Quantum Dots for Conventional and Topological Qubits

Abstract

This thesis presents a series of quantum dot studies, performed with an eye towards improved conventional and topological qubits. Chapters 1-3 focus on improved conventional (spin) qubits; Chapters 4-6 focus on the topological Majorana qubits. Chapter 1 presents the first investigation of Coulomb peak height distributions in a spin-orbit coupled quantum dot, realized in a Ge/Si nanowire. Strong spin-orbit coupling in this hole-gas system leads to antilocalization of Coulomb blockade peaks, consistent with theory. In particular, the peak height distribution has its maximum away from zero at zero magnetic field, with an average that decreases with increasing field. Magnetoconductance in the open-wire regime places a bound on the spin-orbit length (lso < 20 nm), consistent with values extracted in the Coulomb blockade regime (lso < 25 nm). Chapters 2 & 3 demonstrate operation of improved spin qubits. Chapter 2 continues the investigation of Ge/Si nanowires, demonstrating a qubit with tenfold-improved dephasing time compared to the standard GaAs case. e combination of long dephasing time and strong spin-orbit coupling suggests that Ge/Si nanowires are promising for a spin-orbit qubit. In Chap. 3, multi-electron spin qubits are operated in GaAs, and improved resilience to charge noise is found compared to the single-electron case. Chapters 4 & 5, present a series of studies on composite superconductor/semiconductor Al/InAs quantum dots. Detailed study of transport cycles and Coulomb blockade peak spacings in zero magnetic field are presented in Chap. 4, and the parity lifetime of a bound state in the nanowire is inferred to exceed 10 milliseconds. Next, in Chap. 5, finite magnetic field behavior is investigated while varying quantum dot length. Coulomb peak spacings are consistent with the emergence of Majorana modes in the quantum dot. The robustness of Majorana modes to magnetic-field perturbations is measured, and is found to be exponential with increasing nanowire length. Coulomb peak heights are also investigated, and show signatures of electron teleportation by Majorana fermions. Finally, Chap. 6 outlines some schemes to create topological Majorana qubits. Using experimental techniques similar to those in Chap.’s 2 & 3, it may be possible to demonstrate Majorana initialization, readout, and fusion rules.