Control over the motional degrees of freedom of atoms, ions, and molecules in a field-free environment enables unrivalled measurement accuracies but has yet to be applied to highly charged ions (HCIs), which are of particular interest to future atomic clock designs and searches for physics beyond the Standard Model. Here, we report on the Coulomb crystallization of HCIs (specifically (40)Ar(13+)) produced in an electron beam ion trap and retrapped in a cryogenic linear radiofrequency trap by means of sympathetic motional cooling through Coulomb interaction with a directly laser-cooled ensemble of Be(+) ions. We also demonstrate cooling of a single Ar(13+) ion by a single Be(+) ion-the prerequisite for quantum logic spectroscopy with a potential 10(-19) accuracy level. Achieving a seven-orders-of-magnitude decrease in HCI temperature starting at megakelvin down to the millikelvin range removes the major obstacle for HCI investigation with high-precision laser spectroscopy.

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

Mechanical oscillators based on levitated particles are promising candidates for sensitive detectors and platforms for testing fundamental physics. The targeted quality factors for such oscillators correspond to extremely low damping rates of the center-of-mass motion, which can only be obtained if the particles are trapped in ultra-high vacuum (UHV). In order to reach such low pressures, a non-contaminating method of loading particles in a UHV environment is necessary. However, loading particle traps at pressures below the viscous flow regime is challenging due to the conservative nature of trapping forces and reduced gas damping. We demonstrate a technique that allows us to overcome these limitations and load particles into a Paul trap at pressures as low as 10 −7 mbar. The method is based on laser-induced acoustic desorption of nanoparticles from a metallic foil and temporal control of the Paul trap potential. We show that the method is highly efficient: More than half of trapping attempts are successful. Moreover, since trapping attempts can be as short as a few milliseconds, the technique provides high throughput of loaded particles. Finally, the efficiency of the method does not depend on pressure, indicating that the method should be extensible to UHV.

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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Preparing highly charged ions (HCIs) in a cold and strongly localized state is of particular interest for frequency metrology and tests of possible spatial and temporal variations of the fine structure constant. Our versatile preparation technique is based on the generic modular combination of a pulsed ion source with a cryogenic linear Paul trap. Both instruments are connected by a compact beamline with deceleration and precooling properties. We present its design and commissioning experiments regarding these two functionalities. A pulsed buncher tube allows for the deceleration and longitudinal phase-space compression of the ion pulses. External injection of slow HCIs, specifically Ar(13+), into the linear Paul trap and their subsequent retrapping in the absence of sympathetic cooling is demonstrated. The latter proved to be a necessary prerequisite for the multi-pass stopping of HCIs in continuously laser-cooled Be(+) Coulomb crystals.

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Electron beam ion traps used for spectroscopy of highly charged ions (HCI) produce a deep trapping potential leading to high temperatures of the stored ions, and thus limiting the achievable spectral resolution. A novel device at the Max-Planck-Institut für Kernphysik, the Cryogenic linear Paul Trap Experiment (CryPTEx), attached to an electron beam ion trap, provides a new experimental platform to overcome these limitations. The trap assembly operates at a temperature of 4 K and offers optical access for quantum manipulation and imaging of the trapped ions. Since forbidden optical transitions in HCI do not support direct laser cooling, sympathetic cooling with Coulomb crystals of singly charged ions such as Be+ or Mg+ will be applied in order to reach the natural linewidth of optical forbidden transitions in HCI of interest. With the added advantage of long ion trapping times resulting from residual gas pressures of H2 at 4 K below 10−15 mbar, CryPTEx has been commissioned in collaboration with the Ion Trap Group in Århus using rovibrationally cooled MgH+ ions. Strong suppression of the black body radiation at the trap center, ion storage times of about 28 hours, and largely enhanced population of the rovibrational ground state were achieved