Electric, magnetic fields confine antimatter near absolute zero, enabling future precision comparisons with electrons’ magnetic moments

Physicists at Northwestern University have captured a single positron — the antimatter twin of an electron — and successfully held it in a trap for months.

Led by Gerald Gabrielse, the team had cooled the particle to its lowest possible quantum energy state inside a Penning trap, creating what they call a “one-positron quantum cyclotron.” The results were published in Physical Review Letters on 21 October 2026.

The central challenge was confining antimatter without letting it touch ordinary matter, because contact with an electron would cause annihilation. Instead of physical walls, the researchers used carefully controlled electric and magnetic fields to suspend the positron in a near-perfect vacuum.

Once cooled, the particle behaved like an artificial atom whose energy levels could be observed and manipulated.

Methodology

The positrons originated from the radioactive decay of sodium-22 and initially carried energies as high as 546 kilo-electron-volts. To slow them, the team directed them through a tungsten crystal, where many were destroyed but the slowest survivors could be selected.

Some of those survivors briefly formed neutral positronium by binding with an electron, allowing them to enter the trap through a 625-micrometer opening. The trap’s electric field then separated the pair, retaining the positron. At present, loading one positron takes roughly 8 to 11 hours.

The apparatus operates at 300 milliKelvin — about 0.3 degrees Celsius above absolute zero — and the researchers demonstrated that tuned microwave pulses could add precise amounts of energy to the positron. They also detected its individual quantum jumps between energy states.

A clue to matter-antimatter imbalance

Gabrielse said observing those one-quantum transitions was especially satisfying after years spent pursuing the device. The long-term goal is to measure the positron’s magnetic moment with far greater precision and compare it with the electron’s. The same group had previously measured the electron’s magnetic moment to about one part in 10 trillion, but the most recent electron-positron comparison dates to 1987 and found agreement only to within 2.1 parts per trillion. ETH Zurich physicist Paolo Crivelli wrote that the new trap could improve the positron measurement’s precision by a factor of 30.

Under CPT symmetry definitions, a particle and antiparticle should have magnetic moments equal in magnitude but opposite in sign. Any confirmed mismatch would challenge the foundations of relativistic quantum field theory and could offer clues to the universe’s matter-antimatter imbalance.

Gabrielse said current theory predicts no difference, but a discrepancy would amount to the biggest physics discovery in a century.