Subnatural linewidth in a strongly-driven closed $F \rightarrow F'$ transition

We observe linewidths below the natural linewidth for a probe laser on a {\it two-level system}, when the same transition is driven by a strong control laser. We take advantage of the fact that each level is made of multiple magnetic sublevels, and use the phenomenon of electromagnetically induced transparency or absorption in multilevel systems. Optical pumping by the control laser redistributes the population so that only a few sublevels contribute to the probe absorption. We observe more than a factor of 3 reduction in linewidth in the $D_2$ line of Rb in room-temperature vapor. The observations can be understood from a density-matrix analysis of the sublevel structure.