Feel the force

摘要

That electrons have negative charge and repel each other are concepts familiar to most readers. Less well known is that they also possess a magnetic moment associated with their spin, and therefore exert magnetic forces on one another. However, interactions between individual electron spins have hitherto not been measured. This is mainly because they are dwarfed by other effects: for small, atomic-scale separation between electrons, the Pauli exclusion principle, which states that two electrons cannot occupy the same quantum state, and the Coulomb electric interaction dominate; and for large separation, the strength of the magnetic interaction is vastly reduced and typically fully masked by the force that an electron's magnetic moment experiences in an ambient fluctuating magnetic field. On page 376 of this issue, Kotler et al. report how they have succeeded in detecting the minuscule magnetic interaction between two electrons bound to two ions separated by about 2 micrometres, using ideas from the emerging field of quantum sensing.