  We devise a scheme to characterize tunneling of an excess electron shared by
a pair of tunnel-coupled dangling bonds on a silicon surface -- effectively a
two-level system. Theoretical estimates show that the tunneling should be
highly coherent but too fast to be measured by any conventional techniques. Our
approach is instead to measure the time-averaged charge distribution of our
dangling-bond pair by a capacitively coupled atomic-force-microscope tip in the
presence of both a surface-parallel electrostatic potential bias between the
two dangling bonds and a tunable midinfrared laser capable of inducing Rabi
oscillations in the system. With a nonresonant laser, the time-averaged charge
distribution in the dangling-bond pair is asymmetric as imposed by the bias.
However, as the laser becomes resonant with the coherent electron tunneling in
the biased pair the theory predicts that the time-averaged charge distribution
becomes symmetric. This resonant symmetry effect should not only reveal the
tunneling rate, but also the nature and rate of decoherence of single-electron
dynamics in our system.
