Transfer of arbitrary two-qubit states via a spin chain

We investigate the fidelity of the quantum state transfer (QST) of two qubits by means of an arbitrary spin- 12network, on a lattice of any dimensionality. Under the assumptions that the network Hamiltonian preserves themagnetization and that a fully polarized initial state is taken for the lattice, we obtain a general formula for theaverage fidelity of the two qubits QST, linking it to the one- and two-particle transfer amplitudes of the spinexcitations among the sites of the lattice. We then apply this formalism to a 1D spin chain with XX-Heisenbergtype nearest-neighbour interactions adopting a protocol that is a generalization of the single qubit one proposedin Paganelli et al. [Phys. Rev. A 87, 062309 (2013)]. We find that a high-quality two qubit QST can be achievedprovided one can control the local fields at sites near the sender and receiver. Under such conditions, we obtainan almost perfect transfer in a time that scales either linearly or, depending on the spin number, quadraticallywith the length of the chain.