Spontaneous emission spectrum from a V-type artificial atom in a strong-coupling regime: Dark lines and line narrowing

We employ a time-dependent variational approach with the multiple Davydov ${D}_{2}$ ansatz and two approximate analytical methods to study the spontaneous emission of a V-type artificial atom in the strong-coupling regime, where the decay rate of the emitter becomes a considerable fraction of its transition frequencies, and focus on quantum-interference-induced effects. The variational approach is found to be accurate in certain strong-coupling regimes and is used as the benchmark to address the validity of the analytical methods: the rotating-wave approximation (RWA) and the transformed RWA (TRWA). It is found that the TRWA is fairly accurate in the strong-coupling regime where the RWA breaks down. By using the numerical and analytical methods, we illustrate that there are dark lines and line narrowing in the emission spectra in the strong-coupling regime and in a wide accessible range of the emitter parameters. We also illustrate how the emission spectrum is altered by the counter-rotating couplings. The present results offer insights into the experimental observation of the quantum-interference-induced effect in the strong-coupling regime and in the context of artificial atoms.