We propose a special cavity design that is constructed by terminating a one-dimensional waveguide with a perfect mirror at one end and doping a two-level atom at the other. We show that this atom plays the intrinsic role of a semi-transparent mirror for single photon transports such that quasi-normal modes (QNMs) emerge spontaneously in the cavity system. This atomic mirror has its reflection coefficient tunable through its level spacing and its coupling to the cavity field, for which the cavity system can be regarded as a two-end resonator with a continuously tunable leakage. The overall investigation predicts the existence of quasi-bound states in the waveguide continuum. Solid state implementations based on a dc-SQUID circuit and a defected line resonator embedded in a photonic crystal are illustrated to show the experimental accessibility of the generic model.
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