Relevant paragraphs, near figure 4, page 3:

> In order to determine ωz and the number of trapped positrons, the axial oscillation of a positron is driven with oscillating potentials applied to the trap electrodes. The frequency of the drive ωd is swept through resonance with ωz.

> For the center-of-mass axial motion of more than one positron, the damping (and hence the observed signal width) increases in proportion to the number of particles. A confirmation that only one positron is trapped comes from spin flip observations that are discussed presently