Keeping antimatter around is notoriously hard, because the moment it touches ordinary matter both vanish in a flash of energy. Physicists at Northwestern University near Chicago have nevertheless held one positron, the electron’s antimatter twin, in a trap for months and chilled it into its lowest quantum state. The work was published this Wednesday in the journal Physical Review Letters.
The particle begins life as a by-product of radioactive sodium-22 and leaves its source far too fast to catch. The team slowed a stream of them by firing them through a tungsten crystal, losing many along the way, and steered the most sluggish survivors into a vacuum chamber. There, electric fields and a 5.6-tesla magnet keep the positron floating in the middle, never brushing a wall, at roughly 300 millikelvin.
Using carefully tuned microwaves, the researchers can now nudge the particle up a single energy level and watch each quantum jump as it happens. Team leader Gerald Gabrielse describes his device as a quantum cyclotron holding just one positron, a kind of artificial atom whose levels can be steered at will. Any worries about safety are misplaced, he noted, since there is far too little antimatter for an explosion.
What the group really wants is a head-to-head comparison. For the ordinary electron, the same lab has already measured how strongly it behaves like a tiny magnet, with an uncertainty of roughly a ten-trillionth. Repeating that measurement with the positron would test one of the cleanest predictions of the standard model, which says the two numbers should be identical. Spotting a gap between the two would turn physics upside down, Gabrielse told Nature, and might hint at why the cosmos is built almost entirely of matter.
Colleagues in Europe are impressed. Barbara Maria Latacz of CERN called the result a “remarkable achievement”, while Stefan Ulmer of Heinrich Heine University Düsseldorf said he was keen to see the first precision results.
