Physicists at Northwestern University have trapped a single positron, the antimatter twin of the electron, and held it for months while reading its quantum states one jump at a time.
The work, published in Physical Review Letters on 21 October, describes the first "one-positron quantum cyclotron". Gerald Gabrielse, a professor of physics, led the team; Thomas Myers, a doctoral student in his laboratory, is first author.
Why antimatter is hard to hold
A positron disappears the instant it meets ordinary matter. To study one, it has to be slowed, captured, and kept away from every surface. The team collected positrons produced in radioactive decay, slowed them, and loaded a single particle into a Penning trap: a set of electric and magnetic fields inside a sealed vacuum can.
Loading takes between eight and eleven hours. Once captured, the particle stayed for months, which is what makes repeated measurements possible.
Cold enough to see quantum steps
A dilution refrigerator cooled the trap to about 300 millikelvin, a fraction of a degree above absolute zero. At that temperature the positron's circular motion settles into its lowest quantum level, so each step up or down can be driven with microwaves and detected as a small shift in frequency.
The same technique took the group about 20 years to perfect with a single electron. Doing it with antimatter adds a problem: the vacuum must be good enough that the particle never touches a stray atom, and the trap must be stable for months without adjustment.
The measurement still to come
The electron's magnetic moment has been measured to about one part in ten trillion, among the most precise numbers in physics. Gabrielse now plans the same measurement for the positron, then a head-to-head comparison.
That comparison tests CPT symmetry, the rule that matter and antimatter should behave identically when charge, parity and time are reversed. If the two magnetic moments differ, the difference would point to physics beyond the standard model and could help explain why the universe is made of matter with almost no antimatter left.
The team has not found such a difference yet. For now the result is control: one antiparticle, held still enough to watch, and a measurement platform that did not exist before.