So we all learn in E&M and Optics courses that light is an EM wave. We play with simple models of light in which we treat a beam as a single frequency sine wave occupying the entire space. This of course is wrong since all physical signals have finite support (they are non-zero only on a finite interval) and are thus more accurately called wave packets. Anyhow, any wave packet(adsbygoogle = window.adsbygoogle || []).push({}); canbe turned into a superposition of sine waves over some distribution of frequencies each of which has a well defined phase velocities. So now comes a sequence of questions:

1) Are the component waves physical? Does each of these sine waves have an interpretation in terms of solely photons?

-If YES:

What is the relation between the EM wavesphase velocityand the properties of the photons (can EM phase velocity be derived from the photons wave function)?

-If NO:

What is the correct way of decomposing a signal into its physical constituents (photons)? Using this method, can we determine how many photons are in a signal? What does the signal of a single photon look like?

2) Reverse question: If we have a whole ensemble of photons, and we measure it classically as a signal, what would it look like? Assume that we know the states of each photon, and if necessary assume that they are propagating in the same direction.

Its a lot of questions and a tall order, but the reason I ask this is because I want to see the connection between quantum and classical optics. Phase velocity is such an important thing in classical optics that I figure that its crucial that I grasp this concept. Thanks in advance guys!

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# Phase velocities and photons

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