Coherence Length of Cold Exciton Gases in Coupled Quantum Wells
  A Mach-Zehnder interferometer with spatial and spectral resolution was used to probe spontaneous coherence in cold exciton gases, which are implemented experimentally in the ring of indirect excitons in coupled quantum wells. A strong enhancement of the exciton coherence length is observed at temperatures below a few Kelvin. The increase of the coherence length is correlated with the macroscopic spatial ordering of excitons. 
Sen Yang, A.T. Hammack, M.M. Fogler, L.V. Butov, and A.C. Gossard