The researchers used a vapor cell containing spatial overlapping nuclear spins of noble gas (e.g., xenon-129) and atomic spins of alkali-mental atoms (e.g., rubidium-87) to establish ultrasensitive quantum sensors for the detection of weak magnetic fields.

For the first time, they found that the nuclear spins can act as a pre-amplifier that effectively enhances a coherently oscillating measured magnetic field by at least two orders of magnitude.

They showcased the capability of the spin-based amplifier to surpass the photon-shot-noise limit of the rubidium magnetometer itself, approaching the spin-projection-noise limit of the latter. This discovery encouraged them to achieve an ultrahigh magnetic sensitivity of femtotesla level, which has a significantly better performance than that of other magnetometers demonstrated with nuclear spins limited to the sensitivity of a few picotesla.

Then, they extended spin amplification in the Floquet system, which can simultaneously enhance and measure multiple magnetic fields with at least one order of magnitude improvement, offering the capability of femtotesla-level measurements. Moreover, they developed a novel "Floquet maser" on this hybrid system of nuclear and atomic spins, which enables femtotesla-level Floquet maser magnetometry for ~mHz ultralow frequency. The achieved magnetic sensitivity reaches ~700fT/Hz^1/2 below 60mHz, which is so far the highest magnetic sensitivity in the millihertz range.

These techniques will enable laboratory-scale "tabletop" experiments to explore the frontiers of fundamental physics. New particles and forces can generate an exotic magnetic field oscillating at its Compton frequency on the nucleus (e.g., xenon), which can be amplified and then detected by this quantum sensor with spin amplification.