Determining the squeezing in multimode fields using nonlocal quantum   probes
  We present a scheme allowing to access the squeezing parameter of multimode fields by means of the dynamics of nonlocal quantum probes. The model under consideration is composed of two two-level systems which are coupled locally to an environment consisting of nonlocally correlated field modes given by two-mode Gaussian states. Introducing independently switchable interactions, one observes revivals of nonlocal coherences of the two-qubit system which are unambiguously connected to the squeezing parameter of the Gaussian environmental states. Thus, the locally interacting two two-level systems represent a dynamical quantum probe for the squeezing in multimode fields. It is finally demonstrated that perfectly reviving nonlocal coherences also persists for nonentangled correlated field modes and an explanation for this phenomenon is presented by connecting it to the correlation coefficient of the environmental coupling operators. 
Steffen Wi{\ss}mann and Heinz-Peter Breuer
quant-ph