Statistical exchange-coupling errors and the practicality of scalable   silicon donor qubits
  Recent experimental efforts have led to considerable interest in donor-based localized electron spins in Si as viable qubits for a scalable silicon quantum computer. With the use of isotopically purified $^{28}$Si and the realization of extremely long spin coherence time in single-donor electrons, the recent experimental focus is on two-coupled donors with the eventual goal of a scaled-up quantum circuit. Motivated by this development, we simulate the statistical distribution of the exchange coupling $J$ between a pair of donors under realistic donor placement straggles, and quantify the errors relative to the intended $J$ value. With $J$ values in a broad range of donor-pair separation ($5<|\mathbf{R}|<60$ nm), we work out various cases systematically, for a target donor separation $\mathbf{R}_0$ along the [001], [110] and [111] Si crystallographic directions, with $|\mathbf{R}_0|=10, 20$ or 30 nm and standard deviation $\sigma_R=1, 2, 5$ or 10 nm. Our extensive theoretical results demonstrate the great challenge for a prescribed $J$ gate even with just a donor pair, a first step for any scalable Si-donor-based quantum computer. 
Yang Song, S. Das Sarma
cond-mat.mes-hall