  Quantum mechanics can strongly influence the noise properties of mesoscopic
devices. To probe this effect we have measured the current fluctuations at
high-frequency (5-90 GHz) using a superconductor-insulator-superconductor
tunnel junction as an on-chip spectrum analyser. By coupling this
frequency-resolved noise detector to a quantum device we can measure the
high-frequency, non-symmetrized noise as demonstrated for a Josephson junction.
The same scheme is used to detect the current fluctuations arising from
coherent charge oscillations in a two-level system, a superconducting charge
qubit. A narrow band peak is observed in the spectral noise density at the
frequency of the coherent charge oscillations.
