Modulation, signal, and quantum noise in interferometers

We discuss the generation of quantum noise in interferometers. A heuristic model is described that enables different characteristics of the noise to be understood and calculated. In particular, the effects of modulation on signal and noise in interferometers are discussed. It is seen that the mixing in of quantum noise from different modes of the vacuum can degrade the signal-to-noise ratio of a measurement in a variety of apparently different physical situations. Michelson interferometers with both internal and external phase modulation are specifically considered, but the results are easily extendable to any type of interferometer. We also comment upon some implications for the use of squeezing in interferometers. For examples, we argue that it is possible, even in lossy narrow-band interferometers, for squeezing to reduce significantly the effective quantum-noise power.