A paper published on October 21, 2026, reports that Northwestern researchers confined one positron in a quantum cyclotron and resolved and controlled its lowest cyclotron and spin states. The paper specifies one month of confinement in this realization; Northwestern’s announcements described the particle as held for “months.” The result establishes a way to control a single positron’s quantum states. A measurement of its magnetic moment and comparison with the electron were still planned, according to Northwestern’s October 22 account. The paper in Physical Review Letters describes the experiment.
What the experiment demonstrated
The apparatus was a cylindrical Penning trap, which uses electric and magnetic fields to confine a charged particle. The team’s trap operated in a 5.6-tesla magnetic field and at about 300 millikelvins, or 0.3 kelvins. The researchers resolved the positron’s lowest cyclotron and spin energy levels, then used tuned microwaves to drive transitions between them. (The paper; the authors’ manuscript)
The published paper describes one month of confinement for this realization. Northwestern’s announcements on October 21 and 22 used the broader description “months.”
How a positron gets into the Penning trap
The loading process starts with radioactive sodium-22, which supplies positrons. A tungsten moderator slows them down. One positron then binds temporarily with an electron to form neutral positronium, which can pass through a 625-micrometer opening in the trap. Inside, an electric field ionizes the positronium: the electron leaves, and the positron is captured. Loading one positron took about 8 to 11 hours in this setup. (The authors’ manuscript; the paper)
How the team identified one positron
The researchers monitored the positron’s axial motion, meaning its oscillation along the trap’s axis. Quantum-nondemolition measurements let them detect changes in the positron’s spin and cyclotron states without changing the measured states themselves. The authors’ manuscript reports an axial-frequency shift of about 3.8 hertz for one positron. For the center-of-mass motion of two positrons, the shift would be half as large. The paper also describes spin-flip observations that confirmed single-particle occupancy. (The authors’ manuscript; the paper)
The measurement Northwestern planned next
A particle’s magnetic moment is a property measured in experiments involving magnetic fields. Northwestern said the team planned to measure the positron’s magnetic moment and compare it with the electron’s. The comparison would test CPT invariance in the lepton sector, a symmetry incorporated into the Standard Model. Northwestern cited an earlier electron magnetic-moment measurement with a precision of about one part in 10 trillion; the new experiment established a platform for a comparable positron measurement, rather than reporting that measurement itself. (The paper; Northwestern’s account)
