Dephasing in fluxonium qubits from coherent quantum phase slips

Phase slips occur at all Josephson junctions (JJs) at a rate dependent on the junction impedance. In superconducting qubits comprised of JJ array superinductors, such as fluxonium, phase slips in the array can lead to decoherence. In particular, phase slip processes at the individual array junctions can coherently interfere, each with an Aharonov-Casher phase dependent on the offset charges of the array islands. These coherent quantum phase slips (CQPS) perturbatively modify the qubit frequency, therefore charge noise on the array islands will lead to dephasing. By varying the impedance of the array junctions, we specifically engineer a set of fluxonium qubits to be sensitive to CQPS-induced dephasing over three orders of magnitude. We characterize the coherence times of these qubits in regimes dominated by CQPS or flux noise and show agreement with theoretical models. A detailed understanding of this dephasing mechanism has implications for improvements to coherence in fluxonium and is relevant for the many novel qubit designs involving JJ arrays. 

 This material is based upon work supported under Air Force Contract No. FA8702-15-D-0001. Any opinions, findings, conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the U.S. government or the U.S. Air Force.