The cold atomic physics research group of Key Laboratory of Quantum Information, CAS, led by academician GUO Guangcan, has achieved significant progress for high-dimensional quantum memory: the group leader, Prof. SHI Baosen and his collaborators realize the storage and release of a single photon with a spatial structure, carrying the orbital angular momentum (OAM) in a cold atomic ensemble for the first time. This progress clearly demonstrates the possibility for high-dimensional quantum memories and makes the first important step towards realizing a long-distance quantum communication with large information-carrying capability based on high-dimensional quantum repeaters. The main results have beenpublished in Nature Communications online on Oct. 2nd.
Usually quantum information is encoded in a two-dimensional space spanned for example by orthogonal polarizations of a photon, a robust quantum information carrier. In this case, each photon could carry at most a bit information. If the photon could live in a high-dimensional space, for example, spanned by the inherently infinite-dimensional OAM, then the information carried by each photon could be increased significantly (improved from a bit to log2d bits). Moreover, in comparison to a two-dimensional state, high-dimensional states show many interesting properties: enable more efficient quantum-information processing, and afford a more secure flux of information in quantum key distribution, etc. Quantum repeaters are indispensable for increasing the transmission distance and improving the quantum information processing efficiency, among which a quantum memory is the key component consisting of the quantum repeater. If we could realize the reversible transfer of a high-dimensional quantum state between a true single photon and a matter used as a quantum memory with high fidelity and reliability, then we may have the potential solution in enhancing the channel capacity significantly in addition to overcoming distance limitations of quantum communication schemes through transmission losses, a high-dimensional quantum network may become practical. Therefore many groups and researchers are devoted to performing the storage of a light lived in a high-dimensional space. Although some works have reported on the storage of a light carrying OAM or a spatial structure in different physical systems, these works involve bright lights. So far there is no any work reporting on the storage of a photon encoded in a high-dimensional space in any physical system. Constructing such a quantum memory is a hot topic and big challenge.
SHI Baosen and his Ph. D candidates, DING Dongsheng and others are devoted to solving the above problem, and have achieved some progresses on the storage of a light carrying a spatial structure (see PRA, 87, 013835, 013845, 053830, (2013)). Recently, they make big progress along this research direction: report on the first experimental realization of a true single-photon-carrying OAM stored via electromagnetically induced transparency (EIT) in a cold atomic ensemble, demonstrating the possibility for building up high-dimensional quantum memories. In the experiment, they prepare two cold atomic clouds by laser cooling and trapping techniques in two magnetic-optical traps. One atomic cloud is used to as a nonlinear media to prepare a heralded single photon. Then this single photon, imprinted a special structure and carried OAM by a spiral phase plate, is stored through EIT in the second atomic cloud and retrieved after a programmed storage time. The experimental results clearly demonstrate that not only the single photon with OAM could be stored with high fidelity, but also with the help of a well-designed Sagnac interferometer and quantum tomography technique, the superposition state of OAMs carried by the single photon could be also well preserved during the storage.
Figure caption: A heralded single photon is generated through the spontaneous four-wave mixing in Mot 1 (a), then this single photon, imprinted a special structure and carried OAM by a spiral phase plate, is stored through electromagnetically induced transparency in Mot 2 (b). (c) is the image of the photon carrying. We experimentally measure the image carried by the photon before storage and after that along the transverse position, the results are shown in (d) and (e) respectively, exhibiting strong similarity. The solid lines are theoretical fits./ Image by SHI Baosen's Group.
Before published in a peer-reviewed journal, the main results are firstly public in the academic website of arXiv (arxiv:1305:2675), immediately attracts people’s attention: MIT TechnologyReview comments online: "First Quantum Memory That Records The Shape of a Single Photon Unveiled in China" and "The world’s first quantum memory that stores the shape and structure of single photons has been built in a Chinese lab". Subsequently, some other websites reprint these comments. Now, these results have been published online in journal Nature Communications with positive comments as "This is extremely impressive work, and establishes a very high standard in the rapidly growing field of quantum memories. In fact, the authors could have probably split the results into two papers. But taken together, this demonstration of the ability to generate, store, and read out on-demand, arbitrary OAM qubits encoded onto true single photons, represents an exciting watershed in the development of quantum-enabled technologies. The results will have a large impact within the quantum information and quantum atom optics communities, and should be of general interest to a wider physics audience. I am therefore very happy to recommend publication in Nature Communications. I look forward to the future results from this research group."...
This work is supported by the National Natural Science Foundation of China, the National Fundamental Research Program of China.
Contact: Prof.SHI Baosen, drshi@ustc.edu.cn
(School of Physical Sciences)