We theoretically investigate the quantum phase transition in the collective systems of qubits in a high quality cavity, where the cavity field is squeezed via the optical parametric amplification process. We show that the squeezed light induced symmetry breaking can result in quantum phase transition without the ultrastrong coupling requirement. Using the standard mean field theory, we derive the condition of the quantum phase transition. Surprisingly, we show that there exists a tricritical point where the first- and second-order phase transitions meet. With specific atom-cavity coupling strengths, both the first- and second-order phase transition can be controlled by the nonlinear gain coefficient, which is sensitive to the pump field. These features also lead to an optical switching from the normal phase to the superradiant phase by just increasing the pump field intensity. The signature of these phase transitions can be observed by detecting the phase space Wigner function distribution with different profiles controlled by the squeezed light intensity. Such superradiant phase transition can be implemented in various quantum systems, including atoms, quantum dots, and ions in optical cavities as well as the circuit quantum electrodynamics system.

We theoretically study the quantum-interference-induced photon blockade and antiblockade phenomenon in an atom-cavity QED system, where the destructive [constructive] interference between two different transition pathways prohibits [enhances] two-photon excitation. To begin with, we study a single-atom–cavity QED system where the atom or cavity is driven by a coherent pump field. We show that the cavity-driven case will lead to the quantum-interference-induced photon blockade under a specific condition, but such an interference-induced photon blockade cannot be realized in the atom-driven case. This is an important consequence of the pathways possible for the two types of drives. Then, we investigate the two-atom case, and find that an additional transition pathway appears in the atom-driven case. We show that this additional transition pathway results in the quantum-interference-induced photon blockade only when the atomic resonant frequency is different from the cavity mode frequency otherwise antiblockade occurs. Moreover, in this case, the condition for realizing the interference-induced photon blockade is independent of the system's intrinsic parameters, which can be used to generate antibunched photons in both weak and strong coupling regimes.

The single-photon routing properties in the system composed of two waveguides and two coupled cavities interacting with a two-level system are investigated theoretically. The phases of the coupling strengths are considered. The analytical expressions for the single-photon routing probabilities are obtained. The study shows that single-photon routing in a single waveguide or between two waveguides can be switched on or off by modulating the phase difference between the coupling constants. The analytical results also exhibit that the coupling between the two nanocavities plays an important role in the phase-dependent single-photon routing. We also show numerically the influence of the dissipation of cavities and atoms on routing properties. Our results may be useful in controlling and manipulating light-matter interactions based on waveguides at the single-photon level.

We present a study of manipulating and improving the multiphoton blockade phenomenon in a single-mode cavity with two cascade three-level atoms. Using an external control field, we show that the two-photon blockade can be changed to the three-photon blockade by just increasing the control field Rabi frequency when two atoms radiate in-phase. In the case of out-of-phase radiations, we show that the three-photon blockade phenomenon can be significantly improved with an increased mean photon number when the control field is present. We also show that the optical switching from the three-photon blockade $[g^{\left(2\right)}\left(0\right)>1$ and $g^{\left(3\right)}\left(0\right)<1]$ to the super-Poissonian field $[g^{\left(2\right)}\left(0\right)>1$ and $g^{\left(3\right)}\left(0\right)>1]$ can be accomplished by increasing the control field Rabi frequency. The results presented in this work provide potential applications in quantum communication and quantum networking.

We present a proposal to control and improve the nonclassicality of photons in a single-atom cavity-QED system with a strong coupling strength, where the atom is directly driven by external fields. Exploring the eigenvalues and the corresponding eigenstates of the system, we show that the dressed states can be optically manipulated by changing the control field intensity. In particular, tuning the control field frequency to be resonant with the one-photon excitation state, we show that the nonclassicality of cavity photons arising from the two-photon blockade can be actively controlled. We also show that there exists a magic control field Rabi frequency at which the two-photon blockade phenomenon can be significantly improved. The work presented here provides an optical method to control the statistical features of the cavity field that could find utility in nonclassical light generation, photon gateway operation, and exotic quantum-state generation.