Distributing entanglement over long distances is one of the central tasks in quantum networks. An important problem, especially for near-term quantum networks, is to develop optimal entanglement distribution protocols that take into account the limitations of current and near-term hardware, such as quantum memories with limited coherence time. We address this problem by initiating the study of quantum network protocols for entanglement distribution using the theory of decision processes, such that optimal protocols (referred to as policies in the context of decision processes) can be found using dynamic programming or reinforcement learning algorithms. As a first step, in this work we focus exclusively on the elementary link level. We start by defining a quantum decision process for elementary links, along with figures of merit for evaluating policies. We then provide two algorithms for determining policies, one of which we prove to be optimal (with respect to fidelity and success probability) among all policies. Then we show that the previously-studied memory-cutoff protocol can be phrased as a policy within our decision process framework, allowing us to obtain several new fundamental results about it. The conceptual developments and results of this work pave the way for the systematic study of the fundamental limitations of near-term quantum networks, and the requirements for physically realizing them.
High-rate, high-fidelity entanglement of qubits across an elementary quantum network
我们证明了远程纠缠的捕获离子量子比特通过量子光纤链路的保真度和速度接近本地操作。两个${}^{88}$ Sr ${}^{+}$量子比特通过高数值孔径透镜耦合到单模光纤中的两个光子的偏振自由度纠缠在一起，并在分束器上干涉。一种新的几何形状允许高效率的光子收集，同时保持离子光子纠缠的单位保真度。我们以平均速率$182\,\mathrm{s}^{-1}$（成功概率$2.18\times10^{-4}$）生成具有保真度$F=0.940(5)$的远程Bell对。
Probabilistic Cutoffs in Homogeneous Quantum Repeater Chains
QuTech, Delft University of Technology（代尔夫特理工大学 QuTech）；Quantum Computer Science, EEMCS, Delft University of Technology（代尔夫特理工大学 EEMCS 量子计算机科学）；Kavli Institute of Nanoscience, Delft University of Technology（代尔夫特理工大学 卡弗里纳米科学研究所）；College of Information and Computer Science, University of Massachusetts Amherst（马萨诸塞大学阿默斯特分校 信息与计算机科学学院）
