Acting as information carriers, photons can be entangled in single degree of freedom (DOF), or entangled in multiple DOFs independently as hyperentanglement or entangled in multiple DOFs mutually as hybrid entanglement. Entanglement in multiple DOFs shows more advantages than the entanglement in single DOF including enabling high efficiency for Bell measurement and making super dense coding breaking the conventional linear-optics threshold achievable. Meanwhile, it can be utilized in the scenario of asymmetric optical quantum network and generation of multi-qubit entangled states.

Hyperentanglement has many applications in improving the technologies for state purification, obtaining the quantum computing, and tests for the nonlocality. However, memory-memory entanglement in multiple DOFs has not yet been reported, which represents a critical step towards multi-DOFs quantum repeater.

The cold atomic physics research group of Key Laboratory of Quantum Information, University of Science and Technology of China of Chinese Academy of Sciences led by academician GUO Guangcan has made great progress in quantum storage. Prof. SHI Baosen and collaborators established a high-dimensional entanglement and a 2×3 hyper-entanglement between two separated quantum memories. The results have been published in Light: Science & Applications and Nature Communications.

Prof. SHI baosen's group has focused on the research about the quantum storage of high dimensional quantum states for several years. Researchers have realized the first quantum memory of single photon carrying orbital angular momentum (OAM) and the quantum storage of a two-dimensional OAM entanglement. Recently, they experimentally realized a high-dimensional entanglement between the spin-excited states of two cold atomic ensembles, which is a leap forward. The results showed that two quantum memories were entangled up to seven dimensions.

Hyper- and hybrid entanglement in multiple degrees of freedom established by researchers includes path (K-vector) and OAM between two separated atomic ensembles by using quantum storage. The results are promising for achieving quantum communication and computing with many degrees of freedom.

This work is supported by the National Natural Science Foundation of China, the National Fundamental Research Program of China.

Figure: The multiple DOFs entanglement between two separated atomic ensembles. (Image by SHI Baosen)