When a photon traverses a cloud of atoms without scattering, how much time does it spend as an atomic excitation? To address this question, we used the cross-Kerr effect to weakly probe the degree of atomic excitation caused by a transmitted resonant “signal” photon by measuring the phase shift induced on a (separate, off-resonant) “probe” beam, postselected on cases when the signal photon is transmitted. The time integral of this observed phase shift, properly normalized and averaged over many runs in which the photon is detected after transmission, is the excitation time of interest, in a weak-valued sense. We measured mean atomic excitation times ranging from $(-0.82\pm 0.31){\tau }_0$ for the most narrow band pulse to $(0.54\pm 0.28){\tau }_0$ for the most broadband signal pulse, where ${\tau }_0$ is the non-post-selected excitation time, given by the scattering probability multiplied by the atomic lifetime ${\tau }_{\mathrm{sp}}$. Across a range of pulse durations and optical depths, our results are consistent with the recent theoretical prediction that the weak value of the atomic excitation time caused by a transmitted photon equals the group delay experienced by the light. Our experimental results show that negative times are not limited to describing the shift in the peak of a reshaped wave packet (as resulting from group delay), but they also provide the appropriate description of the weak-valued time the photon spends as an atomic excitation.

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• Light propagation, transmission & absorption
• Quantum description of light-matter interaction
• Quantum optics
• Weak values & weak measurements