轨道角动量纠缠的固态量子存储

Photons are the natural carriers of information in quantum networks. However, because photon loss scales exponentially with the channel length, quantum communication protocols that utilize the direct transmission of photons are limited to distances of approximately 300 km. To overcome such limitation, the concept of a quantum repeater has been proposed, which utilizes entanglement swapping and quantum memory to efficiently create long-distance entanglement. The low data rate in quantum communication, which represents one of the main obstacles to the practical realization of quantum networks based on quantum repeater, promises significant speedups by the utilizations of high-dimensional encodings and multimode memories. Two specific methods have been proposed to enhance the data rate, one is the utilization of high-dimensional encodings and the other one is the use of multimode memories. The orbital-angular-momentum (OAM) of a photon describes the helical structure of the wavefront around a phase dislocation. Because it has an unlimited quantum number, it attracts many research interests and has proven to be an outstanding degree of freedom for carrying high-dimensional entanglement and for spatial multimode operations. However, no experiments have addressed the possibility of storage of OAM states in a solid-state quantum memory. Here we planned to realize the quantum storage of 3-dimensional OAM photonic entanglement in a rare-earth ion doped crystal. The entangled photon pairs will be generated through down-conversion process in a nonlinear crystal. The spatial-multimode capacity of the quantum memory will be assessed through the visibility of the stored superposition states in higher dimensional spaces.

光子是未来量子网络的天然信息载体，然而由于光子传输损耗随传输距离呈指数关系增长，目前量子通讯的距离被限制在三百公里左右。基于量子存储和纠缠交换技术的量子中继器可解决这一难题，使五百公里以上的长程量子通信成为可能。通讯速率过低是构建基于量子中继的量子网络所面临的主要困难。两种可行的通信加速方案分别是高维度量子态编码以及量子存储的多模式复用。轨道角动量自由度描述光子的波前相位分布，它具有不受限的量子数，故近些年来这一自由度受到人们的广泛关注。借助它可以实现信息的高维编码以及空间域的多模式复用。目前轨道角动量的固态量子存储研究仍是一片空白。本项目拟在已有平台的基础上，研究轨道角动量量子态的固态量子存储。为体现这一自由度可高维编码的优势，我们计划利用非线性晶体产生三维轨道角动量纠缠的窄带光子对，并实现该高维纠缠的量子存储，实验分析固态量子存储器的空间域多模式存储容量。

光子是未来量子网络的天然信息载体，然而由于光子传输损耗随传输距离呈指数关系增长，目前量子通讯的距离被限制在三百公里左右。基于量子存储和纠缠交换技术的量子中继器可解决这一难题，使五百公里以上的长程量子通信成为可能。通讯速率过低是构建基于量子中继的量子网络所面临的主要困难。两种可行的通信加速方案分别是高维度量子态编码以及量子存储的多模式复用。轨道角动量描述光场的横向相位分布，它具有不受限的量子数， 这一特性使得它可用于高维度信息编码并实现空间域的多模式复用。 项目组首次实验实现了单光子轨道角动量量子态的固态量子存储。 基于非线性晶体可以制备高维度的轨道角动量纠缠态，然而该光源的线宽为THz量级，不适用于与存储器对接。我们首次采用平行平面腔滤波的方式获得了窄线宽、高维度轨道角动量纠缠的光子对。 我们实验实现了对该高维纠缠的存储，且存储保真度高于 99%。通过衡量高维度空间中二维量子叠加态存储后的可见度，我们实验证明固态量子存储器的存储维度/空间多模式数超过51。这些实验系统地研究了轨道角动量的固态量子存储，为未来基于固态系统构建高维度、多模式复用的量子中继打下基础。