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phonon44145
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Does the photon number state of n photons in the same mode (i.e. Fock state |n>) constitute a Bose-Einstein condensate of photons?
Bill_K said:But if you cool a photon gas the number of photons simply decreases and Bose-Einstein condensation never occurs.
Cthugha said:It is realized for a completely delocalized photon number state, but these are pretty theoretical constructs, as far as I know.
Cthugha said:A photon number state is not a BEC because it will lack coherence. If you take for example Legett's book on quantum fluids, you will see that you also need off-diagonal long-range order, which roughly translates into long-range spatial coherence or a fixed phase relationship over larger distances. This is not necessarily realized for a realistic photon number state (remember photon number-phase uncertainty). It is realized for a completely delocalized photon number state, but these are pretty theoretical constructs, as far as I know.
What's the theoretical justification for this? Is it theoretically impossible for the amount of photons to stay the same but for their frequency to decrease? Maybe because of momentum concerns?Bill_K said:No. For a closed system of bosons with a finite mass the number of particles is fixed, and if it is cooled eventually a critical temperature Tc is reached below which a finite fraction of them are forced to occupy the ground state. But if you cool a photon gas the number of photons simply decreases and Bose-Einstein condensation never occurs.
DrDu said:The important point with a BEC being a BEC is that it has to be an equilibrium state at some (low) temperature. A laser is not an equilibrium state.
The Photon BEC described by Martin Weiz is not a condensate of free photons but rather of photons interacting with matter, i.e. polaritons.
DrDu said:Anyhow a BEC is strictly defined only in the thermodynamical limit when N goes to infinity and the density N/V is constant.
Yes, but for particles which vanish upon detection like photons this is somewhere between very hard and impossible to achieve. It is easier for atoms.DrDu said:Off diagonal long range order can also be realized in states with fixed particle number.
Cthugha said:Weitz himself strongly opposes that point of view. To enter the polariton regime, interaction with matter is not enough, but you need strong coupling (or in the semiclassical regime non-perturbative coupling) between the light field and matter excitations which amounts to reversible spontaneous emission. I am quite sure that is not the case in the Weitz-paper as scattering rates are too small and he operates in the weak coupling regime. That of course does not mean no coupling and it should indeed be stressed that these are not free photons, but cavity photons. The question is rather whether he found a good way to have a "masked" common laser.
A Bose-Einstein condensate of photons is a state of matter where a large number of photons, the fundamental particles of light, are in the same quantum state and behave like a single macroscopic particle.
To create a Bose-Einstein condensate of photons, a dilute gas of photons must be cooled to extremely low temperatures, close to absolute zero. This causes the photons to lose their individual identities and form a collective state.
A Bose-Einstein condensate of photons exhibits unique properties such as coherence, where all the photons are in phase with each other, and superfluidity, where the photons can flow without any resistance.
Bose-Einstein condensates of photons have potential applications in quantum computing, optical communications, and precision measurement. They also provide a platform for studying quantum phenomena and testing fundamental physics theories.
Bose-Einstein condensates of photons are unique as they are created from particles with no mass and do not require the use of external forces or confinement to form. This distinguishes them from other types of condensates, such as Bose-Einstein condensates of atoms or excitons.