Montréal Quantum Photonics Seminar Series Abstract: When a pulse of light traverses a material, it incurs a time delay referred to as the group delay. Should the group delay experienced by photons be attributed to the time they spend as atomic excitations? However reasonable this connection may seem, it appears problematic when the frequency of the light is close to the atomic resonance, as the group delay becomes negative in this regime. To address this question, I used the cross-Kerr effect to probe the degree of atomic excitation caused by a resonant transmitted photon by measuring the phase shift on a separate beam that is weak and off-resonant. These results, over a range of pulse durations and optical depths, are consistent with the recent theoretical prediction that the mean atomic excitation time caused by a transmitted photon (as measured via the time integral of the observed phase shift) equals the group delay experienced by the light. Specifically, I measured mean atomic excitation times ranging from (−0.82 ±0.31)τ0 for the most narrowband pulse to (0.54 ±0.28)τ0 for the most broadband pulse. I report these times normalized to the non-post-selected excitation time τ0, which is equal to the scattering (absorption) probability multiplied by the atomic lifetime τsp. These results suggest that negative values taken by times such as the group delay have more physical significance than has generally been appreciated. Bio: Daniela Angulo is a physicist from Colombia who recently completed her PhD at the University of Toronto, focusing on experimental light-matter interaction. Under the supervision of Aephraim Steinberg, her research explored the behavior of photons in atomic clouds using weak measurements. She is passionate about teaching and science communication. Outside the lab, she is an avid cyclist and musician. Local J-1035 Pavillon J. A. Bombardier, Polytechnique Montréal, Montréal, Quebec, Canada