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Journal of Modern Optics

Power spectra and correlations of intensity fluctuations in electromagnetically induced transparency

Vladimir A. Sautenkov, Hebin Li, Yuri V. Rostovtsev, Marlan O. Scully

Published Nov 10, 2007• 12 citations

Abstract

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.

DOI: 10.1080/09500340701639961

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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 demonstrate and characterize two coherent phenomena that can mitigate the effects of laser phase noise for electromagnetically induced transparency (EIT): a laser-power-broadening-resistant resonance in the transmitted intensity cross correlation between EIT optical fields, and a resonant suppression of the conversion of laser phase noise to intensity noise when one-photon noise dominates over two-photon-detuning noise. Our experimental observations are in good agreement with both an intuitive physical picture and numerical calculations. The results have wide-ranging applications to spectroscopy, atomic clocks, and magnetometers.

We report a detailed investigation on the properties of intensity correlation spectra for laser beams interacting with cold atoms under the condition of electromagnetically induced transparency (EIT). We describe the transition of their intensity fluctuations from correlation to anticorrelation as the mean intensity of the fields increases. Such a transition occurs for laser frequencies around the EIT resonance, which is characterized by a correlation peak. The transition point between correlation and anticorrelation is independent of power broadening and provides directly the ground-state coherence time. We introduce a method to extract in real time the correlation spectra of the system. The experiments were done in two distinct magneto-optical traps, one for cesium and the other for rubidium atoms, employing different detection schemes. A simplified theory is introduced assuming three-level atoms in $\ensuremath{\Lambda}$ configuration interacting with a laser with stochastic phase fluctuations, providing a good agreement with the experimental observations.

It is known that cross correlation between the random intensity fluctuations of two lasers forming electromagnetically induced transparency (EIT) exhibits a transition from correlation to anticorrelation. We study the linewidth behavior of this transition and have found the linewidth is below the (effective) coherence lifetime limit and is limited only by competing noises. We established a numerical model which reveals the linewidth dependence on laser linewidth and laser power. Our experiments using lasers with different linewidth showed results in qualitative agreement with the model. This result is useful for quantum optics using EIT and may also have applications in spectroscopy and precision measurements.

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.