This project (TSCAC) builds upon the achievements of previous EMRP and EMPIR projects, most recently 17FUN07 CC4C and 18SIB05 ROCIT. While fundamental techniques such as trapping and cooling of ions of different species simultaneously for clock application have been investigated in CC4C, TSCAC will demonstrate the first clocks making use of the potential benefit of the co-trapped species in clock operation. Due to the potentially higher accuracy and improved stability achievable within TSCAC, fundamental metrology research on composite systems of established optical clocks and new reference transitions will be performed to support the use of multi-species composite atomic clocks as future SI standards.

Novel methodologies for composite systems
The use of multi-species atomic clocks opens new perspectives in terms of accuracy and stability for optical clocks. The first demonstration of an optical atomic clock that uses different atomic species has been the Al+ ion clock that requires an ancillary ion for interrogation. Today, an advanced version of this clock is the first among all atomic clocks reporting a fractional frequency uncertainty slightly below 10^-18. New interrogation methods that provide immunity to frequency shift effects in different combinations of atomic species or automatically correct for those measured in interleaved interrogations on ancillary atoms will be developed by this project. These methods include cooling of the ion by contact to a laser-cooled auxiliary ion (sympathetic cooling) that permit very long coherent laser atom interaction times and thus shorter averaging times. Signal links and real-time data processing will be set up to interconnect atomic clocks of different kinds to obtain a stability and accuracy that would not be achievable with the use of each clock system separately.

Composite clocks for improved stability
Higher clock stabilities are needed, to keep the averaging times required to realise the further reduced fractional clock uncertainties of a few 10^-19 at an acceptable level of a few days. Several proposals to circumvent or reduce limiting laser noise by interrogation of more than one optical clock exist and have been successfully demonstrated in some configurations. This project will systematically investigate the most promising approaches of local-oscillator pre-stabilisation, and interrogation longer than the laser coherence time, apply them to state-of-the-art optical clocks and compare their benefits in order to reduce averaging times of optical clocks.

Improved accuracy enabled by two species or two transitions
Optical clock systems utilising two atomic species in the same trap or multiple transitions within the same atom or ion can be exploited to improve the overall accuracy of the clock by investigating and quantifying systematic frequency shifts using novel interrogation methods. In a variety of atom and ion trapping systems this project will examine the most relevant systematic frequency shifts in depth e.g. those due to atomic motion and exposure to DC and AC magnetic and electric fields and blackbody radiation, through modelling and by experimental observation to enable systematic uncertainties in the 10^-19 level, enabling increased sensitivity in many applications (e.g. geodesy and fundamental physics) well aligned with the accuracy target required by the CCTF roadmap to redefine the SI second. Utilising transitions in ancillary atoms and ions, as well as by way of exciting different transitions within the same ion will allow us to perform real-time calibrate perturbing fields and apply correction of dynamic changes in systematic effects during optical clock operation.

New reference transitions and quantum logic in mixed-species systems
Measurements of optical transitions in highly charged ions (HCI) will be demonstrated that are more than a million times more accurate than the present state of the art, which lies at a fractional uncertainty of 9 x 10⁹, and will bring highly charged ions into the accuracy realm of optical atomic clocks. Also, the first direct XUV laser spectroscopy of an electronic transition in a cold HCI will be performed, establishing a new super-optical frequency range for precision spectroscopy in trapped ions. In parallel, the establishment of the frequency measurement architecture for the ²²⁹Th nuclear transition will be pursued, including the development of a VUV frequency comb and its reference to the composite clock network. This transition has never been directly driven with lasers and possesses the potential to revolutionise ion-based optical atomic clocks, yielding a frequency standard that promises exceptional clock performance with levels of stability and accuracy that surpass the best optical frequency standards operating today.