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

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 into a side mode. This seems to suggest that scattered photons spend one spontaneous lifetime as atomic excitations, while transmitted photons spend zero time as atomic excitations. However, our recent theoretical work (arXiv:2310.00432) challenges this interpretation. We predict that transmitted photons spend a time equal to the group delay as collective atomic excitations, whereas scattered photons spend an average time corresponding to the delay of the scattered light pulse, which consists of a group delay plus a scattering Wigner time delay. While we recently confirmed the prediction for transmitted photons experimentally (arXiv:2409.03680), the prediction for scattered photons remains untested. Here, we report progress towards experimentally testing this prediction. By weakly probing a cloud of cold atoms and post-selecting on photons being scattered to the side, we aim to measure the average time a photon spends as a collective atomic excitation before scattering. This experiment will shed new light on the intricate and surprising histories of photons travelling through absorptive media.