Quantum Storage of Orbital Angular Momentum Entanglement in an Atomic Ensemble

Abstract

Constructing a quantum memory for a photonic entanglement is vital for realizing quantum communication and network. Because of the inherent infinite dimension of orbital angular momentum (OAM), the photon's OAM has the potential for encoding a photon in a high-dimensional space, enabling the realization of high channel capacity communication. Photons entangled in orthogonal polarizations or optical paths had been stored in a different system, but there have been no reports on the storage of a photon pair entangled in OAM space. Here, we report the first experimental realization of storing an entangled OAM state through the Raman protocol in a cold atomic ensemble. We reconstruct the density matrix of an OAM entangled state with a fidelity of $90.3%\ifmmode\pm\else\textpm\fi{}0.8%$ and obtain the Clauser-Horne-Shimony-Holt inequality parameter $S$ of $2.41\ifmmode\pm\else\textpm\fi{}0.06$ after a programed storage time. All results clearly show the preservation of entanglement during the storage.