Subnatural linewidth for probe absorption in an electromagnetically-induced-transparency medium due to Doppler averaging

We obtain subnatural linewidth (i.e., $<\Gamma$) for probe absorption in room-temperature Rb vapor using electromagnetically induced transparency (EIT) in a \Lambda system. For stationary atoms, the EIT dip for a resonant control laser is roughly one-half as wide as the control Rabi frequency $\Omega_c$. But in thermal vapor, the moving atoms fill the transparency band so that the final EIT dip remains subnatural even when $\Omega _c \gg \Gamma$. We observe a linewidth of $\Gamma /4$ for $\Omega _c = 8\Gamma$ in the $D_2$ line of Rb