Observation of tunneling-assisted highly forbidden single-photon transitions in a Ni$_4$ single-molecule magnet

Abstract: Forbidden transitions between energy levels typically involve violation of selection rules imposed by symmetry and/or conservation laws. A nanomagnet tunneling between up and down states violates angular momentum conservation because of broken rotational symmetry. Here we report observations of highly forbidden transitions between spin states in a Ni$_4$ single-molecule magnet in which a single photon can induce the spin to change by several times $\hbar$, nearly reversing the direction of the spin. These observations are understood as tunneling-assisted transitions that lift the standard $\Delta m = \pm 1$ selection rule for single-photon transitions. These transitions are observed at low applied fields, where tunneling is dominated by the molecule's intrinsic anisotropy and the field acts as a perturbation. Such transitions can be exploited to create macroscopic superposition states that are not typically accessible through single-photon $\Delta m = \pm 1$ transitions.