Intermediate states in Andreev bound state fusion

Hybridization is one of the most fundamental quantum mechanical phenomena, with the text book example of binding two hydrogen atoms in a hydrogen molecule. Here we report tunnel spectroscopy experiments illustrating the hybridization of another type of discrete quantum states, namely of superconducting subgap states that form in segments of a semiconducting nanowire in contact with superconducting reservoirs. We discuss a collection of intermediate states with unique (tunnel) spectroscopic fingerprints in the process of merging well-known individual bound states, hybridized by a central quantum dot and eventually coherently linking the reservoirs, carrying a Josephson current. These coupled and fused Andreev bound states can be seen as superconducting analogues to atomic and molecular single electron states in nature, and explain a variety of recent bound state spectra, with specific fingerprints that will have to be winnowed in future Majorana fusion experiments.