UCLA

With the rapid progress in nanofabrication, scientists and researchers are now able to make lateral quantum dots in semiconductor materials. The few electrons confined in these quantum dots provide the possibility of realizing a qubit, the building block of a quantum computer. Tremendous effort has been put in the solid state quantum information field in the last ten years of making single electron spin qubit or singlet triplet qubit based on two electron spin. However, the operation of these types of qubit requires additional engineering by either integrating a microwave loop or an external magnet to creat field difference. This thesis project was inspired by DiVincenzo's proposal of developing qubit based on three electrons controlled by Heisenberg exchange interactions only, which is called "exchange-only" qubit. All the qubit operation can be done in principle via electrical pulses only. We proposed to make the triple quantum device in silicon system. This type of device will have small qubit decoherence, easy integration to industry infrustructure and great chance of scaling up to a real quantum computer. We developed and fabricated the electrostatically defined triple quantum dot (TQD) device in a silicon metal-oxide-oxide structure. We characterized its electrostatic properties using a quantum point contact charge sensing channel nearby. We are be able to obtain the charge stability diagram in the last few elelctron regime that provides the experimental basis of forming a exchange only qubit. We demonstrated the tunability of the TQD by acheiving the quadruple points where all three dots are on resonance. This is the first experimental demonstration of well controlled triple quantum dot device in silicon system. The constant interaction model and the hubbard model for triple quantum dot system are developed to help understand the electrostatic dynamics. Tunnel couplings between quantum dots, which determines the exchange interactions, are extracted using various fitting methods. We implemented the qubit manipulation with three quantum dots in both a linearly and a triangularly arranged geometry. For the first time, we observed coherent oscillation in the Si MOS based triple quantum dot device with oscillation frequency of 2MHz and 7MHz. We suspect the these oscillations are related with spin dynamics in our system. These experimental investigations demonstrate that we have the ability to develop triple quantum dot device for exchange ony qubit and the potential to perform qubit operation in the future.