Retrieving the Covariance Matrix of an Unknown Two-Mode Gaussian State   by Means of a Reference Twin Beam
  A method for revealing the covariance matrix of an unknown two-mode Gaussian state is given based on the interference with a reference twin beam whose covariance matrix is known. In the method, first- and second-order cross-correlation intensity moments are determined varying the overall phase of the reference twin beam. 
Ievgen I. Arkhipov, Jan Pe\v{r}ina Jr
quant-ph