Eleven hours to load, months to observe
Physicists at Northwestern University have held a single positron — the antimatter twin of the electron — in a trap for months and controlled its quantum state one jump at a time. The team calls the device a "one-positron quantum cyclotron." Their results appeared in Physical Review Letters on 21 October.
Getting the particle into place is slow work. Loading takes between eight and eleven hours, because the trap has to capture a positron from a source and cool it without letting it touch ordinary matter, where it would annihilate in a flash of gamma rays. Once captured, it sits at about 0.3 kelvin in a near-perfect vacuum, suspended by combined electric and magnetic fields.
Why antimatter is hard to hold
In a cryogenic Penning trap the positron's motion is quantised. The team resolved its lowest cyclotron and spin states and used microwaves to raise energy levels and flip the spin, watching for tiny frequency shifts. Thomas Myers, a doctoral student in Gerald Gabrielse's laboratory, is the first author. Gabrielse's group used the same approach with a single electron to measure the electron's magnetic moment to about one part in 10 trillion, among the most precise measurements ever made of a particle.
The measurement still to come
That comparison is the point. The Standard Model predicts that a particle and its antimatter twin should have magnetic moments equal in size and opposite in sign. Any measurable difference would break a symmetry called CPT and could help explain why the universe contains so little antimatter. The new trap makes the test possible but has not performed it yet. Independent physicists describe the platform as a powerful probe rather than a finished result.
The next step is to repeat the electron measurement with the positron and set the two numbers side by side, with a precision that would either confirm the standard picture or open a crack in it.