Measuring the time that a photon spends as an atomic excitation before being scattered

When a single photon traverses a cloud of two-level atoms, how much time does it spend as a collective atomic excitation? On average, this time turns out to be given by the product of the atoms' spontaneous lifetime and the probability of the photon being scattered to the side, suggesting that scattered photons spend one spontaneous lifetime as atomic excitations while transmitted photons spend zero time. However, our recent theoretical work (APL Quantum 2, 036108 (2025)) challenges this interpretation, predicting instead that transmitted photons spend a time equal to the group delay as atomic excitations, whereas scattered photons experience an additional Wigner scattering delay time. We have recently verified the prediction for transmitted photons experimentally (arXiv:2409.03680), while the case of scattered photons remains untested. Here we report progress towards testing this prediction by weakly probing the amount of atomic excitation in a cold Rb cloud and post-selecting on side-scattered photons. This experiment aims to directly measure the excitation time associated with scattering and shed light on the intricate histories of photons travelling through absorptive media.