  Donors in silicon, conceptually described as hydrogen atom analogues in a
semiconductor environment, have become a key ingredient of many
"More-than-Moore" proposals such as quantum information processing [1-5] and
single-dopant electronics [6, 7]. The level of maturity this field has reached
has enabled the fabrication and demonstration of transistors that base their
functionality on a single impurity atom [8, 9] allowing the predicted
single-donor energy spectrum to be checked by an electrical transport
measurement. Generalizing the concept, a donor pair may behave as a hydrogen
molecule analogue. However, the molecular quantum mechanical solution only
takes us so far and a detailed understanding of the electronic structure of
these molecular systems is a challenge to be overcome. Here we present a
combined experimental-theoretical demonstration of the energy spectrum of a
strongly interacting donor pair in the channel of a silicon nanotransistor and
show the first observation of measurable two-donor exchange coupling. Moreover,
the analysis of the three charge states of the pair shows evidence of a
simultaneous enhancement of the binding and charging energies with respect to
the single donor spectrum. The measured data are accurately matched by results
obtained in an effective mass theory incorporating the Bloch states
multiplicity in Si, a central cell corrected donor potential and a full
configuration interaction treatment of the 2-electron spectrum. Our data
describe the basic 2-qubit entanglement element in Kane's quantum processing
scheme [1], namely exchange coupling, implemented here in the range of
molecular hybridization.
