  Entanglement enhancement is a key task for quantum technologies. This
operation performed on states produced by parametric down-conversion sources
has been the object of several recent experimental investigations. In
particular, conditional preparation by photon-subtraction has been shown to
improve the entanglement of these states. Here we analyse the role played by
non-Gaussian and Gaussian measurements in more general entanglement
concentration operations performed on a pair of two-mode squeezed vacua. We
find stringent requirements for achieving an improved entanglement enhancement
by measuring jointly these two resource states.
