On measuring coherence in coupled dangling-bond dynamics

We devise a scheme to characterize tunnelling 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 tunnelling 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 mid-infrared laser capable of inducing Rabi oscillations in the system. With a non-resonant 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 tunnelling in the biased pair the theory predicts that the time-averaged charge distribution becomes symmetric. This resonant symmetry effect should not only reveal the tunnelling rate, but also the nature and rate of decoherence of single-electron dynamics in our system.