and a strontium lattice clock – as well as a caesium fountain clock, which is currently used to define the SI second.

One of the most important comparisons was between the indium ion clock (part of the new ion crystal clock system) and the ytterbium ion clock.

The researchers measured the frequency ratio between these two clocks with extreme precision. The level of uncertainty in this measurement was even lower than the strict threshold required for international timekeeping standards.

This achievement is a major step toward officially redefining the second. It demonstrates that the ion crystal clock is reliable and accurate enough to contribute to the future of global time measurement.

Future of ion clocks

This breakthrough paves the way for a new generation of highly stable and precise optical ion clocks. The same approach can be applied to other ions, opening possibilities for advanced clock designs.

Future innovations may include using quantum many-body states or cascaded interrogation of multiple ensembles.

Part of this research was funded by the German Research Foundation (DFG) through the Quantum Frontiers Cluster of Excellence and the DQ-mat Collaborative Research Center.

With these advances, the future of timekeeping is more precise than ever. Optical clocks are set to redefine the way we measure time, and to push the limits of accuracy to an entirely new level.

The study is published in the journal Physical Review Letters.

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