Bose-Einstein Condensation (BEC)

Discovered in space !

According to quantum mechanics atoms have wave-like properties and a corresponding deBroglie wavelength. Recent laser trapping and cooling experiments have succeeded in slowing atoms to such small velocity spreads that their wavelength corresponds to the average interparticle separation. If the atom behaves like a boson, the wavefunction of all the atoms in the condensate collapse into a macroscopic quantum state consisting of a superposition of identical wavefunctions. This 'fifth' state of matter has coherence properties similar to laser light which can also be considered as a condensate of particles called photons. Lasers have been discovered in stellar atmospheres, hence there are naturally occurring BECs in outer space !

Comparison of natural photon BEC and Atom BEC

  1. In both condensates, rapid cooling plays a crucial role.
  2. The cooling method of atom BECs is by laser momentum transfer and evaporative cooling. In natural photon BECs it is the stellar atmosphere as a whole which is 'evaporating' from the photosphere or visible surface of the star. In this process, adiabatic expansion cooling occurs and contact with a colder gas.
  3. Lasers are involved in photon BECs like in most atom BEC's where lasers are used for cooling and trapping.
  4. The matter density in both cases are similar.
  5. Naturally occuring photon BEC's have been around long before we were able to harness them in the laboratory.
  6. Photon BEC are made of masseless particles travelling at the speed of light, as opposed to atom BEC in which the condesate can have an arbitrary macroscopic velocity. (note: photon BEC's can have an arbitrary wavelength and polarization)
  7. Photon condensates can exist at any ambient temperature whereas atom Bose-Einstein condensates must be thermally isolated.
  8. The temperature of atom BECs are near absolute zero, much colder than outer space which is permeated whith radiation of temperature of 3 degrees Kelvin. A photon BEC produced by a star travels trough gases of hundreds of thousands of degree Kelvin then into interstellar space at 3 degrees Kelvin (there is also a hot ion component of interstellar space at temperatures of thousands of degrees Kelvin). Then trough out atmosphere at several hundred Kelvin.
  9. Photons can pass trough gases and transparent solids, whereas atoms must travel trough a vacuum.
  10. The dimensions over which wavefunction coherence of macroscopic quantum state is achieved is many orders of magnitude larger than any condensate in the lab.
  11. As opposed to atom condensates, three body collisions are essential to achieve electron distributions favouring photon condensation into a coherent macroscopic wavefunction.
  12. Although there is no possible experimental control over the attributes of natural Bose-Einstein Condensates, they are however easier to observe: The spectral signature of extraterrestrial photon condensation can be visually observed with an amateur telescope and a diffraction grating.

References

  1. News from the B. Verhaar Group, Eindhoven Univ.
  2. J.T.M. Walraven (ENE July-August 1995)
  3. Intro by P. Ruprecht from Oxford
  4. 'Bose-Einstein Condensation', Griffin,A., Snoke,D.W., Stringari,S. (eds)
  5. Cu_2O Excitonic condesates
  6. BEC page, Georgia Southern University.
  7. JILA (NIST/UColorado).
  8. JILA BEC press release.
  9. R. Hulet Group, Rice University.
  10. Na condensate, Ketterle,W. et al., MIT (old site AMO Physics is down)
  11. D. Heinzen Group, University of Texas. (down)
  12. Biography of S. N. Bose.
  13. C. J. Foot Group, Oxford University.
  14. K. Libbrecht Group, Caltech.
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