Quantum communication enables the implementation of tasks that are unachievable with classical resources. However, losses on the communication channel preclude the direct long-distance transmission of quantum information in many relevant scenarios. In principle quantum repeaters allow one to overcome losses. However, realistic hardware parameters make long-distance quantum communication a challenge in practice. For instance, in many protocols an entangled pair is generated that needs to wait in quantum memory until the generation of an additional pair. During this waiting time the first pair decoheres, impacting the quality of the final entanglement produced. At the cost of a lower rate, this effect can be mitigated by imposing a cut-off condition. For instance, a maximum storage time for entanglement after which it is discarded. In this work, we optimize the cut-offs for quantum repeater chains. First, we develop an algorithm for computing the probability distribution of the waiting time and fidelity of entanglement produced by repeater chain protocols which include a cut-off. Then, we use the algorithm to optimize cut-offs in order to maximize secret-key rate between the end nodes of the repeater chain. We find that the use of the optimal cut-off extends the parameter regime for which secret key can be generated and moreover significantly increases the secret-key rate for a large range of parameters.
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）
1Department of Computer Science, School of Computing \& Information, University of Pittsburgh, Pittsburgh, PA 15260, USA；2Department of Informatics \& Networked Systems, School of Computing \& Information, University of Pittsburgh, Pittsburgh, PA 15260, USA
Distance-Independent Entanglement Generation in a Quantum Network using Space-Time Multiplexed Greenberger-Horne-Zeilinger (GHZ) Measurements
在成功地创建链路、在相邻中继器节点之间共享贝尔状态（在每个时隙中具有概率$p$）并且在具有成功概率$q<1$的节点处执行贝尔状态测量的量子网络中，端到端纠缠生成速率随着消费者之间的距离呈指数下降，尽管有多路径路由。如果中继器可以在GHZ基中以概率$q$成功执行多量子比特投影测量，则速率在特定$(p,q)$区域中不随距离变化，而是在外部呈指数衰减。这个区域的距离无关的速率发生的超临界区的一个新的渗流问题。我们扩展了这个GHZ协议，将时间复用块长度$k$，时隙的数量，中继器可以混合和匹配成功的链接执行fusionon. As $k$增加，超临界区扩大。对于给定的$(p,q)$，纠缠率最初随$k$而增加，并且一旦在足够高的$k$的超临界区域内，它衰减为每个时隙的$1/k$ GHZ态。当记忆体的相干时间与平均$μ$呈指数分布时，可以看出，增加$k$不会无限地增加超临界区域;它有一个硬$μ$相关的限制。最后，我们发现，结合空分复用，即，在高达$d$的断开网络区域中独立地运行上述协议，其中$d$是网络的节点度，可以超过上述随机化本地链路状态协议不能超过的每时隙速率1GHz状态。随着$(p,q)$的增加，可以接近每个时隙的$d$ GHZ态的极限最小切割纠缠生成容量。
Quantum Approaches to the Minimum Edge Multiway Cut Problem
Orange Research；LIST3N, University of Technology of Troyes（特鲁瓦技术大学 LIST3N）；LIMOS - UMR CNRS 6158, ISIMA - Université Clermont Auvergne（克莱蒙奥弗涅大学 ISIMA LIMOS - UMR CNRS 6158）
