# Questions in the quantum theory of radiation

1. Nov 2, 2006

### Abu Abdallah

I have several questions in quantum optics or the quantum theory of radiaition. I begin by the following three questions:

How can a DC field ( static field) be constructed using photons ??

If the spin of photons is related to circular polarization of the constructing fields. How can linear polarized fields be constructed using photons.

What is the temporal and spatial exension of photons? The single photon number state makes the spatial extension of the photon bounded only by the dimensions of the cavity in which we quantize the field !!

2. Nov 3, 2006

### StatMechGuy

First of all, read the "size of the photon" thread earlier. Photons don't have a "size" since you cannot localize a photon.

I'm not sure what you mean by a DC field? A constant electric field is set up by a superposition of photons that yield that field. Write down a general linear superposition of photon number states, write down the electric field operator, and calculate what the coefficients have to be to yield that $$\langle \mathbf{E} \rangle$$ is a constant.

As for the spin of the photon, I think I'll punt on that one and let someone else explain, since I'm a little fuzzy myself about how photon spin works.

3. Nov 3, 2006

### actionintegral

An electric field consists of photons with infinite wavelength.

4. Nov 3, 2006

### Meir Achuz

1. A DC field has no photons.
2. A linear polarized field is a coherent linear combination of R and L fields, classically or QMly.
3. In terms of QED, the photon is a point particle.
The spatial extension is of the wave function of the photon.

5. Nov 3, 2006

### actionintegral

I disagree. Show me any EM field that contains photons. I will then redshift it as far as you please to render it DC. Do the photons disappear? Of course not! Their wavelength approaches infinity, that's all.