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Acta Physica Sinica

REDUCTION OF QUANTUM NOISE BY PHOTON CORRELATION

Li Yong-Qing, Yuzhu Wang, 中国科学院上海光学精密机械研究所

Published Jan 1, 1989• 0 citations
Publisher

Abstract

In this paper, the quantum theory of the measurement of the intensity difference between two light beams is analysed. The quantum noise can be reduced by increasing the photon correlation between two beams. For anti-Cauchy-Schwarz light, another type of nonclassical radiation fields, this quantum noise can be reduced to a level below the classical limit. The sub-shot noise may occur in DFWM between the conjugate wave and the transmit wave.

DOI
10.7498/aps.38.476

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In this paper, the quantum theory of the measurement of the intensity difference between two light beams is analysed. The quantum noise can be reduced by increasing the photon correlation between two beams. For anti-Cauchy-Schwarz light, another type of nonclassical radiation fields, this quantum noise can be reduced to a level below the classical limit. The sub-shot noise may occur in DFWM between the conjugate wave and the transmit wave.
A Cauchy-Schwarz inequality in photon noise spectra is derived and discussed for macroscopic fields of light. Violation of this Cauchy-Schwarz inequality is demonstrated experimentally in quantum noise-correlated light beams radiated from a pair of electrically coupled light-emitting diodes. Our measurements directly demonstrate that the violation of this Cauchy-Schwarz inequality can be observed in the absence of sub-shot noise fluctuations in either the single-beam intensity or the dual-beam intensity difference. Therefore, the dual-beam quantum correlation condition required for the violation of this Cauchy-Schwarz inequality is not associated with sub-shot noise fluctuations.
We present two experiment that demonstrate intensity noise reduction in light below the standard quantum noise level. A first experiment establishes a reduction of 86 % in the intensity difference noise spectrum of twin beams generated by an Optical Parametric Oscillator. The second experiment exploits this intensity correlation between the twin beams and uses one beam to electro-optically control the other, generating a single beam whose intensity noise is reduced 24 % below the shot noise level. General theories are presented for both processes using a semi-classical input-output formalism. Control configurations are examined where the noise reduction is effected directly from one beam to the other (feedforward), or indirectly through the pump beam (feedback). We determine the maximum amount of noise reduction that may be obtained from active control and detail for these configurations the limitations imposed by constraints of causality and stability. Finally, we demonstrate the equivalence of the semi-classical model with a "corpuscular" photon model in the small fluctuation limit, which we substantiate using Monte-Carlo simulation.
The accuracy of optical measurements at low light levels is limited by the quantum noise of the source and by the random nature of the interaction with the measured object. The source noise may be reduced by use of nonclassical photon-number squeezed light. We consider the use of two photon-correlated beams (generated, for example, by spontaneous parametric downconversion) to measure the optical transmittance of an object. The photons of each beam obey a random Poisson process but are synchronized in time. One beam is used to probe the object, and the other is used as a reference providing information on the realization of the random arrival of photons at the object. The additional information available by such measurement may be exploited to improve the accuracy of the measurement. Various estimators, including the maximum-likelihood estimator, are considered, and their performance is evaluated and compared with the measurement based on a single-beam conventional (Poissonian) source and a maximally squeezed (fixed-photon-number) source. The performance advantage that is established depends on parameters such as the intensity of the source, the transmittance of the object, the quantum efficiency of the detectors, the background noise, and the degree of correlation of the photon numbers in the two beams.
Without light, there would be no sight. Light acts as a probe in various measurement techniques ranging from imaging to spectroscopy, from small scale cantilever displace- ment measurement in atomic force microscopy to large scale mirror motion in interfer- ometry, light detection and ranging (LIDAR) and many other fields. As the light probe intensity rises, it not only allows reducing the background noise effect, but it also aids precision to the measurement by reducing the photon noise (shot-noise) contribution. However, increasing intensity beyond certain threshold level is not always advantageous for ultra sensitive measurements. For example in the detection of gravitational waves, the current power circulating in the large scale interferometers can not be increased further without introducing other noise sources like thermal effects on the mirrors, un- wanted scattered photons, and back action due to radiation pressure. In the imaging of delicate photo sensitive sample, high power can causes cell damage or it may lead to disturb the regular process under investigation, viz. favouring certain biochemical reaction, which do not correspond to natural in vitro behaviour. At low intensity, pho- ton noise is an important concern and quantum states of light with correlated photon fluctuation can ideally represent a fruitful way to build specific measurement strategies to surpass the limitation of standard approach based on classical light sources, offering an avenue of solutions for ultra sensitive measurements. This thesis work focuses on two application of quantum enhanced measurement strate- gies. The first part of the thesis work has been dedicated to the realization of the first wide-field microscope with Sub Shot Noise (SSN) sensitivity. This is based on the ex- ploitation of quantum correlations in the Squeezed Vacuum state. In the second part, the investigations have made on the role of the quantum correlated beams in an unusual interferometric scheme, in which two identical optical interferometers are subject to the same phase fluctuation. The scheme, in the classical regime, has been already imple- mented in a large scale experiment devoted to the search of possible quantum gravity (QG) effects at Fermilab.
The accuracy of optical measurements at low light levels is limited by the quantum noise of the source and by the random nature of the interaction with the measured object. The source noise may be reduced by use of nonclassical photon-number squeezed light. This paper considers the use of two photon-correlated beams (generated, for example, by spontaneous parametric downconversion) to measure the optical transmittance of an object. The photons of each beam obey a random Poisson process, but are synchronized in time. One beam is used to probe the object while the other is used as a reference providing information on the realization of the random arrival of photons at the object. The additional information available by such measurement may be exploited to improve the accuracy of the measurement. Various estimators, including the maximum likelihood estimator, are considered and their performance is evaluated and compared with the measurement based on single-beam conventional (Poisson) source and maximally squeezed (fixed photon number) source. The performance advantage established in this paper depends on parameters such as the intensity of the source, the transmittance of the object, the quantum efficiency of the detectors, the background noise, and the degree of correlation of the photon numbers in the two beams.