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Physical Review Letters

Backaction evading measurements for quantum nondemolition detection and quantum optical tapping

Silvania F. Pereira, Z. Y. Ou, Harry J. Kimble

Published Jan 10, 1994• 111 citations
Publisher

Abstract

Backaction evading (BAE) measurements are described for optical parametric amplification with polarization mixing. Quantum correlations between the ``signal'' and ``meter'' outputs from the BAE device lead to an observed photocurrent variance 30% (1.5 dB) below the signal shot-noise level and to an inferred variance 44% below the vacuum-state level for determination of the signal field given the meter reading. Together with its efficient information transfer, this BAE detector functions as a quantum optical tap and is suitable for implementing quantum nondemolition detection.

DOI
10.1103/physrevlett.72.214

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Backaction evading (BAE) measurements are described for optical parametric amplification with polarization mixing. Quantum correlations between the signal'' and meter'' outputs from the BAE device lead to an observed photocurrent variance 30% (1.5 dB) below the signal shot-noise level and to an inferred variance 44% below the vacuum-state level for determination of the signal field given the meter reading. Together with its efficient information transfer, this BAE detector functions as a quantum optical tap and is suitable for implementing quantum nondemolition detection.
Repeated backaction evading measurements have been performed for the first time. The first backaction evading device prepares the field state while the second one measures it again. The conditional variance of the signal at the output of the first device given the measurement is 1.8 dB below the shot noise and the signal to noise ratio transfer coefficient is 1.34. The conditional variance of the final signal given the first device measurement is also 1.8 dB below the shot noise and the normalized correlation between the two meter outputs is 0.3. These figures are clearly in the quantum regime, and they constitute a full demonstration of the quantum nondemolition principle.
Backaction-evading measurement of an optical intensity has been observed, using a three-level system in a sodium atomic beam as a nonlinear medium. The principle of this experiment is to use the cross-phase-modulation effect, which in principle allows one to read nondestructively the quantum intensity fluctuations of the ``signal'' beam when detecting the phase fluctuations of the ``meter'' beam. Using a cavity-enhancement scheme, the quantum noise measured when recombining the signal and meter photocurrents is 0.8 dB below the shot-noise level.