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Journal of Physics B Atomic Molecular and Optical Physics

Detection of atoms in a beam: filtering of the atomic shot noise and velocity dispersion contribution

N. Dimarcq, C. Audoin

Published Jun 14, 1995• 5 citations
Publisher

Abstract

We calculate the contribution of the atomic shot noise in the detection of the laser-induced fluorescence signal emitted by atoms in a beam. We point out a filtering of such a contribution which is the consequence of the finite transit time of the atoms through the laser beam. Moreover, we demonstrate that the distribution of the atomic velocities in the beam induces an extra noise which degrades the measured signal-to-noise ratio and modifies the observed statistics of the fluorescence photons emitted by the whole interaction zone.

DOI
10.1088/0953-4075/28/11/008

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We calculate the contribution of the atomic shot noise in the detection of the laser-induced fluorescence signal emitted by atoms in a beam. We point out a filtering of such a contribution which is the consequence of the finite transit time of the atoms through the laser beam. Moreover, we demonstrate that the distribution of the atomic velocities in the beam induces an extra noise which degrades the measured signal-to-noise ratio and modifies the observed statistics of the fluorescence photons emitted by the whole interaction zone.
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This thesis describes the laser resonance fluorescence correlation technique for single-atom velocity measurement. Using this technique, we have detected individual sodium atoms diffusing through a laser beam in a slow helium flow. From the width of the fluorescence bursts detected, the transit time for the diffusing atom is determined. This is the first measurement of the motion of a single atom in a buffer gas. A probability analysis was developed which allowed us to estimate the average burst size of emitted fluorescence photons by an atom traversing the laser beam. All these results were in general agreement with the theoretical predictions. With improvements, we will be able to measure the velocity of a single-atom either in a flow or in a vacuum. By averaging over many sodium atoms, the diffusion coefficients of sodium atoms in helium and argon buffer gases were investigated using this technique. The measured diffusion coefficients were found to be in reasonable agreement with theoretical predictions and previous experimental results. To our knowledge, this is the first application of resonance fluorescence correlation technique to the measurement of diffusion coefficient of fast moving atoms in gases.
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The detection of optical resonances by observation of the fluorescence light emitted from a fast atomic beam of Na interacting with a collinear laser beam is limited in sensitivity by optical pumping processes. In the experiment reported here, we take advantage of the optical pumping by using a second collinear laser beam, achieving then a gain in fluorescence signal. The effects of the lasers power and polarization on line shapes have been studied in the single laser excitation case as well as in the two laser beams one.