The behavior of conventional transistors derives from large numbers of acceptor and donor impurities that promote carriers into the valence and conduction bands. More recently, nano-electronic devices based on the bound states of individual dopant impurities in silicon have received considerable attention for quantum computation, due to the long spin coherence times of dopants in silicon. This invariably requires control over dopant wavefunctions and the interactions between individual dopants [1]. Recently, we have developed a scheme for spatially resolved single-electron transport, providing the first direct measurements of the wave-functions of single dopants and interacting dopants in silicon. Previously hidden valley-orbit degrees of freedom of electrons in silicon were directly observed. As required to preserve the long spin lifetimes of donor-bound spins, valley degrees of freedom were found to be robust to very nearby (~ few nm) control interfaces [2]. We also directly observed the wavefunction and the spin excitation spectrum of coupled acceptor dopants, allowing us to map the exchange interaction and Hubbard interaction strength at the atomic scale. The interaction strength was found to be tunable with dopant separation, and essentially hydrogenic, even for non-perturbative couplings associated with exotic physics in many-body spin Hamiltonians [3,4]. I will conclude by briefly discussing new directions on locally probing interacting dopants placed with atomic precision in a crystal, and on spin readout of acceptor-based spin-orbit qubits, where long-distance coupling schemes and long coherence times can be anticipated [5].

[1] F. Zwanenburg et al, (2013), Rev. Mod. Phys., 85, 961.
[2] J. Salfi et al, (2014), Nature Mat., 13 605.
[3] J. Salfi et al, arxiv:1507:06125.
[4] I. M. Georgescu et al, (2014), Rev. Mod. Phys, 86, 153.
[5] J. Salfi et al, arxiv:1508.04259, J. van der Heijden, J. Salfi et al, (2014) Nano Lett. 14, 1492 (2014)