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@PHDTHESIS{Spilla:541434,
author = {Spilla, Samuele},
othercontributors = {Splettstößer, Janine and Terhal, Barbara and Napoli, A.},
title = {{C}oherence properties of superconducting flux qubits},
school = {RWTH Aachen},
type = {Dissertation},
address = {Aachen},
reportid = {RWTH-2015-06588},
pages = {1 Online-Ressource (viii, 100 Seiten) : Illustrationen,
Diagramme},
year = {2016},
note = {Cotutelle-Dissertation. - Veröffentlicht auf dem
Publikationsserver der RWTH Aachen University; Dissertation,
RWTH Aachen, 2015. - Dissertation, Università degli Studi
di Palermo, 2015},
abstract = {The research work discussed in this thesis deals with the
study of superconducting Josephson qubits. Superconducting
qubits are solid-state artificial atoms which are based on
lithographically defined Josephson tunnel junctions
properties. When sufficiently cooled, these superconducting
devices exhibit quantized states of charge, flux or junction
phase depending on their design parameters. This allows to
observe coherent evolutions of their states. The results
presented can be divided into two parts. In a first part we
investigate operations of superconducting qubits based on
the quantum coherence in superconducting quantum
interference devices (SQUID). We explain experimental data
which has been observed in a SQUID subjected to fast,
large-amplitude modifications of its effective potential
shape. The motivations for this work come from the fact that
in the past few years there have been attempts to interpret
the supposed quantum behavior of physical systems, such as
Josephson devices, within a classical framework. Moreover,
we analyze the possibility of generating GHZ states, namely
maximally entangled states, in a quantum system made out of
three Josephson qubits. In particular, we investigate the
possible limitations of the GHZ state generation due to
coupling to bosonic baths. In the second part of the thesis
we address a particular cause of decoherence of flux qubits
which has been disregarded until now: thermal gradients,
which can arise due to accidental non equilibrium
quasiparticle distributions. The reason for these
detrimental effects is that heat currents flowing through
Josephson tunnel junctions in response to a temperature
gradient are periodic functions of the phase difference
between the electrodes. The phase dependence of the heat
current comes from Andreev reflection, namely an interplay
between the quasiparticles which carry heat and the
superconducting condensate which is sensitive to the
superconducting phase difference. Generally speaking, the
flux qubit states are characterized by different values of
the phase difference through their Josephson junctions.
Consequently, the phase-dependent thermal current through a
device subject a temperature gradient is related to the
phase-dependent qubit states. We study how the thermal
currents change according to the state of the qubits hence
yielding a measurement of the qubit state. This in turn
leads to an impact of temperature gradient on the dynamics
of the system. We show that flux qubits in the Delft qubit
design can have limitations of the decoherence time to the
order of microseconds as a result of this newly discovered
source of decoherence. In contrast, the fluxonium qubit is
found to be well protected due to its superinductance.},
cin = {135920 / 130000},
ddc = {530},
cid = {$I:(DE-82)135920_20140620$ / $I:(DE-82)130000_20140620$},
typ = {PUB:(DE-HGF)11},
urn = {urn:nbn:de:hbz:82-rwth-2015-065886},
doi = {10.18154/RWTH-2015-06588},
url = {https://publications.rwth-aachen.de/record/541434},
}
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