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Physical Review Letters

Observation of backaction-evading measurement of an optical intensity in a three-level atomic nonlinear system

Philippe Grangier, Jean-François Roch, Gérard Roger

Published Mar 18, 1991• 93 citations
Publisher

Abstract

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.

DOI
10.1103/physrevlett.66.1418

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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.
We present a theoretical study of the quantum non-demolition (QND) measurement of an optical intensity in an atomic three-level system, using a quantum linear response theory recently developed. The principle of this scheme is to use the two-photon crossed-phase- modulation effect, in order to couple the quantum intensity fluctuations of the `signal' beam to the quantum phase fluctuations of the `meter' beam. The predictions of the complete theory are compared with recent experimental results using a sodium atomic beam, which include the observation of both squeezing and QND effects.
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.
Conventional devices for measuring the amplitude of a light wave absorb the light, thus destroying it. Recently, quantum nondemolition (QND) methods based on the optical Kerr effect have been shown capable of measuring optical amplitude fluctuations with near quantum-limited sensitivity.1 In these experiments, amplitude fluctuations of a signal beam are transferred to the phase fluctuation quadrature of a probe beam which co-propagates with the signal beam, by the intensity-dependent refractive index, without absorption of the signal beam. A pure Kerr nonlinearity can cause only phase modulation of the light wave, and the amplitude of the signal wave is preserved. Such methods should more properly be called back-action evading measurement schemes since the quantity being measured is not perturbed by the back-action that quantum mechanics requires for any measurement. The signal-to-noise ratio achieved in these experiments was limited by the small magnitude of the Kerr nonlinearity relative to the excess phase noise imposed on the probe beam by thermal light scattering processes in the glass fibers that were employed.
In this paper we present a detailed theoretical analysis of a recent quantum-nondemolition experiment in optics using cold atoms in a magneto-optical trap as a nonlinear medium. A signal beam and a meter beam from two independent lasers are coupled within a \ensuremath{\Lambda}-type three-level scheme in the D1D1D1 line of 87Rb{}^{87}\mathrm{Rb}87Rb atoms. The experimental results for the relevant quantum correlations of the fields, that represent up to now the best achievements for a single back-action evading measurement, are found in a remarkably good agreement with the theoretical predictions from a fully quantum model for three-level atoms in a doubly resonant cavity.
We have realized a system which is able to read out information encoded on the amplitude of a laser beam without degrading the signal-to-noise ratio of the carrier. This has been implemented using the two-photon cross-phase-modulation effect of a three-level system in a sodium atomic beam located in a doubly resonant optical cavity. We have achieved an improvement by a factor of 1.35 of the overall information transer compared to any classical optical tap, where a perfect quantum duplicator would reach the ideal value of 2.
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.