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@PHDTHESIS{Botzem:836590,
      author       = {Botzem, Tim},
      title        = {{C}oherence and high fidelity control of two-electron spin
                      qubits in {G}a{A}s quantum dots},
      school       = {RWTH Aachen},
      type         = {Dr.},
      publisher    = {RWTH Aachen University},
      reportid     = {FZJ-2017-05665},
      pages        = {150},
      year         = {2017},
      note         = {http://publications.rwth-aachen.de/record/689507Identification
                      nr: REPORT NUMBER: RWTH-2017-04410; RWTH Aachen, Diss.,
                      2017},
      abstract     = {Electron spin qubits confined in GaAs quantum dots are
                      among the most established and well understood qubit
                      systems. Long coherence times due to their weak
                      interactionwith the environment and the electrical
                      tunability of the semiconductor quantum dot have allowed
                      GaAs-based spin qubits to play a central role in
                      demonstrating the keyoperations of semiconductor spin
                      qubits, such as initialization, read-out, universal control
                      and two-qubit gates. Furthermore, spins confined in
                      semiconductor nanostructuresprovide a solid-state approach
                      to quantum computation which leverages current, well
                      established semiconductor production technology for device
                      fabrication and potential scalability.However, the
                      interaction with nuclear spins in the GaAs host material
                      complicates not only the preservation of qubit coherence,
                      but also the precise control of the electronspins. As both
                      these properties, the coherence time and the fidelity of
                      gate operations, play a crucial role as prerequisites for
                      quantum computing, the focus of this thesis areexperiments
                      addressing these challenges on the basis of two-electron
                      spin qubits. Interesting effects arise from the quadrupolar
                      interaction of nuclear spins with electric field gradients.
                      We show experimentally that quadrupolar broadening of the
                      nuclear Larmor precession reduces electron spin coherence
                      via faster decorrelation of transversenuclear fields.
                      However, this effect disappears for appropriate field
                      directions. Furthermore, we observe an additional modulation
                      of coherence attributed to an anisotropicelectronic
                      g-tensor. These results complete our understanding of
                      dephasing in gated quantum dots and point to mitigation
                      strategies. A key requirement for quantum computation are
                      high-fidelity single qubit operations, which so far have not
                      been demonstrated for encoded qubits in GaAs. Here, we
                      realize such accurate operations by iteratively tuning of
                      the all-electrical control pulses. Using randomized
                      benchmarking, a well established characterization method, we
                      find anaverage gate fidelity of F = (98.5 ± 0.1) $\%$ and
                      determine the sum of gate leakage out of and back into the
                      computational subspace to be L = (0.4 ± 0.1) $\%.$ These
                      results demonstrate that high fidelity gates can be realized
                      even in the presence of nuclear spins as existent in all
                      III-V semiconductors.The potential of a feedback mechanism
                      based on electric dipole spin resonance for narrowing the
                      nuclear hyperfine field and its effectiveness for extending
                      qubit coherencetime is investigated in a last experiment.
                      Compared to a previously developed feedback mechanism, this
                      polarization scheme promises higher and more stable pump
                      rates andthe ability to set local magnetic fields in each
                      quantum dot individually},
      cin          = {PGI-11},
      cid          = {I:(DE-Juel1)PGI-11-20170113},
      pnm          = {144 - Controlling Collective States (POF3-144)},
      pid          = {G:(DE-HGF)POF3-144},
      typ          = {PUB:(DE-HGF)11},
      doi          = {10.18154/RWTH-2017-04410},
      url          = {https://juser.fz-juelich.de/record/836590},
}