Controlling the higher dimensional entanglement between light and cold atoms

更高维的二分纠缠态使我们能够实现更有效的量子信息处理，例如与通常采用的量子比特之间的二维纠缠相比，通过增强量子通信中的光学数据流量。这种高维纠缠态可以通过使用拉盖尔-高斯（LG）模式来创建，因为LG模式中的光子携带轨道角动量（OAM），可以用来定义无限维希尔伯特空间。受使用参量下转换（PDC）的开创性实验的启发，已经使用光子的OAM状态演示了各种协议。然而，PDC是一个自发过程，纠缠光子的产生时间是完全随机的。因此，在两个遥远的长寿命大质量原子之间产生更高维的纠缠是实现量子信息复杂任务的重要一步。在这里，作为第一步，我们证明了集体原子激发和光子之间的OAM态的高维纠缠。利用量子层析技术估计了原子-光子系统的施密特数为3，表明该系统在3 × 3维希尔伯特空间中纠缠.

Higher-dimensional bipartite entangled states enable us to achieve more efficient quantum information processing, for example by enhancing optical data traffic in quantum communications compared to usually employed two-dimensional entanglement between qubits. Such higher-dimensional entangled states can be created by using Laguerre-Gaussian (LG) modes, since photons in LG modes carry orbital angular momenta (OAM) which can be utilized to define an infinite-dimensional Hilbert space. Inspired by the pioneering experiment using parametric down-conversion (PDC), various protocols have been demonstrated using the OAM states of photons. However, PDC is a spontaneous process and the generation time of the entangled photons is completely random. Producing higher-dimensional entanglement between two remote long-lived massive atoms is therefore an important step toward the realization of intricate tasks of quantum information. Here, as the first step, we demonstrated higher dimensional entanglement of OAM states between a collective atomic excitation and a photon. Schmidt number for the atom-photon system was estimated to be 3 through the quantum tomography, which means the system was entangled in 3 X 3 dimensional Hilbert space.