On the information-splitting essence of two types of quantum key distribution protocols

With the help of a simple quantum key distribution (QKD) scheme, we discuss the relation between BB84-type protocols and two-step-type ones. It is shown that they have the same essence, i.e., information-splitting. More specifically, the similarity between them includes (1) the carrier state is split into two parts which will be sent one by one; (2) the possible states of each quantum part are indistinguishable; (3) anyone who obtains both parts can recover the initial carrier state and then distinguish it from several possible states. This result is useful for related scheme designing and security analyzing.