Fragility of multi-junction flux qubits against quasiparticle tunneling

Details Fragility of multi-junction flux qubits against quasiparticle tunneling Fragility of multi-junction flux qubits against quasiparticle tunneling

2011-09-13

arxiv.org/abs/1109.2941v1

Phys. Rev. B 85, 144503 (2012)

Physics

Condensed Matter

Mesoscale and Nanoscale Physics

13 pages, 1 figure

Scientific paper

10.1103/PhysRevB.85.144503

We study decoherence in superconducting qubits due to quasiparticle tunneling which is enhanced by two known deviations from the equilibrium BCS theory. The first process corresponds to tunneling of an already existing quasiparticle across the junction. The quasiparticle density is increased, e.g., because of an effective quasiparticle doping of the system. The second process is quasiparticle tunneling by breaking of a Cooper pair. This can happen at typical energies of superconducting qubits if there is an extended quasiparticle density inside the gap. We calculate the induced energy decay and pure dephasing rates in typical qubit designs. Assuming the lowest reported value of the non-equilibrium quasiparticle density in Aluminum, we find for the persistent-current flux qubit decay times of the order of recent measurements. Using the typical sub-gap density of states in Niobium we also reproduce observed decay times in the corresponding Niobium flux qubits.

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