Quantum noise in ideal operational amplifiers
  We consider a model of quantum measurement built on an ideal operational amplifier operating in the limit of infinite gain, infinite input impedance and null output impedance and with a feddback loop. We evaluate the intensity and voltage noises which have to be added to the classical amplification equations in order to fulfill the requirements of quantum mechanics. We give a description of this measurement device as a quantum network scattering quantum fluctuations from input to output ports. 
Jean-Michel Courty, Francesca Grassia, Serge Reynaud (Laboratoire   Kastler Brossel)