Top-down pathways to devices with few and single atoms placed to high precision

Solid-state devices that employ few and single atoms are emerging as a consequence of technological advances in classical microelectronics and proposals for quantum computers based on spin or charge. The fabrication of devices in both these areas requires the development of techniques for deterministic doping of silicon where few or single dopant atoms must be placed to, typically, nanometre precision. Here we discuss a top-down approach, based on deterministic ion implantation, which can potentially be used to fabricate devices intended to explore the novel challenges of designing, building and measuring solid-state devices at the single atom limit. In particular, we address the potential of fabricating more complex devices that exploit quantum coherence. We propose a prototype triple-donor device that transports electron spin qubits via the coherent tunnelling by adiabatic passage (CTAP) protocol for a scalable quantum computer. We examine theoretically the statistics of dopant placement using ion implantation by employing an analytical treatment of CTAP transport properties under hydrogenic assumptions. We evaluate the probability of fabricating proof of concept devices subject to the limitations of ion implantation. We find that the results are promising with a yield of one in six for 14 keV phosphorus implanted into silicon with a target atom site spacing of 30 nm with even higher yields possible for lower-energy implants. This suggests that deterministic doping is an important tool to fabricate and test near-term practical quantum coherent devices.