Coherence and high fidelity control of two-electron spin qubits in GaAs quantum dots

  • Kohärenz und hochpräzise Kontrolle von Zweielektronen-Spinqubits in GaAs Quantenpunkten

Botzem, Tim; Bluhm, Jörg Hendrik (Thesis advisor); Kuemmeth, Ferdinand (Thesis advisor)

Aachen (2017)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2017


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