Possible explanations for some of the most perplexing phenomena in our universe, such as dark matter and charge-parity violation in strong interactions, may lie in the existence of axion-like particles and unseen axions that have not yet been found. The well-motivated “axion window” ranges from 0.01 meV to 1 meV, and several recent theories have made the prediction that the masses of axions most likely fall within this range. However, most current laboratory studies and astrophysical observations concentrate their efforts on looking for axion particles that are located outside of the axion window.

The research team led by Professor Peng Xinhua from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences, in collaboration with Professor Dmitry Budker from the Helmholtz Institution in Mainz, used a recently developed spin-based amplifier to constrain hypothetical axions within the axion window, providing a way to explore promising parameter space. This was accomplished by using the spin-based amplifier to explore promising parameter space.

An unusual dipole-dipole interaction is produced whenever fermions trade axions with one another; this interaction is something that researchers in the lab might be able to observe. As two different types of fermions for the purpose of this study, the researchers made use of a sizable collection of polarized rubidium-87 electrons as well as polarized xeon-129 nuclear spins. The exotic signal on the xeon nuclear spins might be generated by the rubidium as a result of the exchange of axions, and then the polarized xeon-129 nuclear spins would be utilized to resonantly search for the signal.

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The researchers demonstrated, in particular, that the long-lived xeon-129 spins work as a quantum preamplifier, which can increase the strength of the exotic signal by a factor that is greater than 40. By employing such a method, they were able to give the most rigorous restrictions possible on the neutron-electron coupling that is mediated by axions for the axion mass ranging from 0.03 meV to 1 meV while still remaining within the axion window.

This study gives a sensitive quantum technique to realize the indirect axion searches with a recently developed spin-based amplifier. This is a huge improvement in sensitivity in a mass region that is theoretically attractive for axions, and it was made possible by the work. The spin-based amplifier scheme is a new implementation that, in addition to expanding the capabilities of spin measurements, can also be used to search for hypothetical particles beyond the Standard Model, such as new spin-1 dark photons, in a resonant manner. This is possible thanks to the scheme’s ability to extend the capabilities of spin measurements.

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