Multi-qubit architectures for donor-based silicon quantum computing

Abstract

We investigate multi-qubit device architectures for scalable donor-based quantum computing in silicon fabricated using scanning tunnelling microscope (STM) based lithography for the atomic precision placement of single phosphorus (P) atoms in silicon. Three key results are demonstrated in this thesis: (i) electron transport through a few-donor triple quantum dot, (ii) high fidelity electron spin readout of donor dots containing single, double and triple P donors, (iii) charge sensing of few-donor double quantum dots.

We present the independent electrostatic control of three serially tunnel-coupled few-donor quantum dots in silicon, the smallest system where spin qubit transport protocols can be investigated. Through finite bias spectroscopy, we find that a small asymmetry of ∼1 nm in the position of the three quantum dots results in a strong asymmetric coupling between them highlighting the importance of sub-nm precision placement of donors in these systems.

We present a viable architecture for performing single- and two-qubit gates operations on a pair of donor-bound electron spin qubits, the fundamental building block of a donor-based quantum computer. With this architecture, we demonstrate the single-shot readout of an electron spin bound to a precisely positioned P donor with a record fidelity of 99.6% above the fault-tolerant error threshold. Additionally, we show the readout time can be reduced by nearly two orders of magnitude with little loss in fidelity (98.0%) by employing the singlet spin state of the doubly occupied single donor.

We subsequently demonstrate the independent electrostatic control and high fidelity readout (99.8%) of a pair of electron spin qubits bound to two weakly-coupled 2P and 3P donor dots separated by ∼20nm. We measure significantly longer spin lifetimes in the donor dots (∼30s) compared to single donors (∼4s) which is attributed to their stronger confinement potential leading to less interaction between the electron and the silicon lattice. We estimate the exchange coupling between the pair of electron spins and find it to be too small to perform a two qubit gate indicating that smaller inter-donor separations will be needed in future multi-qubit devices. These results demonstrate the potential of STM lithography for scaling up to multi-qubit devices in silicon.