Coherent Population Trapping Induced by Phase Modulated and Fluctuating Fields

We examine the effects of cross correlated phase fluctuations on the
coherent population trapping (CPT) induced by a pair of phase-modulated
fields with equal modulation frequencies in a three-level Λ system.
The maximal coherence of -0.5, which appears when CPT occurs for equal
modulation indices, is preserved in the presence of the critically
cross-correlated fluctuations. Unexpectedly, the non-maximal coherence,
which is established when CPT is obtained for different modulation indices, is
significantly enhanced due to the critically cross-correlated fluctuations.