  We present the first observation of quantum interference in the process phi
-> KS KL ->pi+pi-pi+pi-.
  This analysis is based on data collected with the KLOE detector at the e^+e^-
collider DAFNE in 2001--2002 for an integrated luminosity of about 380pb^-1.
  Fits to the distribution of Delta t, the difference between the two kaon
decay times, allow tests of the validity of quantum mechanics and CPT symmetry.
  No deviations from the expectations of quantum mechanics and CPT symmetry
have been observed. New or improved limits on various decoherence and CPT
violation parameters have been obtained
