  We make the first study the optical energy level structure and interactions
of pairs of single rare earth ions using a hybrid electro-optical detection
method applied to Er-implanted silicon. Two examples of Er3+ pairs were
identified in the optical spectrum by their characteristic energy level
splitting patterns, and linear Zeeman spectra were used to characterise the
sites. One pair is positively identified as two identical Er3+ ions in sites of
at least C2 symmetry coupled via a large, 200 GHz Ising-like spin interaction
and 1.5 GHz resonant optical interaction. Small non-Ising contributions to the
spin interaction are attributed to distortion of the site measurable because of
the high resolution of the single-ion measurement. The interactions are
compared to previous measurements made using rare earth ensemble systems, and
the application of this type of strongly coupled ion array to quantum computing
is discussed.
