Towards the atomic-scale fabrication of a silicon-based solid state quantum computer

The construction of a scalable quantum computer in silicon, using single phosphorus atoms as qubits, presents a significant technological challenge. This paper describes recent results from a ‘bottom-up’ strategy to incorporate individual phosphorus atoms in silicon with atomic precision using a combination of advanced scanning tunnelling lithography techniques followed by low temperature silicon molecular beam epitaxial overgrowth. To date we have demonstrated (i) placement of individual phosphorus molecules at predetermined sites in the silicon surface using a hydrogen resist strategy, (ii) spatially controlled phosphorus incorporation into the silicon surface, (iii) minimisation of surface segregation by low temperature silicon encapsulation and (iv) complete electrical activation of the donors. Whilst these results bode well for the fabrication of silicon devices with atomically precise dopant profiles, we discuss the challenges that remain before a few qubit P in Si quantum computer prototype can be realised.