The quantum muon

Abstract

Most muon spin rotation (µSR) experiments are based on the coupling between a muon (a quantum, spin- particle) and a macroscopic magnetic field, either applied externally (as is often the case for experiments on superconductors) or produced internally (due to, for example, the alignment of spins in an ordered magnet). This article will review some experiments which have exploited this, essentially classical, interaction, but then will consider cases in which a more intrinsically quantum mechanical approach is needed. In these cases, one cannot ignore the back reaction of the muon’s effect on the system it is probing. It can be profitable to consider the muon as a qubit, evaluating the decoherence of quantum information injected by the muon into the environmental spin system. Experiments focussed on this approach are underpinned by DFT+µ calculations (density functional theory with an included muon) and give rise to an excellent agreement between theory and experiment and open up new ways of using the muon as a probe.