Detailed measurements of the voltage, temperature, and frequency dependence of the nonequilibrium current fluctuations for a diffusive mesoscopic conductor are reported. The data confirm predictions that a mesoscopic conductor shorter than the electron-electron inelastic length will display shot noise. Furthermore, the low temperatures (100 mK) and high frequencies (1–20 GHz) used for the measurements allow tests in the high-frequency regime (i.e., $h\nu \gg \mathrm{eV}$ and $\mathrm{kT}$) of the shot noise, which clearly show the influence of vacuum fluctuations. The quantum noise causes a high-frequency “cutoff” in the shot noise, i.e., the noise is independent of bias voltage for frequencies $\nu >\mathrm{eV}/h$.

• Received 22 November 1996

DOI:https://doi.org/10.1103/PhysRevLett.78.3370