Keeping antimatter alive is a difficult business. A positron, the antimatter equivalent of an electron, disappears when it encounters an electron, releasing energy in the process. Even the walls of a laboratory apparatus are therefore a threat. To study one, physicists must keep it suspended in empty space, away from ordinary matter.
At Northwestern University in Illinois, researchers have now managed to hold a single positron for months. More importantly, they have learned to control its quantum behaviour, a feat that could allow scientists to examine the differences between matter and antimatter with far greater precision than before.
The experiment, led by physicist Gerald Gabrielse and detailed in a paper published in Physical Review Letters on 21 October 2026, required an elaborate arrangement of magnetic fields, extreme cooling and an ingenious way of getting the particle into its trap.
The positrons came from radioactive sodium-22. Initially moving too quickly to capture, they were slowed using tungsten before being introduced into an apparatus known as a Penning trap, which uses electric and magnetic fields to confine charged particles without physical contact.
Getting them inside presented an unusual problem. The trap needed to be almost completely enclosed to preserve the delicate conditions required for quantum measurements. A large opening would compromise its performance.
The researchers found a solution by temporarily combining a positron with an electron to form positronium, an electrically neutral particle that could pass through a tiny opening. Once inside, an electric field separated the pair, leaving the positron behind.
The apparatus was cooled to approximately 0.3 kelvin, barely above absolute zero. Under these conditions, the researchers could bring the positron's circular motion into its lowest quantum energy state. Carefully tuned microwaves then allowed them to move it between individual energy levels and observe those changes without destroying the particle.
This degree of control is what makes the experiment important.
Physicists have measured the electron's magnetic properties with extraordinary accuracy, but equivalent measurements of its antimatter counterpart have lagged behind. The new apparatus could narrow that gap considerably.
According to an analysis by the American Physical Society, the technique could improve the precision of positron magnetic-moment measurements by roughly thirtyfold.
Such comparisons test a principle called CPT symmetry, which predicts that particles and their antimatter counterparts should have matching fundamental properties, with certain quantities reversed in sign. Finding a genuine difference would challenge an important foundation of modern physics.
The Northwestern researchers have not found such a difference, and their experiment does not explain why matter dominates the universe. That remains a much larger question.
What they have achieved is a way to keep an exceptionally fragile particle under control long enough to investigate it in unprecedented detail. The next challenge is to turn that control into measurements precise enough to test whether nature treats matter and antimatter exactly as current theory predicts.
