AbstractsEngineering

As classical computers begin to reach their fundamental performance limits, quantum computers will be an invaluable tool for the advance of science and technology. The trillion dollar silicon electronics industry sets the perfect stage for the evolution of quantum computation, yet it has so far proved to be a tough challenge to implement all the elements needed to build a scalable quantum computer in silicon. This thesis presents the first experimental demonstration of the full operation of single-spin qubits in Si. Our qubits consist of the electron and nuclear spins of a single phosphorus atom, implanted in a Si substrate, and controlled by a gated nanostructure. We describe an experimental setup tailored to minimise electron temperature and perform real-time data acquisition, analysis and instrument control. We present modeling, simulation and characterisation of a novel nanoscale coplanar antenna for spin control, designed to work at frequencies up to 50 GHz. These tools have allowed us to demonstrate the first ever single-atom spin-qubits in natural silicon, leading the way to demonstrating record qubit performances in isotopically purified 28Si: an electron spin qubit with measurement and control fidelities > 97% and coherence times of 0.5 seconds; and a nuclear spin qubit with fidelities > 99.99% and a record single-spin coherence of 30 s. We have performed noise spectroscopy in our system and concluded that decoherence is currently limited by magnetic noise originating from our broadband antenna. We also describe a methodology towards the demonstration of electron-nuclear entanglement in a single atom, through density matrix tomography. Finally, we present the experimental demonstration of one of the key milestones towards implementing two-qubit gates in our system: single-shot readout and relaxation measurements of the singlet-triplet states of coupled electrons from a P donor pair in natural Si; finding agreement of our observed exchange coupling and relaxation to previous theoretical predictions for P dimers in Si. The results presented in this thesis have catapulted silicon qubits onto the main stage of quantum computing systems, and pave the way to the exciting future experiments, that should see two-qubit gates and qubit transport become a near-term reality.