A single positron, held in a vacuum chamber for months and made to jump between quantum states on command, is no longer a thought experiment. It is an apparatus at Northwestern University, described in Physical Review Letters.

Positrons are the antimatter counterpart of electrons: same mass, opposite charge. They are hard to study because electrons are everywhere. When the two meet they annihilate, and the experiment is over. Antimatter is reckoned the most expensive substance on Earth, at an estimated $62.5 trillion to $100 trillion per gram.

"If you want to hold on to antimatter, you have to hold it in a container without walls," Gerald Gabrielse, who led the work, told Scientific American. His group — with doctoral student and first author Thomas Myers — has built one.

How you get one in

The apparatus starts with radioactive sodium-22, which emits positrons at high energy. These pass through a cold tungsten crystal, where collisions bleed off energy; many annihilate on the way. Some survivors capture an electron and form positronium — a loosely bound pair in a highly excited state. The step looks counterintuitive and solves an engineering problem: positronium is electrically neutral, so it slips through a tiny opening into the trap without being deflected by the surrounding fields.

Inside, those fields strip the electron away and leave the positron. The device is a Penning trap, combining a 5.6-tesla magnetic field with electric fields to keep the particle away from any physical surface. A cryostat holds the chamber at 0.3 kelvin above absolute zero, about minus 272.85 °C, which nearly eliminates thermal radiation and lets a near-perfect vacuum suppress collisions.

The cost is time. Loading a single positron currently takes eight to eleven hours. After that it stays for months.

What is actually new

Antiparticles, and whole antihydrogen atoms, have been confined before — CERN's ALPHA collaboration has worked on antihydrogen for years and in May 2026 reported a hundredfold improvement in the precision of a key measurement. What is new in Evanston is the degree of control over one positron's quantum behaviour.

In the magnetic field the particle circles like a tiny artificial atom, its motion restricted to allowed energy levels. The researchers call it a one-positron quantum cyclotron. Carefully tuned microwave pulses moved it from its lowest level to the next and flipped its spin, and — the hard part — the team could detect those changes without disturbing the states they wanted to measure. "It's a real kick to stand in my lab and watch a positron making transitions — quantum jumps we call them — between its lowest energy states," Gabrielse told New Scientist.

What it is for

All of it points at one question: is the positron exactly the electron's mirror in every property physics predicts? The decisive property is the magnetic moment. In 2023 Gabrielse's group measured the electron's to an uncertainty of 0.13 parts per trillion. The most precise direct comparison with the positron dates to 1987 and agreed to within 2.1 parts per trillion.

Physicist Paolo Crivelli of ETH Zurich, writing an independent assessment for APS Physics, estimates the new apparatus could make a comparison more than 30 times as precise. The team still has to make the measurement and account for systematic errors.

The stakes are CPT symmetry: the laws of physics should be unchanged if you swap particles for antiparticles, mirror space and run time backwards. That requires the electron and positron magnetic moments to be equal in magnitude and opposite in sign. Even a tiny discrepancy would unsettle the foundations of particle physics — and could bear on an older puzzle, since the Big Bang should have made matter and antimatter in nearly equal quantities, yet a slight excess of matter survived and became galaxies, stars and us. No proposed mechanism has been confirmed experimentally.

So far matter and antimatter have obeyed the expected symmetries everywhere experiments have looked, and Northwestern has found no discrepancy either. "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."

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