Physicists have trapped a single particle of antimatter for months and, for the first time, demonstrated that they can control its quantum behavior with extraordinary precision.

The particle is a positron, the antimatter counterpart of an electron. When the two meet, they annihilate each other, making positrons exceedingly difficult to study because, you know, electrons are everything.

Researchers at Northwestern University overcame this problem by suspending a single positron in a vacuum using electric and magnetic fields, then cooling it to nearly absolute zero.

“We actually coaxed this very fast positron to sit there and behave for months at a time,” said Gerald Gabrielse, the Northwestern physicist who led the research.

You hear the word ‘breakthrough’ thrown out a lot, but this really is one. It could allow scientists to compare matter and antimatter more precisely than ever before. Any unexpected difference could challenge our understanding of fundamental physics and offer clues to one of science’s biggest mysteries: why the universe is made almost entirely of matter rather than antimatter, even though the two were supposed to have been produced in equal quantities just moments after the Big Bang. The answer to this question may explain why anything exists at all.

How Do You Build a Container for Something That Cannot Touch Anything?

Some of the equipment used to trap a single positron. Credit: Gabrielse group/Northwestern University

Every familiar particle of matter has an antimatter counterpart. Electrons have positrons, protons have antiprotons, and neutrons have antineutrons. Their masses match those of their ordinary counterparts, but certain properties, including electric charge, are reversed.

Positrons are particularly difficult to study because electrons are everywhere. An encounter between the two usually ends in mutual annihilation, and there goes your experiment — and it’s not like you can just try the next moment again. Antimatter costs an estimated $62.5 trillion to $100 trillion per gram to produce, making it the most expensive substance on Earth.

“If you want to hold on to antimatter, you have to hold it in a container without walls,” Gabrielse told Scientific American.

To accomplish that, Gabrielse and his colleagues, including doctoral student and first author Thomas Myers, devised an elaborate procedure that begins with radioactive sodium-22.

When sodium-22 undergoes radioactive decay, it can release a positron traveling at tremendous speed. Capturing such a particle requires slowing it almost to a standstill without destroying it.

The researchers first passed the positrons through a cold tungsten crystal. Collisions inside the crystal robbed them of energy, although many particles annihilated along the way.

Some of the surviving positrons then picked up an electron, forming an unusual, loosely bound particle pair called positronium. In this highly excited state, the pair could travel together long enough to enter the trap but still far apart not to annihilate each other.

This seemingly counterintuitive step solved an important engineering problem. Because positronium is electrically neutral, it could slip through a tiny opening in the trapping apparatus without being deflected by the surrounding electric fields.

Gerald Gabrielse (top right) and his team in the lab where they trapped a positron particle for a month. Credit: Gerald Gabrielse

Once inside, those fields separated the electron from the positron. The electron escaped, leaving the positron safely confined.

The apparatus, known as a Penning trap, combines a powerful 5.6-tesla magnetic field with electric fields to hold the particle away from any physical surface.

Meanwhile, a specialized refrigerator cools the chamber to just 0.3 degrees above absolute zero, approximately minus 272.85 degrees Celsius. At this temperature, thermal radiation is almost completely eliminated, and the near-perfect vacuum minimizes the risk of collisions.

The entire process is excruciatingly painstaking. According to the authors, loading a single positron currently takes between eight and eleven hours.

But once captured, the particle can remain suspended for months.

Watching a Single Particle Make Quantum Jumps

Scientists have confined antiparticles and even entire antimatter atoms before, so that’s not the novel thing about this work.

What makes the Northwestern experiment different is the degree of control over a solitary positron’s quantum behavior.

Inside the trap, the particle circles in the magnetic field, behaving somewhat like a tiny artificial atom. Its motion can occupy only certain allowed energy levels, much as an electron in an atom can occupy distinct quantum states.

The researchers call this system a one-positron quantum cyclotron.

By delivering carefully tuned microwave pulses, they could make the positron jump from its lowest energy level to the next one. They could also flip its spin, an intrinsic quantum property that determines how the particle responds to a magnetic field.

Most importantly, they could detect these changes without disturbing the quantum 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. How much more fundamental, how much neater can it get?” Gabrielse told New Scientist.

“This has the potential to become one of the most precise measurements of a fundamental constant in physics,” Stefan Ulmer, a physicist at the Japanese research institute RIKEN who was not involved in the experiment, told New Scientist.

A Tiny Difference That Could Challenge Our Understanding of the Universe

All this effort is directed toward one of the most important questions in physics: Is an electron truly identical to its antimatter counterpart in every way that physics predicts?

One of the most important properties researchers want to compare is the particles’ magnetic moment, which describes the strength and orientation of their magnetism.

In 2023, Gabrielse and colleagues reported an extraordinarily precise measurement of the electron’s magnetic moment, achieving an uncertainty of just 0.13 parts per trillion.

Measuring the positron with comparable precision would allow the researchers to compare the two particles much more rigorously than before.

Historically, positron measurements have lagged behind electron measurements. The most precise direct comparison dates to 1987 and found agreement within 2.1 parts per trillion.

According to an independent assessment by physicist Paolo Crivelli of ETH Zurich, the Northwestern apparatus could make a comparison more than 30 times as precise as possible. However, the team must still complete the demanding measurements and account for potential experimental errors.

The stakes for theoretical physics are grand.

The Standard Model, physicists’ remarkably successful description of fundamental particles and three of nature’s four fundamental forces, incorporates a principle known as CPT symmetry.

The principle states that the laws of physics should remain unchanged if three transformations occur together: particles are exchanged for antiparticles, spatial directions are reversed as though viewed through a mirror, and time runs backward.

Among other things, CPT symmetry requires the electron and positron to have magnetic moments that are equal in magnitude and opposite in sign.

Even a tiny discrepancy would challenge some of the deepest foundations of modern particle physics.

And it could be relevant to an even older puzzle. According to our current understanding of the early universe, the Big Bang should have produced matter and antimatter in nearly equal quantities. As they encountered one another, most would have annihilated.

Yet somehow, a tiny excess of matter survived. That remnant eventually became every galaxy, star, planet and living thing we know.

Physicists have proposed mechanisms that might explain this imbalance, but none has been conclusively established by experiments and observations.

Experiments elsewhere are also on the lookout. At CERN, for example, the ALPHA collaboration has spent years studying antihydrogen, the antimatter version of hydrogen. In May 2026, researchers announced a hundredfold improvement in the precision of a key antihydrogen measurement. The results remained consistent with ordinary hydrogen.

So far, matter and antimatter have stubbornly obeyed the expected symmetries wherever these experiments have looked.

Nor has the Northwestern team found such a difference yet. The present achievement is the ability to trap, manipulate and observe a single positron’s quantum states. The decisive magnetic-moment comparison still lies ahead.

Nevertheless, the experiment has opened a new avenue for testing nature at an extraordinary level of precision.

“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.”

The findings were reported in the journal Physical Review Letters.