  We propose a theoretical scheme to achieve strong photon blockade via a
single atom in cavity. By utilizing optical Stark shift, the dressed-state
splitting between higher and lower branches is enhanced, which results in
significant increasing for lower (higher) branch and decreasing for higher
(lower) branch at the negative (positive) Stark shift, and dominates the
time-evolution of photon number oscillations as well. Furthermore, the
two-photon excitation is suppressed via quantum interference under optimal
phase and Rabi frequency of an external microwave field. It is shown that the
interplay between the quantum interference and the enhanced vacuum-Rabi
splitting gives rise to a strong photon blockade for realizing high-quality
single photon source beyond strong atom-cavity coupling regime. In particular,
by tuning the optical Stark shift, the second-order correlation function for
our scheme has three orders of magnitude smaller than Jaynes-Cummings model and
correspondingly there appears a large cavity photon number as well. Our
proposal may implicate exciting opportunities for potential applications on
quantum network and quantum information processing.
