(color online) The basic principle of the interaction between the three-level atom trapped in the low-Q cavity and the single-photon pulse. (a) A three-level atom is trapped in the low-Q cavity. (b) The three-level atom has an excited state |e〉 and two degenerate ground states |g _L〉 and |g _R〉, respectively. The states |g _L〉 and |g _R〉 couple with a left (L) polarized and a right (R) polarized photon, respectively.