On-chip microwave spectroscopy of the energy levels in a nanowire-based Cooper-pair transistor

Semiconductor-superconductor hybrid devices have attracted significant attention due to their prime role in the field of topological quantum computation. However, the interplay of charging and Josephson physics in such hybrid structures has to be understood in detail in order to enable the engineering of advanced devices, such as topological quantum bits. Here we present our experimental work addressing the energy level structure of a Cooper-pair transistor (CPT) formed in an InAs nanowire with an epitaxial aluminium shell. In order to probe the energy spectrum of the CPT, we utilize the inelastic Cooper-pair tunnelling of an on-chip, capacitively coupled split Josephson junction that acts as a broadband microwave generator of up to 90 GHz in frequency. Using this technique, we observe 2e-periodic level transitions, demonstrating a long parity lifetime of the system. By tuning the microwave power level emitted by the split junction, we establish that higher order processes lead to single electron charge transfer through the island, indicating parity poisoning assisted by microwave radiation.