% IMPORTANT: The following is UTF-8 encoded.  This means that in the presence
% of non-ASCII characters, it will not work with BibTeX 0.99 or older.
% Instead, you should use an up-to-date BibTeX implementation like “bibtex8” or
% “biber”.

@PHDTHESIS{Hosseinkhani:861540,
      author       = {Hosseinkhani, Amin},
      title        = {{N}ormal-{M}etal {Q}uasiparticle {T}raps {F}or
                      {S}uperconducting {Q}ubits: {M}odeling, {O}ptimization, and
                      {P}roximity {E}ffect},
      school       = {RWTH Aachen University},
      type         = {Dissertation},
      publisher    = {RWTH Aachen University},
      reportid     = {FZJ-2019-01995},
      pages        = {150},
      year         = {2018},
      note         = {Dissertation, RWTH Aachen University, 2018},
      abstract     = {Bogoliubov quasiparticle excitations are detrimental for
                      the operation of many superconducting devices. In
                      superconducting qubits, quasiparticles interact with the
                      qubit degree of freedom when tunneling through a Josephson
                      junction, and this interaction can lead to qubit relaxation.
                      At millikelvin temperatures, there is substantial evidence
                      of nonequilibrium quasi- particles. While there is no agreed
                      upon explanation for the origin of these excess
                      quasiparticles, it is nevertheless possible to limit the
                      quasiparticle-induced relaxation by steering quasiparticles
                      away from qubit active elements. In this thesis, we study
                      quasiparticle traps that are formed by a normal-metal in
                      tunnel contact with the superconducting electrode of a
                      qubit. We develop a model to explain how a trap can
                      influence the dynamics of the excess quasiparticles injected
                      in a transmontype qubit. This model makes it possible to
                      find the time it takes to evacuate the injected
                      quasiparticles from the transmon as a function of trap
                      parameters. We show when the trap size is increased, the
                      evacuation time decreases monotonically and saturates at a
                      level that depends on the quasiparticles diffusion constant
                      and the qubit geometry. We find the characteristic trap size
                      needed for the evacuation time to approach the saturation
                      value. It turns out that the bottleneck limiting the
                      trapping rate is the slow quasiparticle energy relaxation
                      inside the normal-metal trap, a quantity that is very hard
                      to control. In order to optimize normal-metal quasiparticle
                      trapping, we study the effects of trap size, number, and
                      placement. These factors become important when the trap size
                      increases beyond the characteristic length. We discuss for
                      some experimentally relevant examples how to shorten the
                      evacuation time of the excess quasiparticle density.
                      Moreover, we show that a trap in the vicinity of a Josephson
                      junction can significantly suppress the steady-state
                      quasiparticle density near that junction and reduce the
                      impact of fluctuations in the generation rate of
                      quasiparticles. When such normal-metal elements are
                      connected to a superconducting material, Cooper- pairs can
                      leak into the normal-metal trap. This modifies the
                      superconductor properties and, in turn, affects the qubit
                      coherence. Using the Usadel formalism, we first revisit the
                      proximity effect in uniform NS bilayers; despite the long
                      history of this problem, we present novel findings for the
                      density of states. We then extend our results to describe a
                      non-uniform system in the vicinity of a trap edge. Using
                      these results together with the previously developed model
                      for the suppression of the quasiparticle density due to the
                      trap, we find in a transmon qubit an optimum trap-junction
                      distance at which the qubit relaxation rate is minimized.
                      This optimum distance, of the order of 4 to 20 coherence
                      lengths, originates from the competition between proximity
                      effect and quasiparticle density suppression. We conclude
                      that the harmful influence of the proximity effect can be
                      avoided so long as the trap is farther away from the
                      junction than this optimum.},
      cin          = {PGI-2},
      cid          = {I:(DE-Juel1)PGI-2-20110106},
      pnm          = {144 - Controlling Collective States (POF3-144)},
      pid          = {G:(DE-HGF)POF3-144},
      typ          = {PUB:(DE-HGF)11},
      doi          = {10.18154/RWTH-2018-226909},
      url          = {https://juser.fz-juelich.de/record/861540},
}