Need and Objectives

The fundamental properties of atoms provide characteristic microwave or optical frequency references used in atomic clocks to realise the most precise measurement devices available today. Within the SI system of units, the realization of the unit of time with Caesium atomic clocks plays an essential role, as the unit second is contained in the definition of 6 of the 7 base units via the defining constants.Having highly accurate, stable, and reliable reference frequency standards is a pre-requisite not only for the SI System of Units but also for many everyday technologies that rely on precise time keeping such as banking transactions, communication, and navigation.

Resulting from the higher reference transition frequency, highly precise optical clocks have made great progress with a variety of different reference systems with neutral atoms and single ions. The seminal proposals and much of the experimental work to date has been focused on atomic clocks based on single reference transitions. The combination of advantages of established reference systems in two-species composite atomic clocks promises to achieve even better performance as required by the growing needs in communication and navigation.

First realisations of these promising composite clocks, with single systems even at distant locations, require the development of the necessary measurement infrastructure and related methodologies such as interrogation protocols and corresponding data processing (Ob. 1). To compare such highly accurate clocks in measurement campaigns shorter than weeks, their frequency instability needs to be improved. This can be achieved by utilizing clocks based on many atomic absorbers (Ob. 2). The realization of unperturbed transition frequencies as standards for time and frequency requires knowledge of the size of frequency shifts caused by residual perturbing fields at the position of the reference atoms. Sensitive atomic transitions of the same atom or ancillary atoms can provide the means to calibrate residual fields and enable precise corrections to improve clock accuracy (Ob. 3). Alongside the use of established atomic reference transitions, novel systems with greater immunity to the electric or magnetic fields inherent in atomic clock operation are of particular interest. These novel systems include a low-energy nuclear transition in 229Th and transitions in highly charged ions (HCI), but currently their atomic transition frequencies are not known to the required. Additionally, theoretically predicted characteristics need to be experimentally investigated (Ob. 4). To enable specialised research laboratories to search for and perform initial spectroscopy on yet to be discovered clock transitions, frequency references with increased reliability are needed (Ob. 5).


This project will focus on metrology research necessary to support the use of composite atomic clocks as future SI standards. The specific objectives of the project are:

  1. To develop and optimise the interrogation sequences, signal links and real-time data processing and cooling methods that enable composite clocks or different clock types to stabilise one common oscillator, to obtain a stability and accuracy that would not be achievable with the use of each clock system separately. This includes the application to systems that are distributed over two or more different locations.
  2. To reduce the frequency instability of optical clocks with single or few atoms/ions of typically above 1×10-15/√τ(s) and the correspondingly long averaging times due to quantum projection noise to below  1×10-15/√τ(s) using information obtained in simultaneous measurements performed on ensembles of many atomic absorbers.
  3. To improve the frequency accuracy in single-species atomic clocks by using established reference transitions in two-species optical clocks or with atoms possessing two reference transitions using precisely measured relative sensitivities to external fields.
  4. To investigate new reference transitions in two-species composite systems, to enable clock operation and absolute frequency measurements for transitions in so far inaccessible atomic systems (such as highly charged ions) with target uncertainties at the 1 Hz level. This is equivalent to a relative uncertainty of 2x10-15. This includes the direct investigation of theoretically predicted characteristics of these transitions and associated systematic shifts via a readout scheme using an ancillary ion.
  5. To facilitate the take up of the technology and measurement infrastructure developed in the project by the measurement supply chain (NMIs, research laboratories), and possible end users (space, aerospace, telecommunications, energy).