Bulk and edge states in a three-site Kitaev chain

A chain of quantum dots (QDs) coupled via semiconductor-superconductor hybrid regions can form an artificial Kitaev chain hosting Majorana bound states (MBSs). These zero-energy states are expected to be localised on the edges of the chain, at the outermost QDs. The remaining QDs, comprising the bulk, are predicted to host an excitation gap that protects the MBSs at the edges from local on-site perturbations. We highlight this connection between the bulk and edges in a minimal system, a three-site Kitaev chain in a two-dimensional electron gas. Through direct tunnelling spectroscopy at each site, we show that the appearance of stable zero-bias conductance peaks at the outer QDs is correlated with the presence of an excitation gap in the middle QD. Furthermore, we show that this gap can be controlled by applying a superconducting phase difference between the two hybrid segments, and that the MBSs are robust only when the excitation gap is present. Lastly, control over all system parameters allows us to show close agreement to the Kitaev chain model in a large phase space beyond the sweet spots where localised MBSs arise.