On-chip microwave spectroscopy of an InAs nanowire Cooper-pair transistor

Controlling both the charging and the Josephson energies of superconducting islands is a crucial requirement for the operation and readout of prospective Majorana-based topological qubits. Here, we present the study of Cooper-pair transistors (CPTs) fabricated from InAs nanowires with superconducting Al shell, a promising platform for the physical realization of topological qubits. By utilizing on-chip microwave spectroscopy, we are able to induce excitations between energy levels of the CPT in a parity conserving manner. We map both the charge- and the photon-like excitations of the system, and show that it is possible to tune the CPT from a Coulomb-blockaded to a transmon-like regime by adjusting the system parameters. The quantum system is exhaustively modeled, allowing us to extract the effective charging and Josephson energies of the CPT. In addition, we study the ground state charge occupation of the CPT and attribute the observed temperature dependence of the even-odd parity structure to the presence of a sub-gap state on the superconducting island.