Northwestern University physicists have trapped a single antimatter particle for months and controlled its quantum states, creating a new tool for testing whether matter and antimatter behave as fundamental theory predicts.
The experiment involves a positron, the positively charged counterpart of an electron. The team calls its device a one-positron quantum cyclotron, describing the controlled motion of one particle held in electric and magnetic fields.
The research was published October 21 in Physical Review Letters. Gerald Gabrielse led the work, with doctoral student Thomas Myers as first author; the authors’ paper describes both the apparatus and measurements establishing control of the particle’s energy and spin states.
A conventional container would defeat the experiment because a positron encountering an electron can annihilate with it. The researchers instead suspend the particle away from the walls inside a near-perfect vacuum, keeping it isolated from ordinary matter.
The trap operates at about 0.3 degrees above absolute zero. Starting with positrons produced by radioactive sodium-22, the team slowed and cooled a particle until it occupied its lowest cyclotron energy state, according to the paper and the university’s account.
Carefully tuned microwaves then changed the particle’s energy and spin. The researchers detected those changes without destroying the positron, providing the control needed for more demanding measurements of its magnetic properties.
The goal is to compare a positron’s magnetic moment with the electron’s at far greater precision. The authors say such a comparison would test CPT symmetry, a principle underlying the Standard Model of particle physics, more stringently than earlier positron measurements.
The achievement is a preparation for that test, rather than evidence that the symmetry has failed. The team has demonstrated the controlled system; precision spectroscopy and attention to experimental errors are still needed before a new electron-positron comparison can establish whether the theory passes.