This project will investigate laser-cooled trapped ions as the reference for a next generation of optical clocks of the highest accuracy. While today’s most precise optical clocks with trapped ions are based on single ions, this project will investigate large ensembles of ions in a Coulomb-coupled solid-like state. The multi-ion approach will provide higher signal-to-noise for clocks of improved stability, the sensitivity to investigate small frequency shifts caused by collisions with neutral particles or interactions between the ions, and the opportunity to study new kinds of reference transitions for clocks, specifically the 229Th nuclear transition and transitions in highly charged ions.


Clocks and frequency standards are the most precise measurement devices available today. They are used in a wide range of applications for example in communication, navigation and in fundamental metrology. Within the revised SI system of units, the realization of the unit of time will maintain a pivotal role, as the unit second will be contained in the definition of 6 of the 7 base units via the defining constants. Progress in research on optical clocks continues at a rapid pace towards lower systematic uncertainties, now evaluated in the low         10-18 range. Proposals for new reference systems with specific advantages in terms of accuracy or stability require research on new experimental methods as well as on relevant atomic, molecular and nuclear data.

This project addresses topics and objectives expressed in the EURAMET call 2017 on fundamental scientific metrology, in that it plans to study the subjects of ‘New optical clocks, for which there are many potential atomic reference transitions including nuclear transitions and where fundamental research is necessary to define the best candidate’. It extends the work of earlier EMRP and EMPIR projects, especially SIB04 ‘ion clock’ and 15SIB03 ‘OC18’ into the directions of new reference transitions, a multi-ion approach at higher ion numbers, the study of sympathetic cooling of the ions of interest for the clock together with ions that are suitable for laser cooling, and the study of collisional frequency shifts that have so far only been estimated based on simplified models. The project will foster cooperation between groups working in optical frequency metrology at National Metrology Institutes with groups from universities and research institutes that provide relevant complementary expertise in atomic and nuclear physics, including ion sources of radioactive isotopes and electron beam ion traps for highly charged ions.


The specific objectives of the project are:

  1. To minimise the effects of kinetic energy and interaction between the ions of laser-cooled Coulomb crystals to reduce systematic frequency shifts in optical clock applications by investigation of the structure and dynamics of laser-cooled two-species Coulomb crystals of ions of different masses and charge states.
  2. To implement sympathetic cooling of clock relevant ions (including highly charged ions) with suitable coolant ions to reduce systematic frequency shifts.
  3. To develop efficient sources of 229Th ions in charge states Th3+ and Th4+, based on recoil ions from 233U. This should allow loading of an ion trap in ultrahigh vacuum from a source of less than 10 kBq 233U activity.
  4. To provide reliable estimates on collisional frequency shifts due to the background gas in trapped ion optical clocks by investigating collisions of trapped ions with neutral atoms and molecules. To identify and eliminate causes of ion loss caused by charge exchange or chemical reactions that lead to the formation of molecular ions.
  5. To develop transportable equipment for laser cooling, high resolution spectroscopy and precision optical frequency measurements that enable common experiments to be carried out at nuclear physics laboratories and optical metrology laboratories.