  We demonstrate a novel approach to obtain resonance linewidth below that
limited by coherence lifetime. Cross correlation between induced intensity
modulation of two lasers coupling the target resonance exhibits a narrow
spectrum. 1/30 of the lifetime-limited width was achieved in a
proof-of-principle experiment where two ground states are the target resonance
levels. Attainable linewidth is only limited by laser shot noise in principle.
Experimental results agree with an intuitive analytical model and numerical
calculations qualitatively. This technique can be easily implemented and should
be applicable to many atomic, molecular and solid state spin systems for
spectroscopy, metrology and resonance based sensing and imaging.
