Precision spectroscopy of atomic systems 1 is an invaluable tool for the advancement of our understanding of fundamental interactions and symmetries 2. Recently, highly charged ions (HCI) have been proposed for sensitive tests of physics beyond the Standard Model 2-5 and as candidates for high-accuracy atomic clocks 3,5. However, the implementation of these ideas has been hindered by the parts-per-million level spectroscopic accuracies achieved to date 6-8. Here, we cool a trapped HCI to the lowest reported temperatures, and introduce coherent laser spectroscopy on HCI with an eight orders of magnitude leap in precision. We probe the forbidden optical transition in 40 Ar 13+ at 441 nm using quantum-logic spectroscopy 9,10 and measure both its excited-state lifetime and g-factor. Our work ultimately unlocks the potential of HCI, a large, ubiquitous atomic class, for quantum information processing, novel frequency standards, and highly sensitive tests of fundamental physics, such as searching for dark matter candidates 11 or violations of fundamental symmetries 2. Alike a microscope aimed at the quantum world, laser spectroscopy pursues ever higher resolving power. Every increase in resolution enables deeper insights into the subtle effects that all known fundamental interactions have on the atomic wave function. Advances in optical frequency metrology have dramatically improved resolution in the last three decades 1 , and are making laser spectroscopy an extremely sensitive tool for studying open physics questions such as the nature of dark matter, the strength of parity violation, or a possible violation of Einstein's theory of relativity 2. However, only a few atomic and ionic species are currently within the reach of cutting-edge optical frequency metrology. Expanding this field of exploration to systems with high sensitivity to such effects is therefore crucial. Due to their extreme properties, highly charged ions (HCI) are promising candidates for such fundamental tests. Contributions from special
In-vacuo cryogenic environments are ideal for applications requiring both low temperatures and extremely low particle densities. This enables reaching long storage and coherence times for example in ion traps, essential requirements for experiments with highly charged ions, quantum computation, and optical clocks. We have developed a novel cryostat continuously refrigerated with a pulse-tube cryocooler and providing the lowest vibration level reported for such a closed-cycle system with 1 W cooling power for a < 5 K experiment. A decoupling system suppresses vibrations from the cryocooler by three orders of magnitude down to a level of 10 nm peak amplitudes in the horizontal plane. Heat loads of about 40 W (at 45 K) and 1W (at 4 K) are transferred from an experimental chamber, mounted on an optical table, to the cryocooler through a vacuum-insulated massive 120 kg inertial copper pendulum. The 1.4 m long pendulum allows installation of the cryocooler in a separate, acoustically isolated machine room. In the laser laboratory, we measured the residual vibrations using an interferometric setup. The positioning of the 4 K elements is reproduced to better than a few µm after a full thermal cycle to room temperature. Extreme high vacuum on the 10 −15 mbar level is achieved. In collaboration with the Max-Planck-Intitut für Kernphysik (MPIK), such a setup is now in operation at the Physikalisch-Technische Bundesanstalt (PTB) for a next-generation optical clock experiment using highly charged ions.
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