Coulomb blockade in open superconducting islands on InAs nanowires

Electrons in closed quantum systems can exhibit Coulomb blockade (CB) oscillations due to charge quantization. Here, we report CB oscillations in aluminum superconducting islands on InAs nanowires in the open regime. The Al island is connected to source/drain leads through two contacts: One is fully transmitting while the other is tuned into the tunneling regime. This device configuration is typical for tunneling spectroscopy where the charging energy is generally considered negligible. The oscillation periods are 2e or 1e, depending on the gate settings. A magnetic field can also induce the 2e to 1e transition. Our result is reminiscent of the "mesoscopic Coulomb blockade"in open quantum dots caused by electron interference.