  We propose the use of intra-particle entanglement to enhance the security of
a practical implementation of the Bennett-Brassard-1984 (BB84) quantum key
distribution scheme. Intra-particle entanglement is an attractive resource
since it can be easily generated using only linear optics. Security is studied
under a simple model of incoherent attack for protocols involving two or all
five mutually unbiased bases. In terms of efficiency of secret key generation
and tolerable error rate, the latter is found to be superior to the former. We
find that states that allow secrecy distillation are necessarily entangled,
though they may be local. Since more powerful attacks by Eve obviously exist,
our result implies that security is a strictly stronger condition than
entanglement for these protocols.
