  We investigate the entangling capability of passive optical elements, both
qualitatively and quantitatively. We present a general necessary and sufficient
condition for the possibility of creating distillable entanglement in an
arbitrary multi-mode Gaussian state with the help of passive optical elements,
thereby establishing a general connection between squeezing and the
entanglement that is attainable by non-squeezing operations. Special attention
is devoted to general two-mode Gaussian states, for which we provide the
optimal entangling procedure, present an explicit formula for the attainable
degree of entanglement measured in terms of the logarithmic negativity, and
discuss several practically important special cases.
