Scientists Trapped a Single Antimatter Particle for Months — Here's Why It Matters

Antimatter sounds like science fiction. Touch it with ordinary matter and both disappear in a flash of energy. So how do you hold on to a piece of it, and keep it for months?

Physicists at Northwestern University in the United States have just shown how. In a study published on 21 October 2026 in the journal Physical Review Letters, a team led by Gerald Gabrielse trapped a single positron, the antimatter twin of the electron, and controlled it in its lowest quantum energy state.

First, what is a positron?

Almost everything around us is made of three particles: electrons, protons and neutrons. Each one has an antimatter partner. The electron's partner is the positron. It has the same mass but the opposite electric charge.

When an electron and a positron meet, they annihilate: both vanish and turn into energy. Gabrielse's team points out that this is why you cannot simply put a positron in a normal box. The moment it touches the wall, it is gone. The team also stresses that this is not dangerous, because the amount of antimatter involved is far too small to cause any explosion.

A container with no walls

The solution is a device called a Penning trap. It uses carefully shaped electric and magnetic fields to hold a charged particle floating in empty space, so it never touches anything. According to Northwestern, the positron in Gabrielse's lab has stayed trapped like this for months.

The positron comes from the radioactive decay of sodium-22. It is born moving very fast. An expert commentary in the APS magazine Physics, written by Paolo Crivelli of ETH Zurich, notes that these positrons can start with energies up to 546 kilo-electron-volts, and must be slowed by more than ten orders of magnitude before one can be captured.

Sneaking the positron in "undercover"

Getting the positron into the trap was its own puzzle. The trap is almost completely closed, and a big opening would spoil it. Crivelli describes the clever trick the team used.

First, the positron grabs an electron and forms a short-lived, electrically neutral "atom" called positronium. Because it has no charge, this atom can drift through a tiny hole only 625 micrometres wide. Once inside, the trap's electric field pulls the atom apart: the electron escapes, and the positron stays behind.

Loading a single positron this way currently takes about 8 to 11 hours. After that, it can be kept for months.

Colder than outer space

Inside, the trap is cooled to about 300 millikelvin, just 0.3 degrees above absolute zero, using a special refrigerator. The magnetic field is 5.6 tesla, according to the research paper.

At that temperature, the positron settles into its quantum ground state, its lowest possible energy level. The team then uses precisely tuned microwaves to add tiny, exact amounts of energy and watch it jump between levels, without destroying it. Crivelli explains that each jump shifts the particle's up-and-down wobble by about 3.8 hertz, which is how researchers can "see" a single quantum jump or a flip of the particle's spin. The paper notes that this exact shift also confirms they are watching one positron, not two.

Northwestern describes the result as a "one-positron quantum cyclotron": the trapped positron behaves like an artificial atom whose energy levels scientists can control.

The real goal: a mirror test for the universe

Why go to all this trouble? Because of one of the biggest mysteries in science.

According to the Standard Model of particle physics, electrons and positrons should be near-perfect mirror images. Their magnetic strength should match exactly. This idea is tied to a deep rule called CPT symmetry.

But there is a problem. Gabrielse explained to Northwestern that the Big Bang should have made equal amounts of matter and antimatter, which would have wiped each other out, leaving nothing. Yet here we are, in a universe made almost entirely of matter. Something in our theories is missing.

Gabrielse's group previously measured the electron's magnetic strength to about one part in 10 trillion. Now they plan to make the same kind of measurement on the positron and compare the two. Crivelli writes that the new setup opens a path to about a 30-fold improvement in precision for the positron, and notes the best electron–positron comparison so far dates back to 1987.

Gabrielse told Northwestern that current theories predict no difference, but if the two results do not match, it would be the biggest discovery in physics in the last century.

Not the finish line yet

Crivelli is careful to note that this experiment does not yet deliver the high-precision measurement itself. That will need detailed further work. He also mentions a different approach being developed at the Max Planck Institute for Nuclear Physics in Germany, so two teams are now racing toward the same test from different directions.

The Globitz take

One particle. Held floating in nothing, colder than deep space, for months. It sounds tiny, but it could help answer one of the biggest questions there is: why does anything exist at all?