Quantum memory

Understanding collective light–matter interactions in dense atomic ensembles is essential for developing high-efficiency quantum memories and photon–atom interfaces. A long-standing question in quantum optics is whether an initial superposition of excited states is necessary to observe V-type quantum beats. To address this, we perform experiments on cesium ensembles with an ultra-high optical depth (OD > 500) using the D₁ (F = 4 → F′ = 4) transition. After abruptly turning off the probe beam, we observe pronounced quantum beats at 1167.68 MHz, matching the 6P₁/₂ hyperfine splitting. By combining Optical Bloch and Maxwell–Schrödinger models, we show that cooperative emission driven by collective vacuum coupling can produce these beats even without an initial coherence. This vacuum-induced superradiant mechanism reveals new insight into coherent dynamics in optically dense media, providing a foundation for optimizing broadband and high-fidelity quantum light–matter interfaces.

In quantum optics experiments, the optical depth (OD) of cold atomic media is a key parameter for realizing efficient quantum memories, efficient quantum converter and bright photon pairs. The ODs are typically determined by fitting the transmission spectrum to a theoretical model line-shape. We accidently found that the two-level transmission spectra measured at a shorter and a longer distance away from the atomic ensembles differ significantly and thus the determined ODs. We believe that this discrepancy is due to the lensing effect of the atomic ensembles and the collection optics. Another method to determine the OD is through the temporal fitting of the precursor signals. The ODs with precursor signals detected at two distances are consistent to each other, and are also consistent to the ODs determined by spectral fitting but measured at a shorter distance only. To gain deeper insight into how atomic lensing affects two-level absorption spectra, we perform numerical simulations of the in-medium propagation of light through spatially inhomogeneous cold atomic clouds. By systematically varying the system's parameters, the simulations aim to reproduce the experimental transmission spectra and clarify the dependence of the extracted OD on detection geometry. The ongoing results will help bridge the gap between experiment and theory, providing a clearer understanding of OD determination in dense cold-atom systems.