Quantum noise limits on optical monitoring with parametric devices

The limits imposed by quantum noise on optical monitoring with parametric devices are described quantitatively, for adjacent (back-to-back) and distant (post-transmission) monitoring. Formulas are derived, which describe the effects of transmission on the photon-number mean and variance of the signal, and which describe how the mean and variance of the output (monitored) signal and (generated) idler depend on the mean and variance of the input signal. It is possible to monitor many-photon signals without decreasing their signal-to-noise ratios significantly.