We are interested in quantum optics and applying its methodology in studying the collective effects of radiation and many-body systems. We have been working on resonant dipole-dipole interactions in super- and sub-radiance [PRA(2017)]; subradiance from a chiral-coupled atomic chain in an atom /nanofiber or atom/waveguide systems; quantum communication via frequency conversion [Nature Physics(2010)] and spectral entanglement; spin-incoherent Luttinger liquid [PRA(2018)]; EIT as a probe of strongly interacting many-body systems [PRA (Rapid)(2013)]; DMRG study of topological excitation in a 1D chain. These studies show rich opportunities of building an interface to access many-body states as a control over quantum information and to investigate the fundamental aspects of light-matter interactions.