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