Subnatural linewidth using electromagnetically induced transparency in Doppler-broadened vapor

We obtain subnatural linewidth (i.e. $<\mathrm{\Gamma }$) for probe absorption in room-temperature Rb vapor using electromagnetically induced transparency (EIT) in a $\mathrm{\Lambda }$ system. For stationary atoms, the EIT dip for a resonant control laser is as wide as the control Rabi frequency ${\mathrm{\Omega }}_c$. But in thermal vapor, the moving atoms fill the transparency band so that the final EIT dip remains subnatural even when ${\mathrm{\Omega }}_c>\mathrm{\Gamma }$. We observe linewidths as small as $\mathrm{\Gamma }\mathrm{/}7$ in the $D_2$ line of Rb.