Scientists finally trapped a piece of 'impossible' antimatter for months—and it changes everything.

A trapped positron, cooled almost to absolute zero and held away from every wall, can now be nudged one quantum at a time to test antimatter.

Antimatter has a reputation, mostly from science fiction, for disappearing the instant it touches ordinary matter. Even so, a single particle of it, a positron, has been held in a laboratory in Illinois for months.

Physicists cooled it to about half a degree Fahrenheit (0.3°C) above absolute zero. They hold it in the middle of an empty chamber with magnetic and electric fields, so it never touches a wall.

Held that still, the particle of antimatter behaves like a tiny, artificial atom.

Its energy can only change in fixed steps, and the team can now push it up one step with microwaves and watch it fall back.

Such control is what they need for the next job. This involves measuring the positron's magnetism so precisely that any difference from an electron's would show.

Gerald Gabrielse, who directs the Center for Fundamental Physics at Northwestern University, led the work. Thomas Myers, a Ph.D. student in his lab, is the paper's first author.

"Antimatter gets its popular charm from science fiction," Gabrielse said.

Why the positron trap has no walls

The positron particle is the electron's antimatter twin, with the opposite electric charge. Itt disappears the moment it meets an electron, including one in a container wall.

So the Northwestern team suspends the positron in the center of a sealed metal chamber instead.

A superconducting magnet makes it circle in a tight loop, and an electric field from the chamber's own electrodes keeps it from drifting out along the magnet's axis. Physicists call the arrangement a Penning trap.

The chamber is chilled so far that its walls freeze out stray gas, leaving almost nothing inside for the antimatter particle to hit.

"If we want to study antimatter, we must handle it carefully," Gabrielse said. "To be clear, it's not dangerous. We don't have enough antimatter to make an explosion."

Catching the antimatter particle takes hours

The positrons come from a sealed capsule of sodium-22, a radioactive form of sodium. They leave it with up to 546,000 electron volts of energy.

By the time the trapped positron has settled, it has 26 millionths of an electron volt, about 20 billion times less.

The capsule is lowered on a thin, nylon cable down through the refrigerator to the top of the chamber. A positron first passes through a titanium window thinner than a human hair. Then it enters a single crystal of tungsten, where collisions slow it almost to a stop.

On its way out it picks up an electron from the tungsten surface. The positron and the electron, now a neutral pair, drift through a hole in a silver electrode about as wide as a mechanical pencil lead.

Inside the trap the electric field tears the electron away, and the positron is caught.

Loading takes 8 to 11 hours per positron, partly because the source is weak: 18 microcuries, compared with the 500 microcuries used for the best earlier positron measurement.

Energy jumps can be seen

Cooled this far, the positron's circling motion drops to its lowest allowed energy and stays there. The chance that a stray bit of heat kicks it up one level is about one in 100 billion. So nothing happens until the team makes it happen.

To make it move, the team feeds in microwaves at about 156 gigahertz, close to the positron's circling frequency. The positron also wobbles back and forth along the magnet's axis, about 206 million times a second.

Each quantum of energy it absorbs shifts that rate by about 3.8 hertz, enough to tell the team which energy level the positron is in without disturbing it.

The team's records show the positron stepping up one level and dropping back, and once stepping up two levels before falling twice.

Left to itself, a positron with one extra quantum releases it again within about 83 thousandths of a second. The trap's electrodes form a microwave cavity that delays that release to about four seconds, long enough for the faint signal to be read.

Spin flips take two steps

A positron also spins, which makes it a tiny magnet. Its spin points either with the field or against it.

A spin in the higher of its two states would take a calculated 2.1 years to flip on its own, but the cavity probably stretches that further.

To flip the spin on purpose, the team applies a second drive for two seconds that changes the spin and the circling energy together. The positron then gives up the extra circling energy on its own, and the spin is left pointing the other way.

The measurement the team wants compares two rates: how fast the positron circles and how fast its spin precesses.

The two differ by about one part in a thousand, and measuring that difference precisely gives the positron's magnetic moment.

The real test comes next

The same lab's 2023 measurement pinned the electron's magnetic moment to about one part in 10 trillion, the most precisely determined property of any elementary particle.

The best positron measurement so far, a 1987 experiment, used a positron circling as an ordinary charged particle rather than hopping between quantum levels.

Protons and their antimatter partners, antiprotons, have been compared to about one part in a billion.

The Standard Model of particle physics - the theory of the known particles and forces - says a positron and an electron should be mirror images. It also can't explain why the universe holds far more matter than antimatter.

"Current theories suggest that there will be no difference in the two measurements. If it turns out they are not equal, that would be the biggest discovery in physics in the last century," Gabrielse said.

A measurement seems possible

The new paper is the trap and the quantum control, not the measurement.

With more work on the setup and the time it takes to pin down its uncertainties, the measurement now seems feasible, the authors wrote.

At the electron's precision, it would test that symmetry more than 30 times more precisely than any test so far with electrons and their relatives.

Until then, the Standard Model's prediction of no difference at all stays a prediction.

The full study was published in the journal Physical Review Letters.

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