  Quantum key distribution schemes which employ encoding on vacuum-one-photon
qubits are capable of transferring more information bits per particle than the
standard schemes employing polarization or phase coding. We calculate the
maximum number of classical bits per particle that can be securely transferred
when the key distribution is performed with the BB84 and B92 protocols,
respectively, using the vacuum-one-photon qubits. In particular, we show that
for a generalized B92 protocol with the vacuum-one-photon qubits, a maximum of
two bits per particle can be securely transferred. We also demonstrate the
advantage brought about by performing a generalized measurement that is
optimized for unambiguous discrimination of the encoded states: the parameter
range where the transfer of two bits per particle can be achieved is
dramatically enhanced as compared to the corresponding parameter range of
projective measurements.
