  Proposed Silicon-based quantum computer architectures have attracted
attention because of their promise for scalability and their potential for
synergetically utilizing the available resources associated with the existing
infrastructure of the powerful Si technology. Quantitative understanding of and
precise physical control over donor (e.g. Phosphorus) exchange are crucial
elements in the physics underlying the proposed Si-based quantum computer
hardware. An important potential problem in this context is that inter-valley
interference originating from the degeneracy in the Si conduction band edge
causes fast oscillations in donor exchange coupling, which imposes significant
constraints on the Si quantum computer architecture. In this paper we consider
the effect of external strain on Si donor exchange in the context of quantum
computer hardware. We study donor electron exchange in uniaxially strained Si,
since strain partially lifts the valley degeneracy in the bulk. In particular,
we focus on the effects of donor displacements among lattice sites on the
exchange coupling, investigating whether inter-valley interference poses less
of a problem to exchange coupling of donors in strained Si. We show, using the
Kohn-Luttinger envelope function approach, that fast oscillations in exchange
coupling indeed disappear for donor pairs that satisfy certain conditions for
their relative positions, while in other situations the donor exchange coupling
remains oscillatory, with periods close to interatomic spacing. We also comment
on the possible role of controlled external strain in the design and
fabrication of Si quantum computer architecture.
