  We propose a scheme for generating a high-purity single photon on the basis
of cavity quantum electrodynamics (QED). This scheme employs a four-level
system including two excited states, two ground states, and two driving lasers;
this structure allows the suppression of the re-excitation process due to the
atomic decay, which is known to significantly degrade the single-photon purity
in state-of-the-art photon sources using a three-level system. Our analysis
shows that the re-excitation probability arbitrarily approaches zero without
sacrificing the photon generation probability when increasing the power of the
driving laser between the excited states. This advantage is achievable by using
current cavity-QED technologies. Our scheme can contribute to developing
distributed quantum computation or quantum communication with high accuracy.
