Physicists have trapped a single particle of antimatter for months while controlling and detecting its individual quantum states, an advance that could enable the most precise comparison yet between matter and antimatter.
A team at Northwestern University created what researchers call a “one-positron quantum cyclotron,” using electromagnetic fields to suspend a positron — the antimatter counterpart of an electron — in an ultra-cold vacuum without allowing it to touch ordinary matter.
The achievement, detailed in a study published in Physical Review Letters last week, doesn’t show that matter and antimatter behave differently but gives researchers a substantially more sensitive tool for looking for such differences.
The researchers say the system could ultimately improve measurements of the positron’s magnetic moment by more than 30 times compared with previous techniques, allowing a stringent test of CPT symmetry, a fundamental principle of modern particle physics.
Cooling antimatter to near absolute zero
Antimatter is notoriously difficult to work with because a particle and its corresponding antiparticle annihilate when they meet. A positron therefore cannot simply be placed inside an ordinary physical container.
The Northwestern system uses a Penning trap, which combines electric and magnetic fields to confine the positively charged particle without physical contact. The experiment operates inside a sealed vacuum enclosure cooled to about 300 millikelvins, or 0.3 kelvin above absolute zero.
The researchers started with positrons produced by the radioactive decay of sodium-22. Those particles can emerge with energies as high as 546 kiloelectronvolts, requiring the team to slow and cool one by more than 10 orders of magnitude in energy before it reaches the conditions needed for the experiment.
Loading the trap is itself a demanding process. According to an analysis published by the American Physical Society, capturing a single usable positron currently takes roughly eight to 11 hours. Once captured, however, a positron can remain confined for months.
The extremely low temperature also allows the particle’s cyclotron motion — the circular motion of a charged particle in a magnetic field — to settle into its lowest quantum energy state.
That level of control is central to the experiment.
Using a magnetic field of 5.6 teslas and microwave radiation near 156 gigahertz, the scientists could deliberately push the positron between discrete energy levels. They were also able to detect individual changes in its cyclotron state and spin without destroying or ejecting the particle.
In effect, the apparatus turns a single positron into an exceptionally well-controlled quantum system that can be repeatedly measured.
A tougher test of the Standard Model
The next goal is to measure the positron’s magnetic moment — a property describing how it behaves like a tiny magnet — and compare it with the corresponding measurement for the electron.
Under CPT symmetry, a foundational feature of quantum field theory, particles and antiparticles are expected to have precisely related fundamental properties. A reproducible discrepancy between the electron and positron would therefore signal physics that cannot be explained by the Standard Model in its current form.
Earlier one-electron quantum cyclotron experiments measured the electron’s magnetic moment to about one part in 10 trillion. Researchers now hope to bring positron measurements closer to that level of precision, with the new system potentially improving the best existing positron measurement by roughly 30-fold.
That precision matters because experimental physicists often search for new physics not by producing entirely new particles, but by finding minute deviations from predictions in systems that are otherwise exceptionally well understood.
The experiment has not detected such a deviation. Its significance is that researchers can now load, cool and preserve a single positron, manipulate its quantum states one at a time and read those states without destroying the particle — capabilities needed before a much more precise magnetic-moment measurement can be attempted.
The larger matter-antimatter mystery
The research also connects to one of cosmology’s biggest unanswered questions: why the observable universe contains overwhelmingly more matter than antimatter.
Current theories indicate that the early universe should have produced matter and antimatter in closely matched quantities. Yet stars, planets and virtually everything observed around us are composed primarily of ordinary matter.
Scientists have already discovered some differences in how matter and antimatter behave, including forms of CP violation in particle decays, but known effects are not sufficient on their own to explain the observed cosmic imbalance.
A highly precise electron-positron comparison provides another way to test the symmetries built into modern particle theory. A disagreement would have consequences extending far beyond the positron experiment, although it would not by itself automatically explain why matter came to dominate the universe.
The team’s next task is to use the trapped positron for high-precision spectroscopy and a new measurement of its magnetic moment. Reaching the experiment’s target precision will require careful control of systematic uncertainties, but the apparatus now provides the long-term stability and quantum-state control needed for a substantially more sensitive comparison between matter and antimatter.
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