A new type of radiofrequency trap can capture particles with extremely different requirements and could theoretically hold both types of particles at the same time. Researchers in the group of Professor Dmitry Budker from the PRISMA++ Cluster of Excellence and the Helmholtz Institute at Johannes Gutenberg University Mainz (JGU) were able to trap calcium ions or electrons in the same apparatus. The team’s findings, published in Physical Review A, show the potential of this technology for synthesizing antihydrogen.
“Radiofrequency traps, also called Paul traps, have long been used by physicists to trap specific particles,” Dr. Hendrik Bekker explained. “However, they are usually limited to a single frequency.” This means that only one type of particle can be captured at a time in a typical Paul trap. In order to synthesize antihydrogen, however, two types of particles – antiprotons and positrons – would need to be trapped together at the same time. Due to their low mass, positrons require GHz-frequency fields for stable confinement, while antiprotons are typically trapped with MHz-frequency fields. For their current study, the researchers at JGU used electrons and heavy calcium ions (40Ca+) as more readily available stand-ins for antiprotons and positrons.
Catching two birds in the same cage
In order to trap the calcium ions and electrons, the dual-frequency Paul-trap, which is being developed in collaboration with Professor Ferdinand Schmidt-Kaler from JGU as well as the group of Professor Hartmut Häffner at UC Berkeley, has to generate both GHz and MHz frequency fields. Hendrik Bekker and PhD candidate students Vladimir Mikhailovski and Natalija Rajeshri Sheth generate these fields by layering three printed circuit boards (PCB) and separating them with ceramic spacers. The central board is equipped with what is known as a coplanar waveguide resonator which generates the GHz frequency field to trap electrons. The top and bottom PCBs feature segmented DC electrodes used to apply the lower MHz frequency field used for catching ions. Both types of particles are generated by photo-ionizing neutral calcium atoms using a two-step laser scheme (423 nm and 390 nm).
The particles are then caught in the dual-frequency trap for various amounts of time, from milliseconds to several seconds, before extracting them via DC voltage pulses and detecting them. Bekker: “Using this technique, we stored electrons or ions. Trapping both at the same time proved challenging.” Electrons turn out to be highly sensitive to the amplitude of the lower-frequency field used for trapping the ions. The higher the amplitude, the more electrons are lost from the trap. Ions, on the other hand, have proven to be effectively unaffected by the amplitude of the high-frequency field.
Further challenges are posed on the mechanical side: roughness of surfaces, mechanical misalignments and dielectric charging currently limit the effectiveness of the trap. Next-generation equipment will feature laser-etched, smoother electrodes with better thermal stability.
Diversifying the creation of antihydrogen
The ultimate goal of the researchers is to use their new dual-frequency trap to hold both antiprotons and positrons in order to combine them into antihydrogen. Currently, the only source for antiprotons, and thus antihydrogen, is the Antimatter Factory (AMF) at CERN in Switzerland. Bekker: “Antihydrogen is a kind of Holy Grail in antimatter research. Its uniquely simple makeup – just one antiproton and a positron – means we can generate it relatively easily compared to other antimatter.” And since its counterpart, hydrogen, is well-researched, measurements taken from antihydrogen have a strong point of comparison. The transport of antiprotons has recently been proven to work, which means that the chance of this becoming a reality rises. Professor Dmitry Budker is optimistic: “The recent success in transporting antiprotons using a truck has shown that delivering antiprotons to researchers far from CERN is feasible, although there are still technical challenges such as long-term cryogenic cooling to solve.”
Along with their own open questions and challenges, Dr. Hendrik Bekker and his team also look forward to scientific work along the way. “While we develop our trap further, we will be able to run a number of fascinating experiments,” said Bekker. “For example, theoretical physics tells us that positrons should be able to bind to atoms – even if for the briefest of moments. We might be able to test that theory in an experimental setting for the first time.”