Characterization of superconducting devices spanning the charging to Josephson regimes

Charge states in mesoscopic Josephson devices are considered promising candidates for a solid-state implementation of quantum computing. We studied three superconducting single electron transistors (SSETs) fabricated with small Al/AlOx/Al tunnel junctions, which, with a different choice of the construction parameters, can realize both the quantum bit and the readout system for the qubit charge state. We varied the junction capacitance and barrier transparency, making the Josephson energy E-J of the device smaller, comparable and larger than the charging energy E-C: the first case is suitable to build the qubit, while the latter can be suitable for the qubit readout.