Jae-Weon Lee^a, Eok Kyun Lee^a, Yong Wook Chung^b, Hai-Woong Lee^b, and Jaewan Kim^c

^a Department of Chemistry, School of Molecular Science (BK 21), Korea Advanced Institute of Science and Technology, Taejon 305-701, Korea
^b Department of Physics, Korea Advanced Institute of Science and Technology, Taejon 305-701, Korea
^c School of Computational Sciences, Korea Institute for Advanced Study, 207-43 Cheongryangri-dong, Dongdaemun-gu Seoul 130-012, Korea

A quantum cryptography scheme based on entanglement between a single-particle state and a vacuum state is proposed. The scheme utilizes linear optics devices to detect the superposition of the vacuum and single-particle states. Existence of an eavesdropper can be detected by using a variant of Bell's inequality.