a GLYPH<RParen1> 775 780 785 790 795 0 50 100 150 200 250 Λ GLYPH<LParen1> nm GLYPH<RParen1> t D GLYPH<LParen1> ps GLYPH<RParen1> Λ c b GLYPH<RParen1> 785 787 789 791 0 5 10 15 Λ GLYPH<LParen1> nm GLYPH<RParen1> t D GLYPH<LParen1> ps GLYPH<RParen1> Fig. 2. (Color online) Experimental (points) and theoretical (solid lines) values of the wavelength-dependent slow light delay measured a) across the entire region around the D1 and D2 resonances and b) in the region of uniform group index between the resonances. The grey horizontal bar indicates the 6 nm region of minimal group index variation centered on λ$_{c}$ . The data correspond to a probe beam passing three times through a 7 cm vapor cell at a temperature of 280 ◦ C. The error is the same for all points and is due to uncertainties in the vapor temperature measurement. peak at a time t = 0. We find excellent agreement between the model and experimental data for 650 and 250-fs-long pulses centered at 787.5 and 788.4 nm after they have traveled through the cell heated to 326 and 263 ◦ C (see Fig. 3 a) and b), respectively). After propagation through the vapor, the pulses become broader and develop trailing intensity oscillations (we quantify this distortion below). Because we are working so far from the atomic resonances, we find that the main contribution to pulse distortion stems from dispersive effects ( i.e. , absorption-induced distortion is negligible), as described in Appendix. A. We also note that our system is nearly lossless; the decrease in pulse amplitude is a direct result of power redistribution rather than loss due to absorption (we calculate the absorption to be < 1%). We find that our model also describes accurately pulse propagation for a broad range of temperatures. Figure 4 a) shows the output pulse shapes for identical incident 650-fs- long pulses traveling through the vapor at various temperatures. The group delay depends strongly (nearly exponentially) on the temperature, as shown in Figure 4 b), which allows 8