  We propose an entanglement distillation and purification scheme for symmetric
two-mode entangled Gaussian states that allows to asymptotically extract a pure
entangled Gaussian state from any input entangled symmetric Gaussian state. The
proposed scheme is a modified and extended version of the entanglement
distillation protocol originally developed by [Browne et al., Phys. Rev. A 67,
062320 (2003)]. A key feature of the present protocol is that it utilizes a
two-copy de-Gaussification procedure that involves a Mach-Zehnder
interferometer with single-mode non-Gaussian filters inserted in its two arms.
The required non-Gaussian filtering operations can be implemented by coherently
combining two sequences of single-photon addition and subtraction operations.
