Parametric up conversion of the vacuum
  The theory of parametric down conversion of the vacuum, based on a real zeropoint, or "vacuum" electromagnetic field, has been treated in earlier articles. The same theory predicts a hitherto unsuspected phenomenon - parametric up conversion of the vacuum. This article describes how the phenomenon may be demonstrated experimentally. 
Trevor W. Marshal (Dept. of Maths., Manchester Univ., Manchester M13   9PL, U.K)