A team of researchers from the Max Planck Institute for Nuclear Physics in Heidelberg (MPIK), the Physikalisch-Technische Bundesanstalt in Braunschweig (PTB) and the University of Aarhus in Denmark demonstrated for the first time Coulomb crystallization of highly charged ions (HCIs). Inside a cryogenic radiofrequency ion trap the HCIs are cooled down to sub-Kelvin temperatures by interaction with laser-cooled singly charged beryllium ions. The new method opens the field of laser spectroscopy of HCIs providing the basis for novel atomic clocks and high-precision tests of the variability of natural constants.
The procedure was demonstrated for the first time at MPIK in the group under José Crespo López-Urrutia. It involves three steps: First, HCIs are generated in Hyper-EBIT, an ion source that produces and confines ions at a million degrees temperature inside a dense and energetic electron beam in an extreme vacuum. Bunches of HCIs are then extracted from this trap, transferred through a vacuum beamline, slowed down and precooled with a pulsed linear deceleration potential. The ions are very delicately transported into, and eventually confined in, CryPTEx, a cryogenic radiofrequency Paul trap developed at the MPIK in collaboration with Michael Drewsen’s group in Aarhus. Inside this trap, the HCIs bounce back and forth between mirror electrodes, slowly losing speed before they become embedded in a laser-cooled ensemble of light ions (singly charged beryllium), which provide a cooling bath for the HCIs (providing indirect or sympathetic cooling).
In a radiofrequency trap, the confined, mutually repelling ions are forced to share a small volume in space by a combination of electrostatic and oscillating electric fields inside a vacuum chamber. Additionally, the millimeter-sized beryllium ion cloud is cooled by a special laser such that the ions freeze out and form a Coulomb crystal once their thermal motion becomes negligible compared with their electric repulsion. Sophisticated laser systems built at the PTB by Oscar Versolato and colleagues are used at MPIK for this purpose. Once sufficiently cold inside the laser cooled ion ensemble, the HCIs crystallize as well, and can be stored in various configurations.
The efficient cooling of trapped HCIs opens up new fields in laser spectroscopy: precision tests of quantum electrodynamics, measurement of nuclear properties and laboratory astrophysics. The ultimate goal of the MPIK-PTB collaboration will be to test the time dependence of natural constants such as the fine structure constant α, which determines the strength of electromagnetic interaction. For laser spectroscopy, theory predicts that the most sensitive atomic species with respect to α variation is 17-times ionized iridium. In preparation for these future studies, a new highly stable laser system will be installed by the PTB at MPIK to demonstrate the technique with the better known Ar13+ first.