Optical probe of quantum shot-noise reduction at a single-atom contact.

Visible and infrared light emitted at a Ag-Ag(111) junction has been investigated from tunneling to single-atom contact conditions with a scanning tunneling microscope. The light intensity varies in a highly nonlinear fashion with the conductance of the junction and exhibits a minimum at conductances close to the conductance quantum. The data are interpreted in terms of current noise at optical frequencies, which is characteristic of partially open transport channels.