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Topological photonic nanocavity provides a robust platform for realizing nano-photonic devices and studying light–matter interaction. Here, a topological photonic-plasmonic hybrid nanocavity, assembling a topological photonic crystal (PhC) nanocavity with a plasmonic nano-antenna, is proposed to have an ultra-high figure of merit Q/V of 1.5 × 10 6 ( λ / n ) − 3 , which is two orders higher than that of the bare topological PhC nanocavity. The single-atom cooperativity parameter is improved by over 60 times due to the large enhancement of Q/V, which makes the coupling between light and a single emitter enter a strong coupling region in topological photonic realm for the first time. Meanwhile, strong coupling and weak coupling can be easily switched in the topological hybrid system by tuning the structure dimension of plasmonic nano-antennas. This work provides a robust platform to control coupling phase transition between light and a single emitter, which has great potential in topological lasers, quantum optics and quantum information.

The phase field, which originates from the electronic interaction, plays an important role in describing a strongly correlated system in one dimension. However, in higher dimensions the effect of phase field cannot be obviously understood. With the eigenfunctional theory, we calculate the pair distribution function of the three-dimensional electron gas to study the relationship between the phase field and the electronic correlation effect and show that at zero temperature the correlation effect of the electrons is mainly dominated by the phase fluctuation, which is produced by the electronic interaction. We also discuss the failure of random phase approximation in studying the correlation function when the correlation effect is strong in the view of the phase field.