Physicists Hold a Single Positron in a Quantum Trap for Months

Physicists at Northwestern University have held a single positron—the antimatter counterpart of an electron—in a trap for months and manipulated it one quantum jump at a time. They call the device a “one-positron quantum cyclotron.”

Why positrons are difficult to study

Antimatter is difficult to study because it is destroyed when it comes into contact with ordinary matter. A positron that touches the walls of a container encounters electrons, causing the particles to annihilate.

“If you collide an electron with an antielectron, or positron, they both annihilate,” Professor Gerald Gabrielse says.

“We call it annihilation, and it’s a concept that fires the imagination of science fiction writers.”

Trapping a single antimatter particle

Professor Gabrielse and colleagues at Northwestern University overcame this challenge by suspending positrons in a Penning trap. The device uses electric and magnetic fields to hold the particles in the center of a near-perfect vacuum.

The trap was cooled to about 0.3 degrees above absolute zero. The team produced its antimatter particles using sodium-22, a radioactive isotope whose decay releases highly energetic positrons.

The researchers slowed the positrons and loaded them into the trap. Using a simple, low-intensity light source, this process took between 8 and 11 hours.

They then cooled the particle to its lowest cyclotron energy state—the quantum mechanical equivalent of the bottom rung of an energy ladder.

Manipulating the positron one quantum jump at a time

The scientists used precisely tuned microwaves to move the positron up the energy ladder and flip its spin. They observed each change through small shifts in the frequency at which the particle vibrated.

The size of these frequency shifts also showed that only one positron was present in the trap.

“We cool it down to the quantum ground state,” Professor Gabrielse said.

“In fact, we coaxed this very fast positron to sit there and do things for months at a time.”

Testing the symmetry between matter and antimatter

The Standard Model predicts that electrons and positrons should be perfect mirror images of one another under CPT symmetry.

Comparing the magnetic moments of electrons and positrons provides a powerful test of this principle. A discrepancy between the two measurements would indicate that the current theory is incomplete.

Such a difference could also help explain why the universe contains far more matter than antimatter, even though the Big Bang should have produced equal amounts of both.

“The great mystery of the universe is that the Big Bang should have produced equal amounts of matter and antimatter,” Professor Gabrielse says.

“But if that were true, it would be wiped out and we would be left with nothing. There’s obviously something wrong with that theory, and that’s what we’re trying to figure out.”

“Current theory suggests that there is no difference between the two measurements,” Gabrielse said.

“If it turns out that they are not equal, it will be the greatest discovery in physics of the last century.”

More precise antimatter measurements

With further optimization, the researchers say that positron measurements matching the precision achieved for electrons may be possible.

“This will test the invariance of the fundamental CPT symmetry of the Standard Model more than 30 times more precisely than was possible in the leptonic sector,” the authors say.

The team’s work was published in Physical Review Letters.

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T. G. Myers et al. 2026. “One-Positron Quantum Cyclotron.” Physical Review Letters 137, 173001. DOI: 10.1103/gbkc-6nlj.