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 $2.6×2.6μm2$ $Nb∕AlOx∕Nb$ tunnel junctions. In zero magnetic field, the critical current $Ic∼8μA$ and the subgap resistance $R∼0.3MΩ$⁠. 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 $1∕(100μs)$⁠.