Northwestern Physicists Trap One Positron in a Quantum Ground State for Months

For the first time, Northwestern University physicists have coaxed a single positron, the antimatter counterpart of the electron, into an exquisitely controlled quantum state, setting the stage for a sharper test of the Standard Model of particle physics.

The team captured a fast-moving positron from radioactive decay, using nuclei of sodium-22, a rare radioactive isotope of sodium. They slowed it dramatically, cooled it to nearly absolute zero and suspended it in a near-perfect vacuum so it never touches ordinary matter, which would annihilate it. A trap combining electric and magnetic fields holds the particle in empty space. The interior is cooled to about 300 millikelvins, or 0.3 degrees above absolute zero. The researchers call the result a "one-positron quantum cyclotron," in which the positron acts like an artificial atom with quantum energy levels that can be controlled and observed.

"We cool it down so much that it's in its quantum ground state," said Gerald Gabrielse, the Northwestern physicist who led the study, published in Physical Review Letters. "We actually coaxed this very fast positron to sit there and behave for months at a time." Using carefully tuned microwaves, the team also added tiny, precise amounts of energy to the positron and detected its quantum transitions. Thomas Myers, a doctoral student in the lab, is the first author.

The goal is to measure the positron's magnetic properties with extraordinary precision and compare them with the electron's. Gabrielse's team previously used a one-electron quantum cyclotron to measure the electron's magnetic properties to about one part in 10 trillion, among the most precise measurements ever made of an elementary particle. The Standard Model says electrons and positrons should be nearly perfect mirror images.

"Current theories suggest that there will be no difference in the two measurements," Gabrielse said. "If it turns out they are not equal, that would be the biggest discovery in physics in the last century." A mismatch would expose a crack in the Standard Model and could offer clues to why the universe contains overwhelmingly more matter than antimatter, since the Big Bang should have produced equal amounts.

Gabrielse stressed that the work poses no danger. "We don't have enough antimatter to make an explosion," he said. "If we want to study antimatter, we must handle it carefully." The new experiment brings together two lines of his career: the quantum control achieved with single electrons and the methods he pioneered for slowing, capturing and cooling antiprotons. The next step is the measurement itself, with the positron result set head to head against the electron's.