Quantum secure direct communication with SU(2) invariant separable polarisation- OAM states

We propose a quantum secure direct communication protocol using SU (2)- invariant $2\times N$ separable states, identified as separable equivalents of two-qubit entangled Werner states in [Bharath & Ravishankar, Phys. Rev. A 89, 062110}]. These states can be experimentally realized as incoherent superpositions of separable polarisation-OAM states.