Shanghai Branch, National Laboratory for Physical Sciences at Microscale and Department of Modern Physics, University of Science and Technology of China, Shanghai 201315, China.
Nature. 2012 Aug 9;488(7410):185-8. doi: 10.1038/nature11332.
Transferring an unknown quantum state over arbitrary distances is essential for large-scale quantum communication and distributed quantum networks. It can be achieved with the help of long-distance quantum teleportation and entanglement distribution. The latter is also important for fundamental tests of the laws of quantum mechanics. Although quantum teleportation and entanglement distribution over moderate distances have been realized using optical fibre links, the huge photon loss and decoherence in fibres necessitate the use of quantum repeaters for larger distances. However, the practical realization of quantum repeaters remains experimentally challenging. Free-space channels, first used for quantum key distribution, offer a more promising approach because photon loss and decoherence are almost negligible in the atmosphere. Furthermore, by using satellites, ultra-long-distance quantum communication and tests of quantum foundations could be achieved on a global scale. Previous experiments have achieved free-space distribution of entangled photon pairs over distances of 600 metres (ref. 14) and 13 kilometres (ref. 15), and transfer of triggered single photons over a 144-kilometre one-link free-space channel. Most recently, following a modified scheme, free-space quantum teleportation over 16 kilometres was demonstrated with a single pair of entangled photons. Here we report quantum teleportation of independent qubits over a 97-kilometre one-link free-space channel with multi-photon entanglement. An average fidelity of 80.4 ± 0.9 per cent is achieved for six distinct states. Furthermore, we demonstrate entanglement distribution over a two-link channel, in which the entangled photons are separated by 101.8 kilometres. Violation of the Clauser-Horne-Shimony-Holt inequality is observed without the locality loophole. Besides being of fundamental interest, our results represent an important step towards a global quantum network. Moreover, the high-frequency and high-accuracy acquiring, pointing and tracking technique developed in our experiment can be directly used for future satellite-based quantum communication and large-scale tests of quantum foundations.
在大规模量子通信和分布式量子网络中，任意距离上的未知量子态转移是至关重要的。借助长距离量子隐形传态和纠缠分发可以实现这一点。后者对于基本的量子力学定律检验也很重要。虽然已经使用光纤链路实现了中等距离的量子隐形传态和纠缠分发，但光纤中的巨大光子损耗和退相干需要使用量子中继器来实现更大的距离。然而，量子中继器的实际实现仍然具有实验挑战性。自由空间信道最初用于量子密钥分发，提供了一种更有前途的方法，因为光子损耗和退相干在大气中几乎可以忽略不计。此外，通过使用卫星，可以在全球范围内实现超远距离量子通信和量子基础的测试。之前的实验已经实现了纠缠光子对在 600 米（参考文献 14）和 13 公里（参考文献 15）的自由空间分布，以及在 144 公里的单链路自由空间信道上触发单光子的传输。最近，在修改后的方案下，使用一对纠缠光子演示了 16 公里的自由空间量子隐形传态。在这里，我们报告了使用多光子纠缠在 97 公里的单链路自由空间信道上对独立量子位进行量子隐形传态的实验结果。对于六个不同的状态，平均保真度达到了 80.4±0.9%。此外，我们还演示了在两链路信道中的纠缠分发，其中纠缠光子之间的距离为 101.8 公里。在没有局域性漏洞的情况下观察到了 Clauser-Horne-Shimony-Holt 不等式的违反。除了具有基本的兴趣外，我们的结果还代表着迈向全球量子网络的重要一步。此外，我们实验中开发的高频高精度的获取、指向和跟踪技术可以直接用于未来基于卫星的量子通信和大规模量子基础测试。
