Double Quantum Dots as Detectors of High-Frequency Quantum Noise in Mesoscopic Conductors
In this work we propose a measurement set-up for detecting quantum noise over a wide frequency range using inelastic transitions in a tunable two-level system as a detector. Recently, two device structures were realized that can be used as tunable two-level systems. In a superconducting single-electron transistor a Cooper-pair  and in a double quantum dot an electron  can make inelastic transitions between two discrete energy states. The transition rate for levels separated by an energy ∈, is a measure of the spectral density of the fluctuations in the enviroment at a frequency f = ∈/h. In our set-up the frequency-resolving detector consists of a double quantum dot which is capacitively coupled to the leads of a nearby mesoscopic conductor. The inelastic current through the double quantum dot is calculated in response to equilibrium and non-equilibrium current fluctuations in the nearby conductor, including vacuum fluctuations at very low temperatures. As a specific example, the fluctuations across a quantum point contact are discussed.