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Redefining the second: Optical atomic clock achieves record accuracy in comparison measurement

The next generation of atomic clocks "ticks" with the frequency of a laser. This is about 100,000 times faster than the microwave frequencies of the cesium clocks which are generating the second at present. These optical clocks are still being assessed, but already now, some are 100 times more accurate than cesium clocks. They will therefore become the future basis for the worldwide definition of the second in the International System of Units (SI).

However, these optical clocks must first prove their reliability by being tested repeatedly and by participating in worldwide comparisons. PTB is one of the global leading institutions and has, up to now, developed an impressive series of different optical clocks—among which are single ion clocks and optical lattice clocks.

Now, such high accuracy has also been 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 realize the SI second.

For this purpose, the new ion crystal clock was compared to other optical clocks and achieved a new accuracy record. The results of the measurement campaign have been in Â鶹ÒùÔºical Review Letters.

In an optical atomic clock, atoms are irradiated by laser light. If the laser has the correct frequency, the atoms change their quantum-mechanical state. For this purpose, the atoms have to be shielded from any external influences—and remaining influences must be measured accurately.

This works very well for optical clocks with trapped ions. The ions can be trapped by means of electrical fields and kept in place within a few nanometers in vacuum. Thanks to this outstanding control and isolation, we can get very close to an ideal, undisturbed quantum system.

Ion clocks have therefore already reached relative systematic uncertainties beyond the 18th decimal place. Such a clock, if it had been ticking since the Big Bang, would have lost one second at most.

To date, these clocks have been operated with one individual clock ion. Its weak signal must be measured over long periods of time—up to two weeks—in order to measure the frequency with such a low uncertainty. To exploit the full potential, it would even require measuring times of more than three years.

The newly developed clock will drastically shorten this measuring time by parallelizing: multiple ions—often of different kinds—will be simultaneously trapped in one trap. By interacting, they form a new, crystalline structure.

"In addition, this concept allows the strengths of different types of ions to be combined," explains 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 challenges was the development of an ion trap that provides high-accuracy conditions for such a spatially extended crystal, rather than just a single ion. Another challenge was to develop experimental methods to position the cooling ions within the crystal.

Research group leader Tanja Mehlstäubler and her team were able to solve these issues with impressive new ideas: The clock currently reaches an accuracy close to the 18th decimal place.

Two further optical and one microwave clock systems of PTB participated in the comparisons: a single-ion ytterbium clock, a strontium lattice clock, and a cesium fountain clock. The ratio of the indium clock to the ytterbium clock is the first to reach an overall uncertainty lower than the limit required for such comparisons by the roadmap for the redefinition of the second.

The concept promises a new generation of highly stable and accurate optical ion clocks. It is also applicable to other types of ions and opens up new opportunities for entirely new clock concepts, such as the use of quantum many-body states or the cascaded interrogation of several ensembles.