The subgap current in Nb/AlOx/Nb tunnel junctions

We have measured the subgap current at 0.8 K in several hysteretic 1.5×1.6 and 2.6×2.6 μ m2 NbAlOxNb tunnel junctions. In zero magnetic field, the critical current Ic ∼8 μA and the subgap resistance R∼0.3 MΩ. When we apply a magnetic field, Ic can be as low as 1 nA, and R is larger than 1 GΩ. This means that in a superconducting qubit made using Nb tunnel junctions, the decoherence rate from the quasiparticle conductance may be as low as 1 (100 μs). © 2004 American Institute of Physics.