Quantum interference and quantum correlations in single dopants and exchange coupled dopants in silicon

Quantum electronics exploiting the highly coherent states of single dopants in silicon invariably requires interactions between states and interfaces, and inter-dopant coupling by exchange interactions. We have developed a low temperature STM scheme for spatially resolved single-electron transport in a device-like environment, providing the first wavefunction measurements of single donors and exchange-coupled acceptors in silicon. For single donors, we directly observed valley quantum interference due to linear superpositions of the valleys, and found that valley degrees of freedom are highly robust to the symmetry breaking perturbation of nearby (3 nm) surfaces. For exchange-coupled acceptors, we measured the singlet-triplet splitting, and from the spatial tunneling probability, extracted information about the 2-body wavefunction amplitudes and determined the entanglement entropy, a measure of the inseparability (quantum correlations) generated by the interactions between their indistinguishable particles. Entanglement entropy of the J=3/2 holes was found to increase with increasing dopant distance, as Coulomb interactions overcome tunneling, coherently localizing spin towards a Heitler-London singlet, mimicking S=1/2 particles [3]. In the future these capabilities will be exploited to peer into the inner workings of few-dopant devices and shed new light on multi-dopant correlated states, engineered atom-by-atom.