pulsed lens heaters vapor translation det. computer chopper BBO mask grating lock-in filter ref. beam sig. beam b) amp. laser stage cell laser MZM wave meter oscilloscope SG heaters vapor cell /2 det. computer a) mirror flip Fig. 1. Schematics of the experimental setups. a) A weak beam is modulated by a MZM, passes three times through a heated Rubidium vapor cell (Triad Technologies, TT-RB-75-V-Q-CW), and is detected by a fast photoreceiver (New Focus 3051). b) An ultrashort pulse passes once through the vapor cell, overlaps with a tunable reference pulse in a BBO crystal, and the resulting sum-frequency generated light is recorded. In the second experiment, we inject ultrashort laser pulses into the vapor and measure the shape and absolute delay of the outgoing pulses (see Fig. 1 b)). We use a mode-locked Ti:Sapphire laser (Spectra-Physics, Tsunami) to generate ultra-short pulses ( ∼ 55 fs) cen- tered between the D1 and D2 resonances, then split the laser output into a reference and signal beam. The reference beam passes through a tunable optical delay line (used to scan the temporal delay between the signal and reference beam) and chopper, and is then focused into a beta barium borate (BBO) crystal. To tailor the frequency spectrum of the signal beam, we send it to a grating, use a rectangular mask to remove unwanted frequency com- ponents, and then re-compress the signal beam by sending it back to the grating. In this way, we produce nearly Fourier-limited, sinc-shaped pulses with a width between ∼ 100 fs and 2 ps by adjusting the spatial width of the mask. We then pass the signal beam through the vapor cell and focus the transmitted beam down onto the BBO crystal. When the signal and reference beams are spatially and temporally overlapped in the BBO crystal, sum-frequency generated light is produced. By varying the relative delay between the signal and reference 6