基于时间域的量子行走及其应用的实验研究

Particles underlying the laws of quantum mechanics have astonishing properties. Despite their particle nature, they can reveal a wavelike propagation and with this move much faster than a classical particle. This interesting behavior occurs, amongst others, in the so-called quantum walk, the quantum mechanical counterpart of a classical random walk. While a classical particle takes its path randomly during a random walk, a quantum particle spreads wavelike over all possible paths and removes with this the randomness from the propagation...With their extraordinary properties quantum walks already enabled the development of new quantum algorithms that outperform all classical algorithms. In addition to their applications in quantum computation, quantum walks help us to understand physical phenomena based on the propagation of quantum particles...Although the theoretical works to quantum walks are well advanced, their high demands on resources allowed only for a handful basic realizations. Implementing quantum walks is a very difficult task. The intrinsic complexity of quantum walk experiments originates from the three essential properties any implementation has to provide that the setup has to be scalable, flexible and free of decoherence. The straightforward approach to implement a position space is to split the particle's wave function into discrete spatial positions and place a detector at each position. By employing a beam splitter cascade to divide a coherent laser beam or single photons and placed detectors at each outcome of the cascade one can realize a photonic quantum walk. Since this requires an additional row of beam splitters for each step, the number of optical elements is increasing quadratically with the number of steps...This project presents a new method to experimentally implement quantum walks. The optical experiment employs a newly designed interferometric architecture that transfers the spatial propagation of the particles into the time domain, which circumvents the high demands. This quantum walk in time is specifically designed to overcome the typical challenges of scalability, stability and flexibility that limit the performance of the first quantum walk experiments...Furthermore, we can prove that quantum walk can be used as a platform of quantum information processing since based on quantum walk, one can implement any unitary operations. Thus in an architecture of quantum walk, we can realize quantum state engineering and transfer, quantum logical gates and quantum measurements. Our work will be useful for experimental realization of general quantum information protocols.

量子行走以其优于经典的特性在量子信息中有着广泛的应用，如针对搜寻无序数据库的量子行走算法等，因而利用现有实验条件和基础实现量子行走凸显出其重要性和迫切性。目前量子行走主要在线性光学体系中得以实验实现，但其所需光学元件的数量与演化的步数呈几何式增长，受到不可扩展性的制约。本项目另辟蹊径，采取时间域上的量子行走，利用不等臂的光纤干涉仪可实现量子行走在时间域上多次循环演化，从而既拥有光子相干时间长易操控等优势，又拥有光纤体系的可集成实用化的特点。本项目的创新之处在于系统地证明时间域上的量子行走可以作为普适的平台实现一切幺正演化，进而实现量子态制备和传输，量子逻辑门操作，及量子测量等，从而实现量子信息处理的所有步骤。本项目目标为以时间域的量子行走作为普适的量子信息处理平台，实验实现可集成的全光纤量子计算和量子测量装置。上述研究取得成功，必将推动量子信息技术实用化、集成化和产业化的进程。

量子行走以其优于经典的特性在量子信息中有着广泛的应用，如针对搜寻无序数据库的量子行走算法等，因而利用现有实验条件和基础实现量子行走凸显出其重要性和迫切性。本项采取时间域上的量子行走，利用不等臂的光纤干涉仪可实现量子行走在时间域上多次循环演化，从而既拥有光子相干时间长易操控等优势，又拥有光纤体系的可集成实用化的特点。本项目的创新之处在于首次系统地证明量子行走可以作为普适的平台实现一切幺正演化，进而实现量子态制备和传输，量子逻辑门操作，及量子测量等，从而实现量子信息处理的所有步骤。.在项目资助期间，首先实验实现时间域的全光量子行走，并将其应用于量子模拟，量子测量及验证量子力学基础原理等方面，取得了一定成果，共在Nature Physics, Physical Review Letters等主流学术期刊上发表高水平学术论文三十七篇，会议论文一篇，均标注项目资助。研究工作进展顺利，进度符合项目预期计划，超过预期研究成果。取得的主要研究进展和重要结果主要集中在利用全光量子信息处理器验证量子力学基础理论和实现量子模拟两个方面。.1全光量子信息处理器验证量子力学基础理论。团队首次在实验上观测到单体演化中的信息亏损(information deficit)，之前普遍认为信息亏损只存在于两体纠缠中，而我们是首次在单体的上下文关系的情景之中观测到信息亏损。Physical Review Letters刊发了这一项关于量关联的重要进展。论文一经发表引起了国内外专家同行的广泛关注。.2全光量子信息处器实现量子模拟。团队首次在开放系统中实验实现非厄米的量子行走并观测到新型一维拓扑保护边界态，修正了体边对应关系，为基于量子行走平台实现量子计算提供了新的依据。该系列研究成果以长文（article）形式于2017年和2020年发表于Nature Physics，2017-2019年发表于Physical Review Letters（三篇），2019年发表于Nature Communications。研究成果一经发表引起国内外专家同行的广泛关注，多篇论文入选ESI高引论文，系列工作入选“2017年度中国光学十大进展”。