Storing High-Dimensional Quantum States in a Cold Atomic Ensemble

Abstract

The reversible transfer of a high-dimensional quantum state between a true single photon, an information carrier, and a matter used as a quantum memory with high fidelity and reliability could enhance the channel capacity significantly in addition to overcoming distance limitations of quantum communication schemes through transmission losses. In the Chap. 2, we have introduced how we demonstrated single photon storage encoded with two-dimensional OAM degree of freedom. Due to the infinity Hilbert space spanned by OAM degree of freedom, photon encoded in OAM space can extend to high-dimensional state. Quantum memories have been realized with different physical systems, such as atomic ensembles and solid systems etc., many of them only realize the storage and retrieval of the single photons spanned in a two-dimensional space for example, orthogonal polarizations, therefore only a quantum bit could be stored there. In this Chapter, I will introduce an experimental realization of a quantum memory storing a heralded photon lived in a three-dimensional space spanned by OAM via EIT in a cold atomic ensemble. We reconstruct the storage process density matrix with fidelity of $85.3\%$ by the aid of a 4-F imaging system experimentally. The ability to store a high-dimensional quantum state with high fidelity is very promising for building a high-dimensional quantum network.