Northwestern Physicists Hold a Single Positron in a Trap for Months and Control Its Quantum States

Antimatter vanishes the moment it touches ordinary matter. Northwestern University physicists say they have kept one positron, the antimatter twin of the electron, alive in a trap for months.

The team, led by Gerald Gabrielse, reported the result Wednesday, Oct. 21, in Physical Review Letters. Doctoral student Thomas Myers is the paper's first author.

They call the device a "one-positron quantum cyclotron."

How you catch a particle that fast

The positrons come from sodium-22, a radioactive isotope with a half-life of about two and a half years. When an atom decays, it spits out a positron at tremendous speed. ScienceAlert puts the top energy at roughly 546 kiloelectronvolts, about 88 percent of the speed of light.

Slowing one down is the hard part. "Most of the positrons that emerge from the source are not successfully slowed and captured," Gabrielse said. "They end up 'annihilating' with an electron in our apparatus."

The team sends positrons through a sliver of cold tungsten crystal, where collisions strip away their energy. "Every time they bang into something, there's a chance of annihilation, but it doesn't always happen," Gabrielse said. "If they make it to the end, they get popped out."

Electromagnetic fields then steer the survivors into a trap. With the simple, low-intensity source the lab uses, loading one positron takes 8 to 11 hours.

A container without walls

The trap is a Penning trap. Electric and magnetic fields hold the particle in the middle of a near-perfect vacuum, chilled to about 0.3 degrees above absolute zero.

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

At that temperature the positron settles into its lowest cyclotron energy state, the quantum equivalent of the bottom rung of a ladder. The researchers then used tuned microwaves to nudge it up the ladder and flip its spin, watching each change as a tiny shift in the particle's oscillation frequency. The size of those shifts also showed that only one positron was in the trap.

"We actually coaxed this very fast positron to sit there and behave for months at a time," Gabrielse said. Scientific American's account describes isolating the particle for a month and says the method could let them do it indefinitely.

The measurement doesn't destroy the particle, so the team can repeat it.

Outside physicists took notice. Stefan Ulmer of Japan's RIKEN institute, who was not part of the work, told New Scientist that "trapping an antiparticle is extremely difficult." Barbara Latacz of CERN, also not involved, called it a "remarkable achievement" in comments to Nature.

What the trap is for

The target is the positron's magnetic moment, essentially the strength and orientation of its magnetic field. To get it, the team must measure two energies: what it takes to flip the positron's spin, and what it takes to lift it from the ground state to the first excited state.

Physicists already have extremely precise electron numbers. Matching that precision for the positron would allow a direct comparison.

The Standard Model says the two should be near-perfect mirror images, a principle called CPT symmetry. "Current theories suggest that there will be no difference in the two measurements," Gabrielse said. That is the prediction on the table, and the experiment exists to test it.

The team says that with further optimization, a positron measurement matching the electron's precision now looks feasible. According to Gabrielse's group, that would test CPT symmetry more than 30 times more precisely than has been possible.

If the numbers don't match, Gabrielse said, "that would be the biggest discovery in physics in the last century."

The mystery behind it

The Big Bang should have produced equal amounts of matter and antimatter. They would have annihilated, leaving nothing. "There's clearly something wrong with that theory, and that's what we're trying to figure out," Gabrielse said.

The Standard Model, which has passed experiment after experiment with extraordinary precision, has no explanation for why matter won. A measurable difference between electron and positron would be a crack in it, and a clue to the answer.

No one is claiming that difference exists. The Oct. 21 paper reports a trapping and control technique, not a magnetic moment result.

The open question is the measurement itself. The Northwestern team has not published a timeline for the positron's magnetic moment, and until it does, the electron-positron comparison remains a test the apparatus is built to run, not one it has completed.