Preparation of Pure Gaussian States via Cascaded Quantum Systems
  This paper provides an alternative approach to the problem of preparing pure Gaussian states in a linear quantum system. It is shown that any pure Gaussian state can be generated by a cascade of one-dimensional open quantum harmonic oscillators, without any direct interaction Hamiltonians between these oscillators. This is physically advantageous from an experimental point of view. An example on the preparation of two-mode squeezed states is given to illustrate the theory. 
Shan Ma, Matthew J. Woolley, Ian R. Petersen and Naoki Yamamoto
quant-ph cs.SY math.OC