Page "Coherent states" Paragraph 61

Roy J. Glauber's work was prompted by the results of Hanbury-Brown and Twiss that produced long-range ( hundreds or thousands of miles ) first-order interference patterns through the use of intensity fluctuations ( lack of second order coherence ), with narrow band filters ( partial first order coherence ) at each detector.

( One can imagine, over very short durations, a near-instantaneous interference pattern from the two detectors, due to the narrow band filters, that dances around randomly due to the shifting relative phase difference.

With a coincidence counter, the dancing interference pattern would be stronger at times of increased intensity to both beams, and that pattern would be stronger than the background noise.

) Almost all of optics had been concerned with first order coherence.

The Hanbury-Brown and Twiss results prompted Glauber to look at higher order coherence, and he came up with a complete quantum-theoretic description of coherence to all orders in the electromagnetic field ( and a quantum-theoretic description of signal-plus-noise ).

He coined the term coherent state and showed that they are produced when a classical electrical current interacts with the electromagnetic field.