For the first time, Northwestern University physicists have coaxed a single positron — the antimatter counterpart of an electron — into an exquisitely controlled quantum state.
My rough understanding is that the particle was kept in a non-eigenstate (e.g. laser perturbed eigen state or some perturbed state), and the temperature is defined as the /beta derived as the thermal weight there real world state vs the eigen states (see link).
What does near absolute zero mean for a single quantum particle?
Edit:
https://physics.stackexchange.com/a/491213
My rough understanding is that the particle was kept in a non-eigenstate (e.g. laser perturbed eigen state or some perturbed state), and the temperature is defined as the /beta derived as the thermal weight there real world state vs the eigen states (see link).
Slow particle. Better chance of observing it.