Figure 1: Measuring magnetic forces

Kotler et al.¹ have measured the tiny interaction between two electron spins bound to two strontium ions about 2 micrometres apart. a, The electron spins of both ions can be illustrated by magneticcompass needles, here aligned opposite to each other; blue indicates a south pole and red a north pole. b, By repeatedly flipping the needles’ directions rapidly and simultaneously, the authors could cancel out the interactions of the ions with a fluctuating external magnetic field (not shown). c, This allowed them to measure the magnetic interaction between both needles from the rotation that they undergo in the previous configurations.