Northwestern team realizes first one-positron quantum cyclotron, enabling sharper antimatter tests

Northwestern University physicists say they have built the first “one-positron quantum cyclotron,” a key experimental milestone that could enable much sharper comparisons between matter and antimatter than earlier positron studies allowed.

The result is described in a paper, “One-Positron Quantum Cyclotron,” by T. G. Myers, L. Soucy, B. A. D. Sukra, B. Sinha and G. Gabrielse of Northwestern’s Center for Fundamental Physics in the Department of Physics & Astronomy. The manuscript, posted on arXiv as 2605.08147 and last revised Aug. 21, appears on the Physical Review Letters accepted papers list with an acceptance date of Aug. 19.

The advance is important because it establishes the experimental platform needed for a more precise measurement of the positron’s magnetic moment, often written as g/2. That comparison between the positron and the electron — its matter counterpart — is a direct test of CPT invariance, a foundational symmetry that says matter and antimatter should obey the same underlying laws. The paper does not report that next-generation positron magnetic-moment result yet. It reports the successful trapping, control and quantum-state readout needed to pursue it.

In the abstract, the authors write: “A one-positron quantum cyclotron is realized with a single positron suspended indefinitely in the magnetic field of a Penning trap.” In the body of the paper, they are more specific that a single positron has been trapped for months, a very long confinement time for an antimatter particle.

A Penning trap uses electric and magnetic fields to hold a charged particle in place. In this case, the team confined one positron in a 5.6-tesla magnetic field produced by a persistent superconducting solenoid operating at 4.2 kelvin. The trap electrodes and sealed vacuum enclosure were cooled to about 300 millikelvin using a dilution refrigerator, creating the ultracold, quiet environment needed to resolve the particle’s quantum behavior.

The central evidence that this is a true quantum-cyclotron system, rather than simply a trapped positron, is the reported quantum nondemolition detection of the positron’s cyclotron and spin states. In plain terms, the researchers say they could detect single-quantum jumps in the positron’s circular motion and flips of its intrinsic spin without destroying the state they were measuring.

A major technical hurdle was keeping those quantum states alive long enough to observe and control them. The team says the trap acted as a low-loss microwave cavity that suppressed spontaneous emission. Without that cavity effect, the positron’s single-quantum cyclotron lifetime would have been about 83 milliseconds, the paper says. With it, the lifetime stretched to about 4 seconds.

The positrons came from a sealed sodium-22 source. Using the loading method described in the paper, capturing a single positron typically took about eight to 11 hours.

The broader significance comes from what this setup makes possible next. Gabrielse and collaborators previously used a single-electron quantum cyclotron to make extremely precise measurements of the electron magnetic moment. Extending that quantum-trap approach to a positron matters because earlier positron measurements relied on classical cyclotron motion, which limited precision.

If the Northwestern team can carry through to a new positron magnetic-moment measurement using this apparatus, the authors say the resulting electron-positron comparison could improve the relevant CPT test by more than a factor of 30 over previous limits.

For now, the milestone is the apparatus itself. As the authors write in their summary, “In summary, a one-positron quantum cyclotron is realized for the first time.”