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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Quantum nondemolition measurements and squeezing in atomic three-level systems

J.-Ph. Poizat, Philippe Grangier

Published May 10, 1996• 2 citations
Publisher

Abstract

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.

DOI
10.1117/12.239481

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We present a detailed analysis of the quantum-nondemolition (QND) properties of a measurement scheme employing coherently driven three-level atoms in the \ensuremath{\Lambda} or cascade configuration inside an optical cavity. We propose to use a strong signal field to empty one of the transitions and dress the signal-transition levels. The atoms become transparent for the signal beam, which sees a ``ghost transition.'' When a probe is applied and tuned to the vicinity of one of the light-shifted levels, the probe phase is extremely sensitive to fluctuations of the signal intensity. This enhances the QND coupling and enables the system to operate at lower cooperativities.
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.
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We consider two modes of the electromagnetic field interacting via a three-level atom in a ladder configuration. We calculate the squeezing spectra of the sum and difference of the two output beams. The usefulness of this system as a quantum-nondemolition-measurement scheme is analyzed and a prediction is made using the parameters of a recent experiment by Grangier et al. [Phys. Rev. Lett. 66, 1418 (1991)]. We use a full three-level model in the most general case and in particular the influence of both the one-photon and the two-photon detunings are investigated.
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