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Physical Review A

Enhancement of the two-photon blockade in a strong-coupling qubit-cavity system

Wenwu Deng, Gao‐xiang Li, Hong Qin

Published Apr 22, 2015• 50 citations
Publisher

Abstract

The output incoherent spectrum and the photon blockade for the two-photon near-resonant excitation are numerically investigated by using the modified theories which are valid for an arbitrary degree of the qubit-cavity interaction. Owing to the effects of strong coupling of the system, the relaxation coefficients of the system are very sensitive to the coupling strength between the qubit and the cavity mode, which results in the redistribution of the populations and the appearance of the population inversion between the ground state and the first excited state, and it is demonstrated by the intensity of the output incoherent spectrum. The redistribution of the populations leads to one cascaded decay channel being weakened and the other possibly being enhanced significantly. Moreover, by adjusting the detuning between the qubit and the cavity mode, the phenomenon of the intercrossing in the energy levels appears. Through choosing the proper two-photon near-resonant excitation of the external classical driving field, the strong-coupling qubit-cavity system effectively causes one cascaded decay channel to realize the two-photon blockade. It is shown that the two-photon blockade can be further enhanced by one cascaded decay channel.

DOI
10.1103/physreva.91.043831

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The output incoherent spectrum and the photon blockade for the two-photon near-resonant excitation are numerically investigated by using the modified theories which are valid for an arbitrary degree of the qubit-cavity interaction. Owing to the effects of strong coupling of the system, the relaxation coefficients of the system are very sensitive to the coupling strength between the qubit and the cavity mode, which results in the redistribution of the populations and the appearance of the population inversion between the ground state and the first excited state, and it is demonstrated by the intensity of the output incoherent spectrum. The redistribution of the populations leads to one cascaded decay channel being weakened and the other possibly being enhanced significantly. Moreover, by adjusting the detuning between the qubit and the cavity mode, the phenomenon of the intercrossing in the energy levels appears. Through choosing the proper two-photon near-resonant excitation of the external classical driving field, the strong-coupling qubit-cavity system effectively causes one cascaded decay channel to realize the two-photon blockade. It is shown that the two-photon blockade can be further enhanced by one cascaded decay channel.
In a coherently-driven nanocavity QED system, the one-photon blockade via quantum interference is investigated by the modified Lindblad master equation and without using the secular approximation as well. Based on the dressed bases of the Rabi Hamiltonian, a modified Lindblad master equation is obtained, which is valid for any arbitrary degree of the qubit-cavity interaction. It is found that the damping coefficients are very sensitive to interaction strength between the qubit and the cavity mode. How to enhance the one-photon blockade by using the quantum interference effect is discussed with the generalized second-order correlation function and the second-order perturbation in the five-state truncation of the Hilbert space. It is found that, under suitable pump or detection conditions, a strong one-photon blockade can be realized by completely eliminating the two-photon emission. Moreover, even for a strong cavity damping rate, there exhibits a large number of cavity photons by utilizing the quantum interference mechanism.
The antibunching effect generated in a cavity containing first a single two-level atomic system and then two identical atoms is explored for off-resonant interactions, in strong- and weak-coupling regimes. The cavity is driven by coherent and squeezed sources. Using the second-order correlation function analytically derived in the weak-excitation regime, we show that squeezed light achieves and improves strong antibunching, leading to a photon blockade. This enhancement is due to a destructive quantum interference mechanism induced by the nonclassical light, creating a supplementary transition pathway. The best effect is obtained for equal frequency detunings in the strong-coupling regime. More interestingly, our investigation reveals that the two-atom-cavity system can further improve the antibunching effect compared to the single-atom cavity. It turns out that the additional couplings appearing in the cavity due to the implementation of the two atoms, combined with the squeezed light, could substantially enhance the photon-blockade effect.
A two-level system interacting with a cavity field is an important model for investigating the photon blockade (PB) effect. Most work on this topic has been based on the assumption that the atomic transition frequency is resonant with the fundamental mode frequency of the cavity. We relax this constraint and reexamine PB in a more general atom--cavity system with arbitrary atomic and cavity detunings from a driving field. The results show that when the signs of the atomic and cavity detunings are the same, PB occurs only in the strong-coupling regime, but for opposite signs of the atomic and cavity detunings, strong photon antibunching is observed in both the weak- and strong-coupling regimes and a better PB effect is achieved compared with the case when the signs are the same. More interestingly, we find that this PB arises from quantum interference for both weak and strong nonlinearities. These results deepen our understanding of the underlying mechanism of PB and may be help in the construction of single-photon sources with higher purity and better flexibility using atom--cavity systems.
A two-level quantum emitter interacting with a cavity field is an important model for investigating the photon blockade (PB) effect. Most work on this topic relies on the assumption that the emitter transition frequency is resonant with the fundamental mode frequency of the cavity. In this work we relax this constraint and reexamine the PB effect in a more general emitter-cavity system with arbitrary emitter and cavity detunings from a driving field. The results show that when the signs of the emitter and cavity detunings are the same, a PB occurs only in the strong-coupling regime, but for opposite signs of the emitter and cavity detunings, strong photon antibunching is observed in both the weak- and strong-coupling regimes, and better antibunching is achieved compared with the case when the signs are the same. More interestingly, we find that this PB arises from quantum interference for both weak and strong nonlinearities. These results deepen our understanding of the underlying mechanism of PBs and may contribute to the construction of single-photon sources with higher purity and better flexibility using two-level emitter-cavity systems.
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.
We investigate theoretically the model of a cavity-quantum-electrodynamics (QED) system that consists of two two-level atoms coupled to a single-mode cavity in the weak coupling regime, where the system is driven by quantum light. The dynamics behavior of the entire system is tackled in the framework of a cascaded quantum system. We find that the two-photon blockade with two-photon bunching and three-photon antibunching can be obtained even when the strong system dissipation is included. This result shows that our work has potential for realizing entangled photon pairs in a weakly coupled cavity. Moreover, we also analyze the photon statistics of the system in the case of out-of-resonance coupling between cavity and two nonidentical atoms. Here, an unconventional photon blockade effect with suppression of two-photon correlation and enhancement of three-photon correlation can be realized, which shows many quantum statistical characteristics of a cavity QED system in weak coupling.