  Coherent light-matter interaction at the single photon and electronic qubit
level promises the remarkable potential for nonclassical information
processing. Against the efforts of improving the figure of merit of the
cavities, here we demonstrate strong anharmonicity in the polariton dressed
states via dark state resonances in a highly dissipative cavity. It is shown
that vacuum Rabi oscillation occurs for a single quantum emitter inside a
cavity even with bosonic decay-to-interaction rate ratio exceeding $10^{2}$,
when the photon field is coupled to an auxiliary high-$Q$ cavity. Moreover,
photon blockade is observable in such a highly-dissipative cavity quantum
electrodynamics system. This study provides a promising platform for overcoming
decoherence and advancing the coherent manipulation of polariton qubits.
