Thin-film aluminium for superconducting qubits

We study single-Cooper-pair-transistors in which the island is made from an ultra-thin (10nm) aluminum film. The value of the superconducting gap (2Delta) changes dramatically with film thickness therefore allowing different superconducting gaps for the islands and leads simply by using different film thicknesses. Devices engineered in this way can be made less susceptible to having a single quasiparticle trapped on the island (often known as the quasiparticle poisoning problem). This is an advantage for charge-based superconducting qubits where quasiparticle poisoning is a source of decoherence. We perform radio-frequency measurements and observe 2e-periodic supercurrent peaks as a function of gate charge. With the device biased on a supercurrent oscillation, a fast random-telegraph signal due to individual quasiparticle poisoning events is observed. By analyzing the statistics of the poisoning and un-poisoning events we determine the energetics and thermal activation of these processes