Quantum key distribution using superposition of the vacuum and single   photon states
  B92-type and BB84-type quantum cryptography schemes using superposed states of the vacuum and single particle states which are robust against PNS attacks are studied. The number of securely transferred classical bits per particle (not per qubit) sent in these schemes is calculated and found to have upper bounds. Possible experimental realizations using the cavity QED or linear optics are suggested. 
Jae-Weon Lee, Jaewan Kim, Yong Wook Cheong, Hai-Woong Lee, Eok Kyun   Lee