  Subnatural-linewidth single photons are ofvital importance in quantum optics
and quantum information science. According to previous research, it appears
difficult to utilize resonance fluorescence to generate single photons with
subnatural linewidth. Here we propose a universally applicable approach to
generate fluorescent single photons with subnatural linewidth, which can be
implemented based on {\Lambda}-shape and similar energy structures. Further,
the general condition to obtain fluorescent single photons with subnatural
linewidth is revealed. The single-photon linewidth can be easily manipulated
over a broad range by external fields, which can be several orders ofmagnitude
smaller than the natural linewidth. Our study can be easily implemented in
various physical platforms with current experimental techniques and will
significantly facilitate the research on the quantum nature of resonance
fluorescence and the technologies in quantum information science.
