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Physical Review A

Optical feedback from quantum non-demolition measurements: How to realize a measurement apparatus to observe macroscopic quantum coherence

Paolo Tombesi, David Vitali

Published Jan 1, 1995• 0 citations
OpenAlex

Abstract

We show how a squeezed bath can be obtained by means of a suitable feedback of the output signal corresponding to a Quantum Non-Demolition (QND) measurement of an observable. We also show that a quantum non-demolition mediated feedback is able to preserve the interference fringes of the superposition of macroscopically distinguishable quantum states.

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Journal of Modern Optics•Dec 1, 1994•6 citations
We show how a squeezed bath can be obtained by means of a suitable feedback of the output signal corresponding to a Quantum Non-Demolition (QND) measurement of an observable. We also show that a quantum non-demolition mediated feedback is able to preserve the interference fringes of the superposition of macroscopically distinguishable quantum states.
We discuss some theoretical and experimental aspects of quantum non-demolition (QND) measurements in the optical domain. The efficiency of a QND measurement can be characterized using three criteria, which describe respectively the quality of the quantum measurement, the non-destruction of the signal, and the conditional variance of the output signal beam, given the output meter beam (quantum-state-preparation criterion). Quantitative limits can be defined with respect to these criteria, delimiting "classical" and "quantum" domains of operation. We describe the implementation of two experiments which fulfill these criteria, using either three-level atoms inside a doubly-resonant optical cavity, or semiconductors emitters and receivers.
In the proposed quantum optical experiment, the complementarity and entanglement exhibited in quantum non-demolition (QND) measurement of coherent state superposition is analysed. This problem can be considered as a continuously variable analogue of the well known exhibition of complementarity in Young's double-slit interferometer. Analysis of the relationship between distinguishability and interference in phase space, as well as quantum entanglement, is performed using directly measurable quantities obtained using homodyne detection.
We describe the experimental observation and theoretical interpretation of a squeezing effect which occurs through the coupling of two light beams in a three-level atomic system. The origin of this effect can be attributed to the transfer of the intensity fluctuations of one beam to the phase fluctuations of the other one, followed by an optical feedback onto the intensity of the initial beam. The first step of the information transfer is similar to the one which occurs in a quantum non-demolition (QND) measurement of the intensity. The feedback effect is obtained through mixing of the phase and intensity quadratures, due to the detuning of the optical cavity which contains the nonlinear medium. Therefore the information obtained by the QND measurement is used to correct the intensity fluctuations of the signal beam by a build-in mechanism, which does not require any use of external electronics.