Probing axion mediated fermion–fermion interaction by means of entanglement

We propose a new approach in the investigation and detection of axion and axion–like particles based on the study of the entanglement for two interacting fermions. We study a system made of two identical fermions with spin −1/2, and we show that fermion–fermion interaction mediated by axions leads to a non–zero entanglement between the fermions. An entanglement measurement can reveal the interaction, providing an indirect evidence of the existence of axions. We discuss how the other interactions affect the entanglement, and how to isolate the axion contribution. Particular care is devoted to the analysis of the magnetic dipole–dipole interaction, which turns out to be, apart from axions, the most relevant contribution to the entanglement, and we show that it can be suppressed by setting opportunely the duration of the observation. We also introduce a two–body correlation function, which could be directly observed in an experiment, and plays the role of an entanglement witness.