Designing High-performance Qubits using Dopant based Quantum Dots

Motivated by the advancement in phosphorus donor atom qubits in silicon over the last decade, the semiconductor quantum computing community has begun investigating a variety of dopant species as well as more complex dopant structures for embedding enhanced quantum functionalities into quantum processors. The atomistic tight-binding method can simulate such systems capturing the wavefunctions of these substitutional atoms over a 1-30 nm domain without any apriori assumptions about the electronic structure. It has been hugely successful in describing the phosphorus atom qubits in silicon with atomistic detail. In this thesis, we extend this methodology to a variety of dopants in Group IV semiconductors (Si and Ge), including both donor and acceptor species. We achieve excellent calibration with known experimental data and obtain insights about the interplay between multiple bands, orbitals, spins, and valley states. We show how these features modify the critical spin qubit properties, such as hyperfine couplings, g-factors, energy splittings, magnetic and electric field dependencies, and discuss the ultimate limits of the modeling capability. We apply the simulation framework to investigate more complex dopant structures, such as phosphorus donor quantum dots (QD), which arise from the deterministic placement of a few dopant atoms within a small lithographic patch of 1-3 nm by Scanning Tunneling Microscope. High tunability in electronic confinement can be achieved in these systems by varying the number and arrangement of the donors inside the dot. We show that atomic confinement engineering enables control of qubit properties crucial for quantum operations, including contact hyperfine coupling, g-factors, relaxation time, decoherence time, and tunnel coupling strengths. In addition, we develop a Broyden mixing scheme with exchange-correlation corrections to self-consistently obtain tight-binding states and wavefunctions with different electron filling in the donor QDs. This provides us with a knob to engineer quantum confinement and the spin qubit properties post-fabrication. We assess the metrics of these multi-electron donor QDs as spin qubits and compare them with relevant experimental measurements. In summary, this thesis has generalized the simulation capabilities to investigate more complex dopant species and structures as potential high-performance qubits