From classical to quantum noise

An account is given of noise in electrical circuits and its relation to quantum noise connected with Heisenberg’s uncertainty principle. A phase-insensitive amplifier must introduce noise (most commonly the spontaneous emission noise) in order to satisfy the constraint imposed by quantum mechanics on the simultaneous measurement of two observables represented by noncommuting operators. Nonstationary, phase-sensitive amplifiers can employ squeezed radiation and reduce the noise below that of phase-insensitive amplifiers. Experiments on squeezing in optical fibers and the attendant noise reduction are presented. The squeezing apparatus can also be employed as a quantum nondemolition measurement of photon number. A thought experiment of a two-slit interferometer is described, in which the fringes are washed out progressively with the accuracy of the photon number determination in one of its arms. Implications of measurements of this kind for the resolution of the Einstein–Podolsky–Rosen paradox are discussed.