The Signal
Northwestern Team Traps One Positron in a Quantum Cyclotron
Northwestern University researchers report a device that confines a single positron and resolves its lowest cyclotron and spin energy levels. The demonstration sets up a future quantum measurement of the positron’s magnetic moment, but does
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“A one-positron quantum cyclotron is realized with a single positron suspended indefinitely in the magnetic field of a Penning trap.”
The authors, T. G. Myers and colleagues at Northwestern University, write in Physical Review Letters.
Northwestern University researchers report the first one-positron quantum cyclotron, a device that confines a single antimatter particle while resolving its lowest cyclotron and spin energy levels. The paper appeared in Physical Review Letters on October 21, 2026. The team has not yet measured the positron’s magnetic moment; it has demonstrated the control and readout needed to attempt that measurement.
A narrow entrance into a cold Penning trap
The apparatus holds the positron in a Penning trap cooled to about 300 millikelvins inside a sealed vacuum enclosure. A superconducting solenoid supplies a 5.6-tesla magnetic field, separating the particle’s energy levels. At this temperature, thermal photons are scarce enough that the positron can stay in its lowest cyclotron state until researchers drive it into an excited state.
Getting the particle into the trap without compromising the measurement posed a particular engineering problem. The electrodes also form a microwave cavity that suppresses spontaneous emission, giving researchers time to detect an excited state before it decays. A large opening would weaken that cavity. The team instead used positronium—a neutral pairing of a positron and an electron—to carry the positron through a 640-micrometer hole; inside the trap, an electric field strips away the electron and captures the positron.
Hours to load, months to store
The positrons come from radioactive sodium-22. After a tungsten moderator slows them, the experiment’s weak 18-microcurie source supplies particles that can enter the trap by the positronium route. Loading one positron takes eight to 11 hours, a slow rate the authors say is workable once the trap is tuned: they report keeping a single positron confined for months.
The trap’s magnetic field sets the cyclotron frequency at about 156 gigahertz. Without the cavity, an excited cyclotron state would decay in roughly 83 milliseconds; in this apparatus, the lifetime reaches about four seconds. That extra time lets the researchers read out the particle’s state without changing it, using a magnetic-field gradient that shifts the positron’s axial oscillation frequency.
Quantum jumps show the particle is alone
Researchers applied microwaves near the cyclotron frequency to drive transitions between energy levels, then tracked the resulting frequency changes. Each cyclotron quantum shifts the axial oscillation by about 3.8 hertz. The paper reports observing single-quantum excitations and decays, as well as instances where the positron gained two quanta in quick succession before emitting them.
Measurements of spin flips supplied another check that the signal came from one positron. The authors say the observed frequency shift matches the expected single-particle value; a two-positron signal would produce half that shift. The readout is quantum nondemolition: it reveals the cyclotron and spin states without disturbing them.
The magnetic-moment measurement still lies ahead
Resolving those states provides the ingredients for quantum-jump spectroscopy, which can determine a particle’s magnetic moment from the frequencies associated with its cyclotron motion and spin. The electron’s magnetic moment has already been measured to about one part in 1013 using a one-electron quantum cyclotron. The Northwestern team says its positron apparatus could reach that precision after further optimization and a fuller accounting of uncertainties.
That comparison matters because CPT symmetry requires a particle and its antiparticle to have equal magnetic-moment magnitudes. The authors say a positron measurement at the electron’s precision would test CPT in the lepton sector more than 30 times more precisely than previous tests. For now, the result is the experimental platform: one positron, held for months, with individual cyclotron and spin transitions resolved—not yet a new value for the positron’s magnetic moment.