  We present a scheme to realize a quantum key distribution using vacuum-one
photon entangled states created both from Alice and Bob. The protocol consists
in an exchange of spatial modes between Alice and Bob and in a recombination
which allows one of them to reconstruct the bit encoded by the counterpart in
the phase of the entangled state. The security of the scheme is analyzed
against some simple kind of attack. The model is shown to reach higher
efficiency with respect to prior schemes using phase encoding methods.
