The building blocks of a quantum computer based on donor spins in silicon

摘要

A computer with quantum mechanical building blocks, or qubits, promises a new class of computational capability and is a natural platform on which to simulate microscopic systems. Building a quantum computer, however, is a formidable challenge. The system chosen must be engineered to balance contradicting requirements. The fragile quantum states must be sufficiently protected from noise in the environment, while being accessible for accurate measurement and control. Moreover, the system must be scalable to millions of qubits that can be made to interact in complex sequences. Recent experimental breakthroughs have highlighted spin qubits in silicon as a promising platform. Firstly, fabrication of these semiconductor devices would capitalize on the expertise developed by the microelectronics industry. Secondly, and more fundamentally, silicon can provide a virtually noiseless environment for spin qubits while maintaining qubit addressability. This thesis builds upon the foundational work on phosphorus donor electron spin qubits in silicon. We achieve dramatic improvements in the fundamental single qubit operations: the fidelities of readout, initialization and control of the electron spin are all enhanced to 99.9% simultaneously and we report a new record coherence time of 0.98 seconds. Furthermore, we provide novel theoretical proposals for the next steps in scaling this technology. We present a two-qubit operation that can be performed with fidelity exceeding 99%, robust to variability in donor positioning, and we devise a scheme for high-fidelity short-range qubit transport that may prove invaluable in the design of a quantum processor. The experimental setup required for scale-up is also addressed, as we report the first measurement of the silicon spin qubit device in a cryogen-free dilution refrigerator and present techniques to mitigate the noise induced by mechanical vibrations. Our work has solved several crucial problems on the road-map towards a full-scale processor. It highlights the scalability and promise of this technology, which may result in the world's first quantum computer.