Physicists Trap a Single Antimatter Particle for Months

Keypoints:

• Northwestern University physicists trapped a single positron and cooled it to its quantum ground state
• The positron stayed in the trap for months
• The result appeared in Physical Review Letters on 21 October
• The goal is to compare the positron's magnetic properties with the electron's
• No precision measurement has been made yet

A positron is the antimatter twin of the electron. Physicists at Northwestern University have now held one in a trap for months and calmed it into its lowest energy state. The result was published in Physical Review Letters on 21 October. The team calls the device a one-positron quantum cyclotron. In that state the particle behaves like an artificial atom, with energy levels that scientists can control and read.

The setup is demanding. Positrons from a radioactive sodium source are slowed by a tungsten crystal. They then enter a Penning trap, which uses electric and magnetic fields to hold a charged particle without touching any wall. A superconducting magnet supplies a field of 5.6 tesla. A dilution refrigerator cools the trap to about 0.3 kelvin. Loading a single positron takes eight to eleven hours.

Control is the achievement. Team leader Gerald Gabrielse said they coaxed a very fast positron into sitting still and behaving for months. The team used carefully tuned microwaves to add tiny, exact amounts of energy. They then detected the resulting quantum jumps. A Physics magazine commentary described the particle as loaded, cooled, stored, manipulated one quantum at a time and read out without being destroyed.

The motivation is a symmetry test. The Standard Model predicts that the positron's magnetic moment should match the electron's. Gabrielse's group has already measured the electron's to about one part in ten trillion. A matching positron measurement would provide an exceptionally stringent test of CPT symmetry. A mismatch would point to physics beyond the Standard Model, including clues about why matter dominates the universe.

Outside experts are enthusiastic but patient. Barbara Maria Latacz of CERN called the work a remarkable achievement. Stefan Ulmer of Heinrich Heine University Düsseldorf said he cannot wait to see the first precision measurements. The paper does not yet deliver one. That will require detailed spectroscopy and careful control of systematic effects. The commentary says the method opens a path to a 30-fold improvement in precision for the positron.