Fig. 1. Schematic diagram of collective attack in E91-QKD

Fig. 2. Schematic diagram of bipartite coupling. (a) Bipartite phase coupling; (b) Bipartite energy level transfer coupling

Fig. 3. Simulation of ${\chi }_{\mathrm{E};\mathrm{A}}$ with respect to $〈p|q〉$and $〈r|s〉$

Fig. 4. Comparison of the key rate for the security proofs of E91-QKD based on collective attack and entanglement purification protocol