Non-exponential energy decay and quasi-particle fluctuations in a superconducting flux qubit

We measure pronounced non-exponential energy relaxation in a superconducting flux qubit, observing a decay function that exhibits a fast initial decay followed by a much slower decay for long times. When applying a sequence of pi pulses to the qubit and measuring the decay after the last pi pulse, we observe strong modifications to the decay function, including a slow-down of the fast initial decay and a three-fold increase of the 1/e-time. If we attribute the non-exponential decay to quasiparticle number fluctuations, we speculate that the improvements in T1 are due to a qubit-mediated shuffling of quasiparticles between the metallic islands of the device, which will eventually pump them away from the Josephson junctions to a larger ground plane where their contribution to qubit energy relaxation become negligible.