Andreev bound states, molecules and fusion

In a sequence of tunneling spectroscopy measurements we show intermediate states in the process of merging two Andreev bound states (ABSs), separated by an in-situ grown quantum dot (QD), previously used to investigate the superconducting proximity effect [1] and magnetic field independent subgap states in semiconducting InAs nanowires (NWs) [2]. The QD couples the two adjacent NW segments carrying individual ABSs, or “Andreev atoms”. For decreasing barrier strengths, these ABSs sequentially hybridize with the central QD, forming different types of “Andreev molecules”, with zero-bias conductance peaks as precursors of the Josephson effect. At even larger gate voltages, we then find gate tunable Josephson currents, suggesting an ABS spanning the complete NW, which one might call a "fused" ABS or “Andreev helium”. Our results illustrate the merging of subgap states and may serve as a guide in future Majorana bound state fusion experiments.

[1] Jünger et al., Commun. Physics 2, 76 (2019)

[2] Jünger et al., Phys. Rev. Lett. 125, 017701 (2020)