  We isolate a novel four-wave mixing process, enabled by Coherent Population
Trapping (CPT), leading to efficient phase sensitive amplification. This
process is permitted by the exploitation of two transitions starting from the
same twofold degenerate ground state. One of the transitions is used for CPT,
defining bright and dark states from which ultra intense four-wave mixing is
obtained via the other transition. This leads to the measurement of a strong
phase sensitive gain even for low optical densities and out-of-resonance
excitation. The enhancement of four-wave mixing is interpreted in the framework
of the dark-state polariton formalism.
