The Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig has demonstrated high accuracy in a new type of clock. According to the PTB, it has the potential to measure time and frequency 1000 times more accurately than the cesium 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.
Vacuum shields all external influences
In an optical atomic clock, atoms are irradiated with laser light. If the laser has the right frequency, the atoms change their quantum mechanical state. All external influences on the atoms must be shielded or precisely measured. Optical clocks with trapped ions do this very well. The ions can be localized to a few nanometres in a vacuum using electric fields. Thanks to excellent control and isolation, this comes very close to the ideal of an undisturbed quantum system. Ion clocks have therefore already achieved systematic uncertainties beyond the 18th decimal place. Such a clock, if it had been ticking since the Big Bang, would be at most one second off today.
Current ion clocks are operated with a single clock ion. Its low signal must be measured over long periods of time, up to two weeks, in order to determine a frequency at this level. To exploit the full potential, it would even require measurement periods of more than three years.
Parallel measurement saves a lot of time
With the newly developed clock, this measurement time is drastically reduced through parallelization: several ions are captured simultaneously in a trap, often combining different ions. Through their interaction, they form a new, crystalline structure. "This concept also makes it possible to combine the strengths of different ions," says PTB physicist Jonas Keller: "We use indium ions because of their favorable properties to achieve high accuracies. For efficient cooling, ytterbium ions are also added to the crystal".
One challenge was the development of an ion trap that can use such a spatially extended crystal as a clock just as precisely as individual ions. Another challenge was to develop experimental methods to position the cooling ions within the crystal.
Prerequisite for redefining the second given for the first time
For the necessary comparisons with other clock systems, two other optical and one microwave clock from PTB were included: a ytterbium single-ion clock, a strontium lattice clock and a caesium fountain clock. For the first time, the ratio of the indium clock to the ytterbium clock achieved a total uncertainty below the limit required in the roadmap for the redefinition of the second for such measurements. The concept promises a new generation of ion clocks with high stability and accuracy. It can also be applied 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 interrogation of several ensembles.