基于轨道角动量编码的高维态量子中继研究

Quantum network mainly consists of a memory used for storing and manipulating quantum information and an information carrier through which different memories could connect with others. 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 orbital angular momentum (OAM), then the information carried by each photon could be increased significantly (improved from a bit to log2(d) bits, where d is the number of orthogonal basis vectors of the Hilbert space in which the photon is described). 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 transmission distance and improving quantum information processing efficiency, among which a quantum memory usually consisted of atomic ensembles or solid systems 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. Moreover, a quantum memory for storing a high-dimensional state can reduce its sensitivity to memory coherence time. It can lead to significant improvements in storage capacity, and could potentially find some novel quantum information applications such as quantum holographic teleportation, quantum dense image coding, and holographic quantum computing, etc. Therefore many groups and researchers are active in performing the research on the subject of quantum information with a state lived in a high-dimensional space. . Although works have reported on some progresses, but the research in this field is still in its early stages, there are many basic open questions which should be answered in order to build up a high-dimensional quantum network. In this project, in order to realize a high-dimensional quantum repeater, we will focus our research on the storage of a light carrying OAM, performing the storage of a single photon carrying OAM and a quantum OAM entanglement state, realizing the quantum OAM entanglement swapping and frequency conversion between different wavelength bands, etc. To our best knowledge, there has been no any report on the study contents shown above. We believe the results obtained in this project will be very promising for establishing a high-dimensional quantum network.

量子通信网络主要由用于存储和操纵量子态的存储单元和联络各存储单元之间的信息载体构成。光子是信息载体的最佳选择，存储单元可以由固体材料或气态原子构成。光子携带的信息量取决于光子被编码的空间：若将光子编码于一个高维空间，则可以极大地提高光子携带的信息量。如果能够实现编码于高维空间的量子态存储，则可以大幅度缩短存储单元存储量子态的时间要求，还可以实现大信息量的量子通信网络，因而基于高维编码态的量子中继研究是量子信息领域的热点方向。尽管目前该领域的研究已取得重要进展，但仍处于初始阶段，尤其在基于高维编码的量子态存储方面依然处于起步阶段，基于纠缠交换的中继研究仍然是一片空白。本项目拟以编码于轨道角动量空间的高维量子态存储为核心，以高维单光子态、纠缠态的存储、纠缠交换以及频率变换等为主要研究内容，以实现基于高维编码态的量子中继单元为目标开展研究工作。预期的研究成果将为构建大信息量量子网络奠定坚实基础。

利用光子的轨道角动量可以构成一个无限维的完备的Hilbert空间。将光子编码在轨道角动量空间可以实现高维编码，进而大幅度增加光子的信息携带量。此外，利用光子的高维编码态还可以提高量子密钥传输的安全性，提高信道容量，以及应用于量子力学基本问题的研究等。基于高维量子态的存储、纠缠交换、量子接口以及识别等基本操作可以实现高维量子中继，进而构建大信息量量子通信网络。本项目紧紧围绕高维量子中继这一核心目标，以光子轨道角动量自由度的相干操控为技术手段，以高维量子态的产生、存储、频率变换以及识别等为主要研究内容，系统开展了高维量子中继关键技术研究。经过五年的不懈努力，团队顺利完成了研究任务，达到了预期目标，并在多个方面取得了突破性进展，特别是团队在高维量子态存储方面的系列工作开创了量子存储的新领域，受到国内外量子信息领域的广泛关注；在宽带量子存储方向该团队取得了该领域的第一个标志性成果,为未来构建基于光纤系统的高速量子网络奠定基础；同时该团队通过非线性频率变换技术为解决高维量子接口这一关键问题提供了一种有效手段，等等。系列研究成果被包括量子信息和量子光学领域的权威Zeilinger、Gisin、Padgett、Walmsley等学术同行积极评述，在学术界产生了很大影响，推动和引领了大容量、高速量子通信技术的发展。. 团队已在包括Nat.Photo.、Nat. Commun.、PRL、Light、Optica等顶尖学术期刊发表50多篇学术论文，其中两篇代表性论文入选SCI高被引论文，一篇入选SCI热点论文，google学术引用2000余次。受J. Phys. B邀请撰写发表了关于轨道角动量量子存储的唯一的一篇topic review；受J. Phys. B邀撰写了“Roadmap on quantum light spectroscopy”中一个专题；受邀在《科学通报》撰写发表了轨道角动量频率变换的综述；受《物理学报》邀请撰写并发表了关于Raman存储的综述，以及受《物理》邀请撰写发表了关于量子图像存储的综述。同时项目执行过程中我们也注意知识产权的保护，申报并授权了一项发明专利、四项实用新型专利，另有多项专利正在申请当中。