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Physical Review Letters

Experimental realization of a quantum optical tap

J.-Ph. Poizat, Philippe Grangier

Published Jan 18, 1993• 126 citations
Publisher

Abstract

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.

DOI
10.1103/physrevlett.70.271

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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.
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.
Two configurations involving high quantum efficiency photodetectors and light emitting diodes have been investigated. A light beam was detected with a photodetector and its photocurrent was used to measure the incoming beam. Simultaneously, the photocurrent was used, with and without amplification, to regenerate the absorbed beam. With these schemes photon number amplification, duplication, and information tapping without introducing any significant noise to the downstream propagating beam could be experimentally realized.
We describe an amplifying system using a photodiode and a light emitting diode, which is able to measure and to ``recreate'' the quantum statistics of the intensity of a light beam. This system acts therefore as a nonclassical ``quantum optical repeater.'' The overall information transfer is improved by a factor of 1.8 compared to a classical device, while the ideal quantum value would be 2, and the correlation between the signal and meter outputs is 1.2 dB below the shot-noise limit. We also demonstrate the use of this device for both detecting and recreating an input squeezed state.