## 6 Conclusions  
In summary, we have studied the photon statistics of a coupled cavity QED system that consists of a linear cavity and a nonlinear cavity that consists of a two-level atom. The linear cavity is weakly driven by a continuous laser field, and the two cavities are coupled by hopping strength \(J\). The single photon blockade effect can be observed in the linear cavity, both conventional and unconventional. Compared with existing schemes, we can achieve a conversion between a conventional photon blockade and an unconventional photon blockade in the same system; more importantly, we can also achieve the coexistence of two different photon blockades. In addition, we obtain the coupling between two cavities via photon hopping interactions, which can be realized by current experimental techniques. The photon blockade provides an effective method for obtaining a single photon source, which is the basis of realizing optical quantum information technology. Thus, our work has potential applications in quantum information processing and quantum communication. Being a theoretical work, we provide detailed theoretical derivation and numerical analysis. In our model, these photon blockade effects are mainly achieved by the quantum destructive interference between different excitation paths. We have analyzed the values of the equal-time second-order correlation function in the truncated Hilbert space by solving the master equation of the coupled cavities. In the same parameter regime, we also derived the optimal atom-cavity coupling strength and cavity detuning parameter pairs, which once again verify the symmetric photon blockade regime for the linear cavity. We also extend our model by adding a pump laser to Cavity B. We find that, under the condition of driving these two cavities simultaneously, the conventional photon blockade in Cavity A is dominant, while in Cavity B the unconventional photon blockade is apparent. In our scheme, the introduction of a two-levelatom provides an efficient method for the realization of an unconventional photon blockade in Cavity B. When Cavity B is manipulated by the pump laser, the interaction between the cavity and atom can be effectively controlled, which may affect the nonlinearity of the total system. Thus, we obtain the unconventional photon blockade in the case of weak nonlinearity. Here the pump laser and the two-level atom are well controlled experimentally, and the unconventional photon blockade is not obviously dependent on the decay rate of cavity. Therefore, we provide a feasible scheme for the realization of an unconventional photon blockade.  
Conceptualization, M.-C.L. and A.-X.C.; methodology, A.-X.C.; software, M.-C.L.; formal analysis, M.-C.L.; writing--original draft preparation, M.-C.L.; writing--review and editing, A.-X.C.; supervision, A.-X.C.  
This work was supported by the project of National Natural Science Foundation of China [grant number 11775190] and by Science and Technology of Jiangxi province [grant number 20171 BAB212006]; the project of science and technology awarded by the Education Bureau of Jiangxi Province [grant number JJ160503] and by the Science Foundation of Zhejiang SCI-TECH University (17062071-Y).  
The authors declare no conflict of interest.