Periodic Reporting for period 2 - FunClocks (Testing Fundamental Physics with Highly Charged Ion Clocks)
Periodo di rendicontazione: 2023-07-01 al 2024-12-31
Precision spectroscopy of highly charged ions (HCI) provides insight into atomic systems in which electrons are highly correlated, strongly relativistic, and experience strong internal fields. Thus, HCI are excellent systems to probe and refine our understanding of physics under these extreme conditions. They are the most sensitive known atomic species to probe for possible changes in fundamental constants and offer advantageous properties to study coupling of hypothetical dark matter fields to normal matter. For these applications, high-precision optical spectroscopy of HCI is required. In the past, the spectroscopic resolution of optical transitions in HCI was limited by Doppler-broadening to hundreds of megahertz. We have recently demonstrated the first hertz-level laser spectroscopy of an optical fine-structure transition in highly charged argon using sympathetic cooling and quantum logic with a co-trapped logic ion in a Paul trap, improving the spectroscopic precision by nine orders of magnitude compared to the previous state-of-the-art. Here, we propose to further develop quantum techniques for controlling HCI and to push spectroscopic resolution in order to realise next generation optical clocks based on promising reference transitions in HCI. We will employ these novel types of optical clocks to advance our understanding of atomic structure and to probe for physics beyond the standard model. Sub hertz-level isotope shift spectroscopy of highly charged calcium ions will be performed to improve current bounds on hypothetical fifth forces that couple neutrons and electrons. Furthermore, we will perform optical clock-type spectroscopy on HCI that offer up to a 20-fold higher sensitivity to a possible change in the fine-structure constant and a non-gravitational coupling between dark matter and normal matter than existing clocks. Through frequency comparisons with other clocks, we will improve bounds on these new physics effects.
During the first months of the project, an Ar13+ highly charged ion (HCI) optical clock has been operated and a detailed analysis of systematic frequency shifts has been performed. The relative systematic frequency uncertainty was limited to 2.2e-17 by uncompensatable micromotion from a defect of the employed ion trap. Frequency comparisons against a high accuracy clock at PTB based on an octupole transition in Yb+ yielded a frequency ratio with a relative uncertainty of 1e-16 and an absolute frequency of the Ar13+ transition with a relative uncertainty of 1.7e-16. These results demonstrated the first optical clock based on a highly charged ion and confirmed the potential of these species for high accuracy clocks [King et al., Nature 611, 43-47 (2022)].
In parallel, preparations for isotope shift spectroscopy of the stable even Ca14+ isotopes started. A clock laser system was setup and stabilized to a stable reference cavity, transfer-locked to a Si-cavity stabilized laser at PTB. An extension of the electron beam ion trap was designed, built and commissioned to allow ablation loading of metal targets such as Calcium. Precision spectroscopy of 40, 42, 44, 46 and 48-Ca14+ at a level of 1e-16 (corresponding to <100 mHz resolution) was performed against the Yb+ octupole clock at PTB and the corresponding isotope shifts were extracted and combined with existing data on singly charged Calcium. Together with several other experimental and theory groups the quality of the previously existing data is currently being improved and an analysis in terms of exclusion of possible 5th force candidates is under way.
In preparation for the next HCI clock species, several search strategies for finding mHz narrow transitions within a frequency range of THz have been investigated and benchmarked [Chen et al., arXiv:2406.04015]. Of the investigated techniques based on Rabi spectroscopy, oscillating dipole forces and linear continuous sweep, the latter is by far the most efficient if a logic transition is available. Laser systems for spectroscopy of the next optical clock candidate, Ni12+, have been procured and set up. This includes a fiber laser-based logic laser system at 512 nm and a frequency doubled Ti:Sa laser at 495 nm, which offers the broad scan range required to cover the uncertainty in current calculations of the clock transition frequency of on the order of a terahertz. The logic transition has recently been found using the optical dipole force search technique. We are currently preparing the search for the 8 mHz narrow clock transition using the linear continuous sweep technique.
We have also started investigating the use of multiple HCI. Original attempts to prepare those using our standard technique failed due to loss of the HCIs when heating out the excess Be+ ion. We have therefore developed a splitting & discarding technique in another setup [Zawierucha et al., Phys. Rev. A, accepted] and implemented them successfully with HCI. In a next step we will investigate the preparation of decoherence-free substates in HCI to eliminate the linear Zeeman effect and take advantage of the larger signal-to-noise ratio of having two HCI in the trap.
In parallel, preparations for isotope shift spectroscopy of the stable even Ca14+ isotopes started. A clock laser system was setup and stabilized to a stable reference cavity, transfer-locked to a Si-cavity stabilized laser at PTB. An extension of the electron beam ion trap was designed, built and commissioned to allow ablation loading of metal targets such as Calcium. Precision spectroscopy of 40, 42, 44, 46 and 48-Ca14+ at a level of 1e-16 (corresponding to <100 mHz resolution) was performed against the Yb+ octupole clock at PTB and the corresponding isotope shifts were extracted and combined with existing data on singly charged Calcium. Together with several other experimental and theory groups the quality of the previously existing data is currently being improved and an analysis in terms of exclusion of possible 5th force candidates is under way.
In preparation for the next HCI clock species, several search strategies for finding mHz narrow transitions within a frequency range of THz have been investigated and benchmarked [Chen et al., arXiv:2406.04015]. Of the investigated techniques based on Rabi spectroscopy, oscillating dipole forces and linear continuous sweep, the latter is by far the most efficient if a logic transition is available. Laser systems for spectroscopy of the next optical clock candidate, Ni12+, have been procured and set up. This includes a fiber laser-based logic laser system at 512 nm and a frequency doubled Ti:Sa laser at 495 nm, which offers the broad scan range required to cover the uncertainty in current calculations of the clock transition frequency of on the order of a terahertz. The logic transition has recently been found using the optical dipole force search technique. We are currently preparing the search for the 8 mHz narrow clock transition using the linear continuous sweep technique.
We have also started investigating the use of multiple HCI. Original attempts to prepare those using our standard technique failed due to loss of the HCIs when heating out the excess Be+ ion. We have therefore developed a splitting & discarding technique in another setup [Zawierucha et al., Phys. Rev. A, accepted] and implemented them successfully with HCI. In a next step we will investigate the preparation of decoherence-free substates in HCI to eliminate the linear Zeeman effect and take advantage of the larger signal-to-noise ratio of having two HCI in the trap.
We have demonstrated the first optical clock based on highly charged ions (HCI) and performed a detailed analysis of systematic shifts, resulting in a relative uncertainty of 2.2e-17. For the first time isotope shift measurements of a HCI with sub-Hertz resolution have been performed. Combined with improved mass data and spectroscopy of singly charged ions, this will improve bounds on hypothetical fifth forces by more than an order of magnitude. In the future, an improved ion trap will allow us to demonstrate an HCI clock based on Ni12+ with a systematic uncertainty at 1e-18 or below. The long excited state lifetime of Ni12+ supports long probe times and thus a significantly reduced statistical uncertainty. In the future, entangled HCIs in a decoherence-free substate are free of the linear Zeeman effect and provide a larger signal-to-noise ratio, improving the systematic uncertainty and the stability of an HCI clock. Towards the end of the project a HCI species with high sensitivity to a change in fundamental constants will be investigated, improving corresponding bounds and excluding ultralight dark matter models.