# Article about creating molecules from photon

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1. Jan 12, 2015

### bubblewrap

I read an article about creating molecules from photon, and this part-

As the photons enter the cloud of cold atoms, Lukin said, its energy excites atoms along its path, causing the photon to slow dramatically. As the photon moves through the cloud, that energy is handed off from atom to atom, and eventually exits the cloud with the photon.

this says that a photon, giving off energy, slows down, and I thought that when light loses energy, its wavelength gets longer, is this case in any way different?

2. Jan 12, 2015

### Doug Huffman

"Not even wrong!"

3. Jan 12, 2015

### Staff: Mentor

We have some guys around here who have seen photon Bose-Einsten condensates - I would like to get their view first.

Thanks
Bill

4. Jan 12, 2015

### Doug Huffman

Bose condensates beggar the meaning of molecule.

5. Jan 12, 2015

### Staff: Mentor

The article looks far fetched. But popular accounts of genuine phenomena can be like that. I would simply like the comments of some of the more experienced posters on this forum.

Had a look at the journal article associated with it - that looks far less controversial and far fetched:
http://www.nature.com/nature/journal/v502/n7469/full/nature12512.html

It's quite likely simply a combination of populist 'sensationalism' and misunderstanding.

Thanks
Bill

Last edited by a moderator: May 7, 2017
6. Jan 12, 2015

### Staff: Mentor

I think when reading a populist article we should look at the actual science its based on in the link to the journal write-up.

Thanks
Bill

7. Jan 12, 2015

### light

8. Jan 12, 2015

### Staff: Mentor

Quite likely.

The real article is the journal entry it links to.

Thanks
Bill

9. Jan 12, 2015

### light

Much appreciated guys :)

10. Jan 12, 2015

### Staff: Mentor

11. Jan 12, 2015

### vanhees71

Well, usually the popularized versions of articles are hard to understand for physicists. Sometimes, if you have enough expertise about the subject, to read the original article helps.

In this case, it's hopeless to popularize the issue in a way that it is understandable to laymen. I'll attempt to try this anyway below ;-). What's been talked about are not photons in free space (which are already hard enough to understand; almost all popularizations are wrong at some point; the best I know is the popular book on QED by Feynman, having of course no real "bugs" in its explanation) but about quasi-photons in a medium.

One can only very vaguely explain the ingenious idea of "quasi-particles", invented by Landau in condensed-matter physics (particularly his famous Fermi-liquid theory and the theory on liquid Helium). First, one has to remind that what's called "a particle" in the quantum sense is far from what we understand under "particle" in classical physics. In classical physics it's a macroscopic object, where I consider a situation, for which the finite extension of this object is neglible.

E.g., for the motion of the Earth and the other heavenly bodies around the Sun, you can to a good precision take them as "particles". Then you describe their motion as trajectories in space from the equations of motion, given the forces acting among the "particles" (in this case Newton's gravitational force).

This picture becomes wrong in a pretty dramatic way when it comes to the atomic and subatomic scale, i.e., when you deal with very small things. Then you need quantum theory as discovered in 1925/26 by Heisenberg, Born, Jordan, and Schrödinger, and Dirac. This non-relativistic theory is good enough for atomic physics (of not too large atoms), molecular, condensed-matter, and low-energy nuclear physics, but not for "particles" scattering at each other at higher energies, where you need to take into account relativistic effects.

Photons are relativistic in the extreme. They are massless "particles" and there's not even a conserved number-like quantity, i.e., you can easily create and destroy them in interactions. Because of this you have to work in an even more abstract formalism, called relativistic quantum-field theory (in this case Quantum Electrodynamics).

Now, in a medium everything becomes even more complicated than in the vacuum, but sometimes you are lucky, and to a high precision you can work in a description, which looks pretty much like the formalism of quantum field theory for particles in the vacuum. This means that while you deal with some complicated collective excitations of the medium as a whole, which involves in reality many quantum particles, you can reformulate these excitations in a theory that looks like quantum particles with different properties than their free analoga, but you can apply the same math.

A simple example are the vibrations of a crystal lattice, which can be used as a starting point to describe a solid body as a whole, quantum mechanically. The most simple way to understand it is to start with a pretty classical picture and think about it as an elastic continuous body which can get into oscillations. These are nothing else than sound waves propagating in the solid. In the analysis it turns out that in the approximation valid for small vibrations, that the entire body can be described as a harmonic oscillator with various vibrational modes, and you learn in quantum kindergarten how to deal with such harmonic oscillators! Now, if you consider a quantum field theory of non-interacting quantum particles, it turns out that these particles are described in an equivalent way by assuming a system of independent quantum harmonic-oscillator modes. So in turn you can map the sound wave excitations of the solid to a set of "quasi-particles". These you call "phonons" (Greek for "sound particles") Of course, there are no real particles involved, but the math is totally equivalent. Of course you can refine the model by making the phonons interacting with (quasi-)electrons or defects or what not other quasi-particles might be useful to describe the various phenomena related to the solid body. With the phonon picture, even before the full quantum theory was developed, Einstein and a bit later Debye could finally understand the up to then enigmatic behavior of the heat capacity of solids at very low temperature.

In the present case, the "medium" is a gas of highly excited atoms ("Rydberg states" are just high energy atomic bound states, large principle quantum number). Here, contrary to the behavior of photons (the quanta of the electromagnetic field) in the vacuum the corresponding quasi-photons become effectively very strongly interacting, which you can describe as the scattering of quasi-photons among themselves. In the vacuum such a process also exists, but it's a higher-order correction, i.e., to lowest order in perturbation theory a $\mathcal{O}(\alpha_{\text{em}}^4)$-box Feynman diagram in QED (the socalled Delbrück scattering). In the medium the analogous process, i.e., scattering of "light by light" contrary to this, is pretty strong.

12. Jan 12, 2015

### Staff: Mentor

As usual Vanhees superb explanation.

Thanks
Bill