Shot Noise of a Quantum Dot Measured with Gigahertz Impedance Matching

The demand for a fast high-frequency read-out of high-impedance devices, such as quantum dots, necessitates impedance matching. Here we use a resonant impedance-matching circuit (a stub tuner) realized by on-chip superconducting transmission lines to measure the electronic shot noise of a carbon-nanotube quantum dot at a frequency close to 3 GHz in an efficient way. As compared to wideband detection without impedance matching, the signal-to-noise ratio can be enhanced by as much as a factor of 800 for a device with an impedance of 100 k Omega. The advantage of the stub resonator concept is the ease with which the response of the circuit can be predicted, designed, and fabricated. We further demonstrate that all relevant matching circuit parameters can reliably be deduced from power-reflectance measurements and then used to predict the power-transmission function from the device through the circuit. The shot noise of the carbon-nanotube quantum dot in the Coulomb blockade regime shows an oscillating suppression below the Schottky value of 2eI, as well as an enhancement in specific regions.