  We propose a general scheme to generate entanglement encoded in the
photon-number basis, via a sequential resonant two-photon excitation of a
three-level system. We apply it to the specific case of a quantum dot
three-level system, which can emit a photon pair through a biexciton-exciton
cascade. The state generated in our scheme constitutes a tool for secure
communication, as the multipartite correlations present in the produced state
may provide an enhanced rate of secret communication with respect to a perfect
GHZ state.
