Quantum Non-Demolition Measurements in the Optical Domain

Abstract

A quantum measurement usually perturbs the quantity which is measured, adding ”back-action noise” to the system under study. However, ”quantum non-demolition” (QND) measurements can be performed, leaving the observed quantity unperturbed, while adding the back-action noise into another complementary observable. Quantum non- demolition measurements were first proposed to monitor the motion of mechanical oscillators [1]– [3], but it was then realized that they were easier to implement in the optical domain [4]– [6], [29], [30]. In this case, the measurements are made on a mode of the electromagnetic field considered as a quantum harmonic oscillator, and the coupling responsible for the measurement is crea- ted using either χ(2) or χ(3) optical non-linearities [7]– [9]. In this paper, we will focus on the QND measurements of ”travelling wave” light beams, rather than fields stored in high Q cavities [10]. Moreover, we will assume that the quantum fluctuations of the beams are very small compared to the mean intensities, so that a linear treatment of these quantum fluctuations is possible [11]. A ”travelling wave” optical QND scheme usually involves the coupling of two field modes : one ”signal” mode, which is to be measured, and one ”meter” mode, which is directly detected and yields some informations about the signal mode. A perfect QND measurement device should leave the measured quadrature component of the signal mode unchanged, and turn the detected meter quadrature component into some kind of ”macroscopic copy” of the signal. However, such a device does not exist at present time, and it appears necessary to be able to characterize the ”non-ideality” of practical QND schemes.KeywordsConditional VarianceQuantum FluctuationSignal BeamPhase FluctuationOptic CommThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.