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Chinese Physics Letters

Nonadiabatic Effects of Atomic Coherence on Laser Intensity Fluctuations in Electromagnetically Induced Transparency

Qing Yang Xu, Xiangming Hu

Published Jul 1, 2011• 1 citation
Publisher

Abstract

In an electromagnetically induced transparency system, the atoms have long-lived coherence compared to the cavity lifetime and interact nonadiabatically with the laser fields. We show that the high frequency fluctuations of both the intensities and the intensity difference can be squeezed below the shot noise limit due to the nonadiabatic effects. This noise squeezing can be used to enhance the precision in the short time measurements based on the intensities or the intensity difference.

DOI
10.1088/0256-307x/28/7/074217

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The quantum fluctuations of optical beams are highly correlated in electromagnetically induced transparency (EIT). The intensity fluctuations are enhanced by conversion of phase noise to intensity noise in atomic vapour. We have studied spectra of intensity fluctuations in dense rubidium vapour for different laser beam powers. Observed excess noise spectrum is broader than EIT window at the same optical power. Both spectral widths are proportional to the laser beam intensity. By using signal subtraction scheme we can reduce amplitude of the fluctuations to close the photonic short noise level. This result confirms the high degrees of intensity correlations and can be used to improve quality of optical atomic clocks.
We present a series of experiments measuring intensity fluctuations and correlations of laser beams after propagation through an atomic vapour at room temperature in situations where coherent effects play a major role. In particular, we investigate these properties in the condition of electromagnetically induced transparency (EIT) in three-level lambda systems and in degenerate two-level systems. In a similar way, we also study the Hanle-EIT effect and electromagnetically induced absorption (EIA) in degenerate two-level systems. These results show the great influence of coherent interaction with the atomic system over the noise properties of the macroscopic laser fields, and indicate new sensitive ways of probing an atomic sample.
It has been shown by Imamoglu, Field and Harris1 that electromagnetically induced transparency can be used to achieve laser action without inversion. We have analyzed the quantum noise properties of the laser without inversion and shown that the intensity fluctuations in the laser output may be reduced 50% below the shot noise limit2. In recent work Makuta, Marmet and Stoicheff3 have shown that a resonantly enhanced second order susceptibility with reduced absorption at the second reduced harmonic wavelength may be achieved by electromagnetically induced transparency induced with a dc-electric field. We have calculated the photon statistics of the second harmonic light and found that near 100% amplitude squeezing can be achieved in the output at the second harmonic. We have shown that driven A model atoms in an optical cavity can reduce the amplitude fluctuation of a strong coherent input light field into the cavity by almost 100% without significantly depleting the input signal.
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