Quantum Noise Reduction I

Abstract

In many areas of modern physics, ultrasmall displacements are detected optically. The small displacement is converted into a change of optical path length in an interferometer. This detection scheme is usually done in two ways:

• (1) In a passive detection scheme, laser light, generated outside, is sent trough a cavity and the change in the path length results in a phase shift. The shift is then detected by homodyning the output beam with a reference beam. This phase shift is generally small, since the light spends only a finite time in the cavity, limited by the cavity lifetime. In this type of measurement, the limiting noise source is the photon-counting error or shot noise, reflecting the photon number fluctuations at the detector.

• (2) In the so-called active detection scheme, the laser light is generated inside the cavity and the operating frequency of the system changes due to the change in the path length, which results in a phase shift proportional to the measurement time, leading in general to a bigger signal as compared to the first case. The shift is then detected by heterodyning the output light with that from a reference beam. The limiting quantum noise source, in this case, is the spontaneous fluctuations of the relative phase between the two lasers, or in other words, the relative phase diffusion noise.