de Sitter super-gravity in ambient space formalism

In the de Sitter ambient space formalism the massless fields, which include the linear gravity and massless minimally coupled scalar field, can be written in terms of two separate parts: a massless conformally coupled scalar field and a polarization tensor(-spinor) part. Therefore due to the massless conformally coupled scalar field, there exist an unique Bunch-Davies vacuum state for quantum field theory in the de Sitter space time. In the de Sitter ambient space formalism one can show that the massless fields with spin $sgeq 1$ are gauge invariant. By coupling the massless gauge spin-$2$ and the massless gauge spin-$3/2$ fields and using the super-symmetry algebra in de Sitter ambient space formalism, one can naturally construct a unitary de Sitter super-gravity on Bunch-Davies vacuum state.