This feedback mechanism (confusingly called “radiation damping”) amplifies the spin noise by driving the other 9.99999e19 spins in response to the tiny spin-noise signal. This is analogous to the (usually annoying) audio signals produced by improperly arranged audio equipment amplifying random noise
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
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
I will note that ultra sensitive #NMR experiments being built to search for #Axion-like dark matter are expected to be sensitive enough to measure spin noise without the feedback enhancement. (CASPEr, the cosmic axion spin precession experiment)
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....
Okay, I think I can work with this: www.whitehouse.gov/wp-content/u...
Reminder to re-brand all #NMR research as #Quantum
But wait, ~1mL of a 1M solution should give me ~10^20 spins, so measuring spin noise should mean measuring ~10^10 spins. If we could do that, shouldn’t #NMR be capable of measuring (10s of) picomolar concentrations? Why isn’t @brukercorporation.bsky.social selling us 32-bit ADCs?
Hooray, an excuse to go on and on about spin noise! Predicted correctly by Bloch in 1946