Gianluigi Catelani, Peter Grünberg Institute, Germany
Superconducting qubits based on Josephson junctions are a promising
platform for quantum computation, reaching quality factors of over one
million. Such high quality factors enable the investigation of
decoherence mechanisms with high accuracy. An intrinsic decoherence
process originates from the coupling between the qubit degree of freedom
and the quasiparticles that tunnel across Josephson junctions. In this
talk I will review the general theory of quasiparticle effects, valid
both for equilibrium and non-equilibrium quasiparticles [1-3].
In a transmon qubit, tunnelling of a single quasiparticle is associated
with a change in parity; I will discuss the theory of the parity
switching rate in single-junction transmons [4], compare it with recent
measurements [5], and comment on the implications for the dephasing rate.
In qubits that can be tuned by magnetic flux, such as the ³split²
transmon and the fluxonium, the quasiparticle-induced decoherence rate
depends on the flux; as experimentally shown in fluxonium [6], this
provides a way to differentiate quasiparticle tunnelling from other
sources of decoherence.
[1] G. Catelani, J. Koch, L. Frunzio, R. J. Schoelkopf, M. H. Devoret,
and L. I. Glazman, Phys. Rev. Lett. 106, 077002 (2011)
[2] G. Catelani, R. J. Schoelkopf, M. H. Devoret, and L. I. Glazman,
Phys. Rev. B 84, 064517 (2011)
[3] G. Catelani, S. E. Nigg, S. M. Girvin, R. J. Schoelkopf, and L. I.
Glazman, Phys. Rev. B 86, 184514 (2012)
[4] G. Catelani (unpublished, arXiv:1401.5575)
[5] D. Ristè, C. C. Bultnik, M. J. Tiggelman, R. N. Schouten, K. W.
Lehnert, and L. DiCarlo, Nature Communications 4, 1913 (2013)
[6] I. M. Pop, K. Geerlings, G. Catelani, R. J. Schoelkopf, L. I.
Glazman, and M. H. Devoret (Nature, to appear)