Scientists from the Physikalisch-Technische Bundesanstalt (PTB) and the Max Planck Institute for Nuclear Physics (MPIK) have enabled precise optical measurements of highly charged ions by isolating a single highly charged ion from an extremely hot plasma and bringing it almost to rest inside an ion trap, along with a laser-cooled, singly charged ion. The physicists used quantum logic spectroscopy on the ion pair to increase the relative precision of their measurements by a factor of a hundred million over previous methods. To date, scientists have not been able to apply established measurement techniques, such as those used with optical clocks, to highly charged ions. The main obstacle manifests even before measurement can begin. During the production of highly charged ions, a large amount of energy is needed to remove a significant number of electrons from the atoms, and the ions then exist in the form of a plasma as hot as the sun. To ensure precise measurements, the lowest possible temperatures and well-controlled ambient conditions are required. Highly charged ions cannot be directly laser-cooled, and conventional detection methods cannot be applied due to their atomic structure. Artist's impression of the ion pair: laser-cooled Be+ (left) and highly charged Ar13+ (right). Courtesy of PTB. The physicists from PTB and MPIK combined solutions that addressed each of these problems individually in an experiment at the QUEST Institute for Experimental Quantum Metrology in Braunschweig. They isolated a single highly charged ion (Ar13+) from a hot plasma ion source and stored it together with a singly charged beryllium ion in an ion trap. Beryllium ions can be laser-cooled, and through the mutual electrical interaction between the ions, the temperature of the ion pair is reduced. Eventually, this so-called sympathetic cooling forms a two-ion crystal that completely “freezes” into the quantum mechanical ground state of motion at an equivalent temperature of only a few millionths of a degree above absolute zero. Using an ultrastable laser, the scientists precisely resolved the spectral structure of the Ar13+ ion in a measurement procedure similar to that used in atomic clocks. They applied the concept of quantum logic, in which the spectroscopy signal is coherently transferred from the highly charged ion to the beryllium ion by means of two laser pulses. They used quantum logic spectroscopy to probe the forbidden optical transition in the highly charged ion at a wavelength of 441 nm and to measure its excited-state lifetime and g-factor. Implantation of the Ar13+ ion into the laser-cooled Be+ ion crystal and step-wise reduction to the quantum logic configuration of an ion pair. Courtesy of PTB/MPIK. The researchers said that the quantum state of the beryllium ion is much easier to determine via laser excitation. “Descriptively, the beryllium ion ‘eavesdrops’ on the state of the less communicative, highly charged ion and reports to us about its state,” professor Piet Schmidt, who led the research, said. “Here, we have improved the relative precision for highly charged ions by a factor of one hundred million compared to traditional spectroscopy,” researcher Peter Micke said. By combining methods, the researchers established a general concept that could be applied to most highly charged ions. The beryllium ion can always be used as a so-called logic ion. The production process of the highly charged ions in the plasma, with subsequent isolation of a single ion, is independent of the choice of the atomic type and the charge state. Due to their high positive charge, the outer electrons of the highly charged ion’s atomic shell are strongly bound to the atomic nucleus. Highly charged ions are therefore less sensitive to perturbations by external electromagnetic fields. On the other hand, compared to neutral and singly charged atoms, the effects of special relativity and quantum electrodynamics, as well as the interaction with the atomic nucleus, are enhanced. Highly charged ions are therefore ideal systems for accurate atomic clocks that can be used to test fundamental physics. The outer electrons in these systems serve as sensitive “quantum sensors” for effects such as previously unknown forces and fields. Since every single element of the periodic table provides as many charge states as there are electrons in the atomic shell, there exists a vast variety of atomic systems to choose from. José Crespo, head of the group at the Max Planck Institute for Nuclear Physics, said, “This experiment opens up an unprecedented, extremely extensive area of atomic systems to be used in precision spectroscopy as well as for future clocks with special properties.” The variety of these new, tailored “quantum sensors” could aid in the search for dark matter; could help determine whether the standard model of particle physics is complete; and could help determine whether fundamental constants are really constant. The research was published in Nature (www.doi.org/10.1038/s41586-020-1959-8).