Interacting and hopping spin qubits in germanium

Abstract

In this thesis, we explore the physics and control protocols enabled by hopping spins in the quantum dot arrays. Our approach consists of experimental studies as well as numerical simulations. In chapter 2 we provide the background information relevant to the work in this thesis, including the theoretical description of spins in germanium quantum dots, as well as the measurement setup used in the experiments. In chapter 3 we study the control protocols for spin-spin exchange interactions, and quantify the individual coupling strength in a configuration of 2×2 array when all the nearest neighboring coupling are turned on. We can tune to a regime of equal coupling strength, in which the four-spin entangled states emerge as the resonating valence bond states. In chapter 4 we model the qubit frequency susceptibility to charge noise and predict the optimal magnetic field orientation for extended qubit coherence time. In chapter 5 we show multi-photon transitions in a two-spin system, and discuss opportunities to use this method for qubit addressability in large qubit array based on shared control architecture. In chapter 6 we demonstrate coherent spin qubit shuttling through quantum dots. We quantify the loss of the quantum information during the process. In chapter 7 we demonstrate high-fidelity baseband control of single qubit and two qubit gates, with extended coherence times by operating at low magnetic field. The single qubit gate is realized based on shuttling between the quantum dots, resembling the original spin qubit proposal by Loss and Divincenzo. The two-qubit CPhase gate achieves average fidelity 99.3% in the randomized benchmarking experiments. In chapter 8 we conclude and provide an outlook on the near future for germanium spin-qubits operations.