On the path to defining a new second

On the path to defining a new second
by Robert Schreiber
Berlin, Germany (SPX) Jan 20, 2025

The future of timekeeping is being reshaped by optical atomic clocks, which operate with laser frequencies roughly 100,000 times faster than the microwave frequencies of caesium clocks, the current standard for defining the second in the International System of Units (SI). Some of these optical clocks are already 100 times more accurate than caesium clocks and are poised to redefine the second. However, before this transition occurs, these optical clocks must demonstrate reliability through repeated testing and international comparisons.

Germany's Physikalisch-Technische Bundesanstalt (PTB), a leading global institution in metrology, has developed various optical clocks, including single-ion and optical lattice clocks. Recently, PTB researchers achieved a milestone with a novel ion crystal clock, which exhibits accuracy 1,000 times greater than current caesium clocks. A comparison with other optical clocks has confirmed this unprecedented level of precision. These findings were published in the journal Physical Review Letters.

Optical atomic clocks use laser light to manipulate atoms, causing changes in their quantum states. To achieve this, atoms must be shielded from external influences, with any residual effects meticulously measured. Optical clocks with trapped ions excel in this area, as the ions can be held in place within a vacuum using electric fields, isolated to within a few nanometers. This control allows these clocks to achieve systematic uncertainties beyond the 18th decimal place. In practical terms, such a clock would lose only one second if it had been operating since the Big Bang.

Traditionally, these clocks operate with a single ion, requiring extended measurement periods-sometimes up to two weeks or even longer-to achieve their remarkable accuracy. To reduce this time, the newly developed clock incorporates multiple ions trapped together, forming a crystalline structure. This approach not only accelerates measurement but also combines the strengths of different ion types. As PTB physicist Jonas Keller explains, "We use indium ions as they have favorable properties to achieve high accuracy. For efficient cooling, ytterbium ions are added to the crystal."

Developing this clock involved overcoming significant challenges, such as creating an ion trap capable of maintaining high-accuracy conditions for a larger crystal and devising methods to position cooling ions precisely. PTB's research group, led by Tanja Mehlstaubler, tackled these challenges with innovative techniques. As a result, the new clock has achieved an accuracy near the 18th decimal place.

The clock's capabilities were validated through comparisons with other PTB systems, including a ytterbium single-ion clock, a strontium lattice clock, and a caesium fountain clock. Notably, the indium clock's ratio to the ytterbium clock demonstrated an uncertainty below the threshold outlined in the roadmap for redefining the second.

This ion crystal clock marks a significant advancement in optical timekeeping. Its adaptable design enables the use of other ion types and paves the way for novel clock concepts, such as employing quantum many-body states or cascading multiple ensembles.

This research was supported in part by the German Research Foundation (DFG) through the Quantum Frontiers Cluster of Excellence and the DQ-mat Collaborative Research Center.

Research Report:115In+-172Yb+ Coulomb Crystal Clock with 2.5 + 10-18 Systematic Uncertainty