000861540 001__ 861540
000861540 005__ 20210130000844.0
000861540 0247_ $$2doi$$a10.18154/RWTH-2018-226909
000861540 0247_ $$2Handle$$a2128/21875
000861540 037__ $$aFZJ-2019-01995
000861540 041__ $$aen
000861540 1001_ $$0P:(DE-Juel1)164373$$aHosseinkhani, Amin$$b0$$eCorresponding author
000861540 245__ $$aNormal-Metal Quasiparticle Traps For Superconducting Qubits: Modeling, Optimization, and Proximity Effect$$f - 2018-03-01
000861540 260__ $$bRWTH Aachen University$$c2018
000861540 300__ $$a150
000861540 3367_ $$2DataCite$$aOutput Types/Dissertation
000861540 3367_ $$2ORCID$$aDISSERTATION
000861540 3367_ $$2BibTeX$$aPHDTHESIS
000861540 3367_ $$02$$2EndNote$$aThesis
000861540 3367_ $$0PUB:(DE-HGF)11$$2PUB:(DE-HGF)$$aDissertation / PhD Thesis$$bphd$$mphd$$s1553088126_27382
000861540 3367_ $$2DRIVER$$adoctoralThesis
000861540 502__ $$aDissertation, RWTH Aachen University, 2018$$bDissertation$$cRWTH Aachen University$$d2018
000861540 520__ $$aBogoliubov 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.
000861540 536__ $$0G:(DE-HGF)POF3-144$$a144 - Controlling Collective States (POF3-144)$$cPOF3-144$$fPOF III$$x0
000861540 588__ $$aDataset connected to DataCite
000861540 773__ $$a10.18154/RWTH-2018-226909
000861540 8564_ $$uhttps://juser.fz-juelich.de/record/861540/files/730515.pdf$$yOpenAccess
000861540 909CO $$ooai:juser.fz-juelich.de:861540$$pdnbdelivery$$pdriver$$pVDB$$popen_access$$popenaire
000861540 9101_ $$0I:(DE-588b)5008462-8$$6P:(DE-Juel1)164373$$aForschungszentrum Jülich$$b0$$kFZJ
000861540 9131_ $$0G:(DE-HGF)POF3-144$$1G:(DE-HGF)POF3-140$$2G:(DE-HGF)POF3-100$$3G:(DE-HGF)POF3$$4G:(DE-HGF)POF$$aDE-HGF$$bEnergie$$lFuture Information Technology - Fundamentals, Novel Concepts and Energy Efficiency (FIT)$$vControlling Collective States$$x0
000861540 9141_ $$y2019
000861540 915__ $$0StatID:(DE-HGF)0510$$2StatID$$aOpenAccess
000861540 920__ $$lyes
000861540 9201_ $$0I:(DE-Juel1)PGI-2-20110106$$kPGI-2$$lTheoretische Nanoelektronik$$x0
000861540 980__ $$aphd
000861540 980__ $$aVDB
000861540 980__ $$aUNRESTRICTED
000861540 980__ $$aI:(DE-Juel1)PGI-2-20110106
000861540 9801_ $$aFullTexts