Explain Glauber's theory of optical coherence.
Let us consider the problem of describing light statistically. Previously, light was largely treated as either purely classical electromagnetic waves or as uncorrelated streams of particles. My work introduced a more nuanced approach. I defined 'coherent states' – special quantum states of the electromagnetic field that behave remarkably like classical waves, exhibiting perfect interference. Furthermore, I developed the Glauber-Sudarshan P representation, which allows one to represent any quantum state of light as a mixture of coherent states. This provides a powerful tool for calculating expectation values of normally ordered operators, which are crucial for understanding optical measurements. It is the coherence of the light, its phase and amplitude correlations, that dictates its observable properties.
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