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EMPIR project publishes new papers on optical atomic clocks

Image of a glowing clock face with rotating hands

Clock with rotating hands

The TSCAC project is helping to push time standards towards even lower uncertainties

The project

The second, the unit for time, is a component of six out of seven SI units. Its accurate measurement is vital across almost all areas of metrology, as well as many every-day applications which rely on precise timekeeping like banking and telecommunications.

Since the 1960s the second has been realised using atomic clocks, devices that measure the frequency of photons absorbed when Caesium-133 atoms transition from the lowest to a higher energy level. Using this measured transition frequency, the second can today be derived to an extremely high degree of accuracy.

In more recent years, new optical clocks (which use optical reference transitions of trapped ions or atoms confined in optical lattices) have reduced the relative measurement uncertainty to as low as 10^-19. As the use of optical clocks develops, primary standards which are stable and reliable must be created to support their use, including their possible use in the definition of the second.

EMPIR project Two-species composite atomic clocks (20FUN01, TSCAC) is investigating two-species composite atomic clocks, building on the work of EMPIR projects Coulomb Crystals for Clocks (17FUN07, CC4C) and Robust Optical Clocks for International Timescales (18SIB05, ROCIT).

The project has developed new measurement methods for composite systems, studied clock stabilisation and presented the first realisation of an optical clock using highly-charged ions.

Journal Papers

Further achievements of the project have been published in a number of journal papers:

“Evaluation of a ⁸⁸Sr⁺ Optical Clock with a Direct Measurement of the Blackbody Radiation Shift and Determination of the Clock Frequency”, a joint paper published in Physical Review Letters, which demonstrates both a new technique to determine the frequency of a clock transition and the first comparison of an ⁸⁸Sr⁺ optical clock, enabling measurement uncertainty of 2.3 × 10^-17.
“Quantum Non-Gaussianity of Multiphonon States of a Single Atom”, published in Physical Review Letters
“Improved Limits on the Coupling of Ultralight Bosonic Dark Matter to Photons from Optical Atomic Clock Comparisons”, published in Physical Review Letters, which has been selected as Editor’s Suggestion and additionally published as a Synopsis
“¹⁷¹Yb⁺ optical clock with 2.2 × 10⁻¹⁸ systematic uncertainty and absolute frequency measurements”, published in Metrologia, which includes an assessment of trap-drive-induced AC Zeeman shift within the clock completed by the project

Project coordinator Nils Huntemann (PTB) has said about the work of the project:

“These great results nicely reflect how the fundamental research conducted in the project is helping optical clocks to not only improve metrological capabilities and enable future primary measurement standards, but also to be used in other areas of research such as the search for dark matter.”

This EMPIR project is co-funded by the European Union's Horizon 2020 research and innovation programme and the EMPIR Participating States.

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