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8) a Gaussian function of τ with a width inversely proportional to the frequency spread. For a narrow frequency spread, ∆ν small, we have a large coherence length whilst for a broad spectrum, ∆ν large, the coherence length becomes very short. It is this first order coherence length which is measured by the Stellar interferometer. Now let us consider intensity correlation measurements. As a generic example we consider direct detection of the intensity of two beams that have not been optically mixed, but one of which has a variable time delay imposed on it before detection.

It can, however, determine a transfer of energy from the radiation field to the atoms in the detector. And from this process we can conclude properties of the light intensity. What are the experimental details of the process of photodetection? One fundamental fact is the resonance like property of the interaction. Atoms interact only with radiation that matches their eigen-frequencies. The absorption and emission of light results in very specific spectra. For free atoms, as those in a gas, these spectra have a resonant structure; they consist of several narrow well defined lines.

22) 32 2 Classical models of light where φj represents the phase of the individual contributions. For simplicity we assume that all atoms emit into the same polarisation. For a large number of atoms |α(t)| and φ(t) are effectively randomly varying functions which describe the magnitude and the phase of the total amplitude at any time t. 23) The time interval τ0 is usually very short. If we assume that a single measurement takes a time larger than τ0 , the coherence function can be evaluated as: α∗ (t)α(t + τ ) = α02 exp(i2πν0 τ ) [exp(−iφ1 (t)) + · · · + exp(−iφM (t))] [exp(iφ1 (t + τ )) + · · · + exp(iφM (t + τ ))] In multiplying out the large brackets many cross terms appear which correspond to waves emitted by different atoms.

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AGS Experiments [1994-1996]


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