Dispersive gate-sensing of a quantum dot coupled to a superconducting island

Combining superconductivity with quantum dots in proximitized, semiconducting nanowires has led to many novel phenomena ranging from π-junctions to Cooper-pair splitters. Currently, these hybrid systems are of interest in the context of Majorana-zero modes (MZMs). In particular, it has been proposed that a topological qubit can be made by coupling a superconducting island hosting MZMs to a dot. However, to date, even trivial superconducting island-dot systems are not yet fully understood.

To increase this understanding, we experimentally study the coupling between a quantum dot and a superconducting island. Dispersive gate-sensing is used to readout because it is directly sensitive to the coupling amplitude between the two systems. We focus on two regimes characterized by their coupling to the leads. For weak, but finite, lead coupling, we identify the relevant charge-transfer processes by comparing a phenomenological model to our data. For a closed system without lead coupling, we can access quasiparticle states allowing us to extract the free energy difference of the island.

Combined, these experiments set the stage for future dot-MZMs experiments. This is necessary for distinguishing dot-trivial superconductor coupling from dot-MZM coupling.