Quantum Key Distribution Based on Random Grouping Bell State Measurement

We propose a four-qubit quantum key distribution protocol via two Bell states which constitute a transmitted unit from the sender to the receiver in each communication. An encryption here is designed by randomly grouping four qubits of a unit into two new couples, which is a way to increase the possibility of detecting the eavesdropper. Ultimately, the receiver randomly measures this grouped unit with two Bell state measurements. From the comparison of grouping information of these four qubits, we find that the two sides in a valid communication can discover the illegal party in the channel. In the proposed protocol, the receiver measures the unit when he receives it instantly, which is an efficient way to overcome the ultrashort storage time of quantum state.