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Nature Communications

Experimental realization of entanglement in multiple degrees of freedom between two quantum memories

Wei Zhang, Dong-Sheng Ding, Ming‐Xin Dong, Shuai Shi, Kai Wang, Shi-Long Liu, Yan Li, Zhi‐Yuan Zhou, Bao‐Sen Shi, Guang‐Can Guo

Published Nov 14, 2016• 87 citations
Publisher

Abstract

Entanglement in multiple degrees of freedom has many benefits over entanglement in a single one. The former enables quantum communication with higher channel capacity and more efficient quantum information processing and is compatible with diverse quantum networks. Establishing multi-degree-of-freedom entangled memories is not only vital for high-capacity quantum communication and computing, but also promising for enhanced violations of nonlocality in quantum systems. However, there have been yet no reports of the experimental realization of multi-degree-of-freedom entangled memories. Here we experimentally established hyper- and hybrid entanglement in multiple degrees of freedom, including path (K-vector) and orbital angular momentum, between two separated atomic ensembles by using quantum storage. The results are promising for achieving quantum communication and computing with many degrees of freedom.

DOI
10.1038/ncomms13514

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Abstract Electromagnetic waves have both spin momentum and orbital angular momentum (OAM). Light carrying OAM has broad applications in micro-particle manipulation, high-precision optical metrology, and potential high-capacity optical communications. In the concept of quantum information, a photon encoded with information in its OAM degree of freedom enables quantum networks to carry much more information and increase their channel capacity greatly compared with those of current technology because of the inherent infinite dimensions for OAM. Quantum memories are indispensable to construct quantum networks. Storing OAM states has attracted considerable attention recently, and many important advances in this direction have been achieved during the past few years. Here we review recent experimental realizations of quantum memories using OAM states, including OAM qubits and qutrits at true single photon level, OAM states entangled in a two-dimensional or a high-dimensional space, hyperentanglement and hybrid entanglement consisting of OAM and other degree of freedom in a physical system. We believe that all achievements described here are very helpful to study quantum information encoded in a high-dimensional space.
Quantum memories enabling the storage of an input photonic qubit and its later retrieval with a fidelity beating any classical device constitute essential components in quantum communication networks and optical quantum information processing [1]. Our main goal is to develop a new generation of memories which have a near-unity efficiency for storage-and-retrieval operations, as well as high multiplexing capabilities. In the recent years, we demonstrated for instance the implementation for quantum bits encoded in the orbital angular momentum degree of freedom, which provides an essential capability for future networks with multimode capability [2]. We also realized multiple-degree-of-freedom memory, which can find applications in classical data processing but also in quantum network scenarios where states structured in phase and polarization have been shown to provide promising attributes [3].
A quantum memory for photonic entanglement is the basic component in quantum networks. Photons entangled in multiple degrees of freedom enable the mutual transformation of Hilbert spaces for the same particle and different particles, thereby connecting heterogeneous nodes in the quantum network. So far, the quantum storage for photonic entanglement is limited to two degrees of freedom. Here, we demonstrate the heralded storage of single-photon hybrid entanglement involving three degrees of freedom (polarization, orbital angular momentum, and path) using a solid-state medium. The fidelity of the output state with the ideal maximum entangled state is $94.0\ifmmode\pm\else\textpm\fi{}0.8%$. The quantum property of the retrieved state is verified by testing the hypothesis of noncontextual hidden-variable theories. These results demonstrate the entanglement interface between light and matter involving multiple degrees of freedom, paving the way for the construction of hybrid quantum networks.
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%±0.8% and obtain the Clauser-Horne-Shimony-Holt inequality parameter S of 2.41±0.06 after a programed storage time. All results clearly show the preservation of entanglement during the storage.
Combining the multiple degrees of freedom of photons has become topical in quantum communication and information processes. This provides advantages such as increasing the amount of information that is be packed into a photon or probing the wave-particle nature of light through path-polarisation entanglement. Here we present two experiments that show the advantages of using hybrid entanglement between orbital angular moment (OAM) and polarisation. Firstly, we present results where high dimensional quantum key distribution is demonstrated with spatial modes that have non-separable polarisation-OAM DOF called vector modes. Secondly, we show that through OAM-polarisation entanglement, the traditional which-way experiment can be performed without using the traditional physical path interference approach.