Northwestern University physicists have trapped a single positron—the antimatter counterpart of the electron—and kept it suspended for months at a time, allowing quantum measurements on a lone antimatter particle for the first time.

The team, led by Gerald Gabrielse with doctoral student Thomas Myers as first author, reported the result October 21 in Physical Review Letters.

Because antimatter annihilates the instant it meets matter, no ordinary container can hold it: as Gabrielse put it, 'you have to hold it in a container without walls.'

The positrons came from the radioactive decay of sodium-22, a rare isotope with a half-life of about two and a half years, and emerged at energies of up to 546 kiloelectronvolts—roughly 88 percent of the speed of light, ScienceAlert reports.

To slow them, the team fired them through a cold tungsten crystal, where most annihilated but a scant few survived and slowed enough to be guided into a Penning trap.

There, held by electric and magnetic fields in a near-perfect vacuum chilled to 300 millikelvins—just 0.3 degrees above absolute zero—the particle never touches anything.

Once cooled to its quantum ground state, the positron behaves like what the team calls a one-positron quantum cyclotron—essentially an artificial atom with controllable energy levels, according to phys.org.

Using microwaves tuned near 156 gigahertz, the researchers nudged the particle between discrete cyclotron energy levels and flipped its spin, observing one-quantum jumps in both directions.

The Brighter Side of News reports the team used quantum nondemolition detection, reading the states without changing them, and stretched the excited-state lifetime to about four seconds.

Theory says the Big Bang should have produced equal amounts of matter and antimatter, which would then have annihilated—leaving nothing behind. Yet a small excess of matter survived to seed stars and planets, and the Standard Model cannot explain why.

According to ScienceAlert, Gabrielse and colleagues hope to measure the positron's magnetic moment as precisely as their earlier one-electron cyclotron measured the electron's—to about one part in 10 trillion—then compare the two head-to-head, a test of the CPT symmetry that underpins the Standard Model.

Current theory predicts the positron's magnetic moment should exactly mirror the electron's, equal in magnitude and opposite in sign. 'If it turns out they are not equal, that would be the biggest discovery in physics in the last century,' Gabrielse said, according to phys.org.

The Brighter Side of News notes the authors estimate such a comparison could sharpen tests of CPT symmetry in the lepton sector by more than 30 times.

Even a perfect match would confirm the Standard Model's precision while deepening the mystery of how it can be so accurate yet unable to explain the universe's matter surplus.

The magnetic-moment measurement itself remains ahead: the current demonstration establishes the platform rather than the comparison, and The Brighter Side of News cautions that turning this control into a precision measurement will require further optimization and a detailed accounting of uncertainties.

Gabrielse told ScienceAlert the team hopes to report a result early next year, though it could take longer. Notably, the experiment does not yet show any difference between matter and antimatter—it supplies the tool for making that comparison for the first time.