Development and Testing of a Persistent Flux Bias for Qubits

We have realized an on-chip persistent flux bias for our flux based qubit by adding a switchable aluminum shunt to the niobium flux bias coils. This inductive shunt can create a persistent current loop by trapping the bias flux at the superconducting transition of the Al. When the Al is superconducting, the noise spectral density from the flux bias and the damping due to the flux bias should both be reduced by a factor of 100, since the inductance ratio,L_sh/L_p, is 0.1. This reduces one of the sources of decoherence for the qubit. To permit changes to the persistent current in the superconducting loops at mK temperatures, a local heater has been coupled to the Al shunt. This makes it possible to briefly drive the shunt to its normal state to change to flux without heating the entire device.