Excess noise acquired by a laser beam after propagating through an atomic-potassium vapor.
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We have found that an intense shot-noise-limited laser beam tuned near the 4 2S1/24 2P3/2 potassium resonance transition acquires excess noise after passing through an atomic-potassium vapor cell. The noise is maximum for laser detunings of approximately 1 GHz and falls to nearly the shot-noise limit for detunings greater than 3 GHz. We describe the production of this noise in terms of a forward four-wave mixing process involving the laser field and its side modes, which are initially in the vacuum state. We present a fully quantum-mechanical theory of forward four-wave mixing in a system of two-level atoms and use it to predict the noise properties of the transmitted laser beam. The predictions of this theory are in good agreement with the experimental data. 1995 The American Physical Society.