Spin and Charge Qubits in SiliconSilicon Germanium Quantum dots

In this thesis, we study the spin and charge properties of a Si/SiGe double dot system and demonstrate coherent quantum control of few-electron quantum states. We use a pulsed magneto-spectroscopy method to measure the excited state spectrum of a Si/SiGe quantum dot containing two valence electrons. We extract the singlet-triplet energy splitting, an essential parameter for spin qubits, and we present calculations showing the data are consistent with a spectrum in which the first excited state of the dot is a valley-orbit state. We then propose a quantum dot qubit architecture that has an attractive combination of speed and fabrication simplicity. The proposed hybrid qubit consists of a double quantum dot with two electrons in one dot and one electron in the other. The hybrid qubit formed from three electrons in a double quantum dot has potential for great speed due to the presence of level crossings where the qubit becomes charge-like. Thus, as a stepping stone, we demonstrate coherent manipulations in a three-electron charge qubit. Finally, we demonstrate fast coherent manipulation of three-electron states in a double quantum dot as progress towards the implementation of the pulse-gated hybrid qubit. We demonstrate that tailored pulse sequences can be used to induce coherent rotations between 3-electron quantum states.