Turns out inductive (“normal”) NMR can only measure spin noise because it is amplified by radiation damping. Because NMR probes are high-Q tuned resonant circuits, any small nuclear magnetization signal induces a small current in the circuit, and this current then acts on all the spins in the sample
So inductive #NMR can indeed measure the signal from 10^10 spins, but only if there are 10^20 identical ones around to help amplify the signal
In fact, it will be hard to distinguish dark-matter-induced signals from spin noise, so this is expected to be the eventual sensitivity limit for such measurements. doi.org/10.1088/2058....