Electrical engineers and physicists are naturally very interested in noise in circuits, amplifiers and detectors. With the advent of quantum computation at low temperatures, we have entered a regime where quantum noise and quantum-limited detectors are important. I report on measurements of the backaction of a superconducting single electron transistor (SSET) measuring a Cooper-pair box qubit. During the weak, continuous measurement made by the SSET, the charge noise of the current running through the SSET acts on the Cooper-pair box. Due to the quantum nature of that noise, the SSET is able to dephase, relax and even excite the qubit.