Single-Photon-Level Quantum Image Memory Based on Cold Atomic Ensembles

An important step towards the successful development of network that allows the distribution of quantum information is the storage of light in a matter at the single-photon level. Encoding photons in high-dimensional photonic states can significantly increase their information-carrying capacity. So far, there is no any report on the storage of an image at true single photon level, realizing a quantum memory that can record the shape of a single photon is a challenging work. In this work, we report on the first experimental realization of storing a true single photon carrying an orbital angular momentum via electromagnetically induced transparency in a cold atomic ensemble and retrieved later after a given storage time. We experimentally not only prove the preservation of the property of the single photon, but also show very good similarity on the spatial structures of the input and the retrieved photons. More importantly, we also demonstrate the preservation of the coherence of the single photon during the storage using a Sagnac interferometer. The demonstrated capability of storing a spatial structure at single-photon-level opens the possibility to the realization of a high-dimensional quantum memory.