  Recently some alternatives of the measurement device independent quantum key
distribution(MDI-QKD) based on the single-photon Bell state measurement (SBSM)
have been proposed. Although these alternatives are not precisely as secure as
MDI-QKD, they possess the advantage of high key rate of traditional BB84-like
protocol and avoid the technical complexity of two-photon interference required
in the MDI-QKD. However, the setups of these proposed schemes are rather
complicated compared to commonly used BB84 systems. Here we propose a simple
implementation of SBSM-based QKD which is directly built on the existing
realization of BB84 QKD. Our proposal exhibits the hidden connection between
SBSM-based QKD and traditional phase-coding QKD protocols. This finding
discloses the physics behind these two different types of QKD protocols. In
addition, we experimentally demonstrate the feasibility of our protocol.
