Can we use photons in a Bose-Einstein condensate? If not then why? If yes then how? Which kind of boson are we using in Bose-Einstein condensation?
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Wikipedia: “BECs have also been realized using molecules, quasi-particles, and photons.” – G. Smith May 20 '21 at 04:37
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For the “how”, read Bose-Einstein condensation of photons in an optical microcavity. – G. Smith May 20 '21 at 04:41
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Laser light IS the photon equivalent of a Bose-Einstein condensation, or rather the Bose-Einstein condensate is the matter-wave equivalent of laser light. Both of which are a collection of bosons in the same quantum state. – R. Rankin May 20 '21 at 07:25
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@R.Rankin A laser is not a BEC. They can both be described by a coherent state, sure, but a BEC is an equilibrium state, a laser is a steady-state. – SuperCiocia May 21 '21 at 20:32
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One should point out that while the experimental mass limit on photons is extremely small, there is the possibility that photons are massive, after all, in which case a continued expansion of the universe may condense even photons... after a very long time and at an extremely low temperature. – FlatterMann Jun 15 '23 at 16:55
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Which kind of boson are we using in Bose-Einstein condensation?
Honestly, any boson that is experimentally feasible to trap, cool, and manipulate. To get a BEC, you need to start lowering the temperature, for which you need laser cooling. For laser cooling, you need lasers that address the specific atomic transition, and you look for ones that are commercially available. Up to now, people have Bose condensed H, He, Cs, Li, Na, Yb, Sr, Er, Dy, Ca and Cr, Rb, and K. Multiple isotopes. I heard they are trying Ti and Fe. The other day there was a paper on arxiv about Europium in a magnetic trap. Molecules formed by Fermi-Fermi or Bose-Fermi mixtures of the species listed above. Also, quasi-particles like excitons, polarons, etc.
Can we use photons in a Bose-Einstein condensate?
See this question and the answer.
Short answer: no, you cannot use free photons. Because they have zero chemical potential. You can, however, use photons in a cavity and dye in order to force them to interact among each other.
SuperCiocia
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Photons cannot cool as they always move with velocity c. They also have a very small probability to interact with each other, they only superpose forming interference patterns collectively, see this https://ocw.mit.edu/resources/res-6-006-video-demonstrations-in-lasers-and-optics-spring-2008/demonstrations-in-physical-optics/destructive-interference-2014-where-does-the-light-go/ . do you have a link for your last statement? – anna v May 20 '21 at 06:08
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OK, I found it in your previous answer "in which photons are repeatedly absorbed and re-emitted by the dye molecules." note, "absorbed and remitted" , not the same photon. A condensate may be made, but it is not really a photon condensate, but a collective optical effect creates the possibility. – anna v May 20 '21 at 06:29
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Yes ok 1) it's not really about cooling but about increasing phase space density. I was trying to be make a quick realistic example. 2) in that answer and especially in the comments we go through what the difference of "having photons in the ground state" and "forcing photons into the ground state", which is the process of Bose-Einsten condensation. For the latter, you need to artificially change the density of state from that of free space, so yes you play tricks with cavities, dyes etc. – SuperCiocia May 20 '21 at 07:04