Passive versus active interferometers: Why cavity losses make them equivalent

Active and passive interferometers (as used, for example, for rotation-rate sensing or gravita- tional-wave detection) are known to have essentially the same ultimate sensitivity, although they appear to work differently, have signals of different sizes, and be limited by different kinds of noise (shot noise for the passive case, spontaneous emission for the active case). This paper explains this remarkable coincidence. The underlying physics common to both systems is brought forth and the role of the losses in limiting the sensitivity is clarified. The possibility of squeezing the field is explicitly considered; it is shown when it can or cannot help, and why.