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.

Figure 1

(a) Conductance (in units of the conductance quantum $G_0=2e^2/h$) recorded during tip displacement $\Delta z$ from tunneling ($\Delta z=0$) to contact. Sample voltage $V=1.6\mathrm{V}$. (b) Series of 60 luminescence spectra recorded simultaneously. Intensities (counts per s and eV) are represented as false colors on a logarithmic scale. Arrows indicate the formation of a single-atom contact.

Figure 2

Intensity of $1e$ light vs conductance. Data recorded at $V=1.6\mathrm{V}$. The $1e$ intensity was integrated over $1.19\mathrm{eV}<h\nu <1.47\mathrm{eV}$. (a) Double-logarithmic scale covering conductances from ${10}^{-3}{G}_0$ to $1.6G_0$. For comparison, a straight line indicates a linear relation. (b) The same data displayed on linear scales. Occasionally, intensity data close to integer multiples of $G_0$ could be recorded and exhibited nonmonotonous variation. Here, data for $G\gtrsim 3G_0$ are included.

Figure 3

Yield of $1e$ photons (defined as $1e$ intensity divided by the current, normalized to 1 at low conductances) vs conductance on (a) logarithmic and (b) linear scales. Experimental data (dots) from Fig. 1b are compared to the Fano factor from Eq. (1). Inset to (b) shows data near $3G_0$. A yield of 1 corresponds to an estimated quantum efficiency of $3\times {10}^{-6}$ photons per electron [37].

Figure 4

Yield of $2e$ light ($2e$ intensity divided by current, arbitrarily normalized) vs conductance. The intensity was integrated over $1.6\mathrm{eV}<h\nu <2.08\mathrm{eV}$. Experimental data (dots) from Fig. 1b are compared to the Fano factor from Fig. 3. A yield of 1 corresponds to an estimated quantum efficiency of $3\times {10}^{-7}$ photons per electron [37].