Centre for Quantum Computation and Communication Technology, Laser Physics Centre, The Australian National University, Canberra, Australian Capital Territory 0200, Australia.
1] Centre for Quantum Computation and Communication Technology, Laser Physics Centre, The Australian National University, Canberra, Australian Capital Territory 0200, Australia [2] Department of Physics, Princeton University, Princeton, New Jersey 08554, USA.
Nature. 2015 Jan 8;517(7533):177-80. doi: 10.1038/nature14025.
Space-like separation of entangled quantum states is a central concept in fundamental investigations of quantum mechanics and in quantum communication applications. Optical approaches are ubiquitous in the distribution of entanglement because entangled photons are easy to generate and transmit. However, extending this direct distribution beyond a range of a few hundred kilometres to a worldwide network is prohibited by losses associated with scattering, diffraction and absorption during transmission. A proposal to overcome this range limitation is the quantum repeater protocol, which involves the distribution of entangled pairs of optical modes among many quantum memories stationed along the transmission channel. To be effective, the memories must store the quantum information encoded on the optical modes for times that are long compared to the direct optical transmission time of the channel. Here we measure a decoherence rate of 8 × 10(-5) per second over 100 milliseconds, which is the time required for light transmission on a global scale. The measurements were performed on a ground-state hyperfine transition of europium ion dopants in yttrium orthosilicate ((151)Eu(3+):Y2SiO5) using optically detected nuclear magnetic resonance techniques. The observed decoherence rate is at least an order of magnitude lower than that of any other system suitable for an optical quantum memory. Furthermore, by employing dynamic decoupling, a coherence time of 370 ± 60 minutes was achieved at 2 kelvin. It has been almost universally assumed that light is the best long-distance carrier for quantum information. However, the coherence time observed here is long enough that nuclear spins travelling at 9 kilometres per hour in a crystal would have a lower decoherence with distance than light in an optical fibre. This enables some very early approaches to entanglement distribution to be revisited, in particular those in which the spins are transported rather than the light.
纠缠量子态的类空分离是量子力学基础研究和量子通信应用中的一个核心概念。在纠缠的分配中，光学方法无处不在，因为纠缠光子很容易产生和传输。然而，由于在传输过程中散射、衍射和吸收引起的损耗，将这种直接分配扩展到几百公里以外的全球网络是被禁止的。克服这种范围限制的一种方案是量子中继器协议，它涉及在沿传输通道设置的许多量子存储器之间分配纠缠的光模对。为了有效，存储器必须长时间存储光模上编码的量子信息，与通道的直接光传输时间相比，这段时间要长得多。在这里，我们测量了在 100 毫秒内的退相干率为 8×10(-5)每秒，这是全球范围内光传输所需的时间。该测量是在掺镝硅酸钇（(151)Eu(3+):Y2SiO5）的基态超精细跃迁中使用光检测磁共振技术进行的。观察到的退相干率至少比任何其他适合光学量子存储器的系统低一个数量级。此外，通过采用动态去耦，在 2 开尔文时实现了 370±60 分钟的相干时间。人们几乎普遍认为光在长距离量子信息传输中是最佳载体。然而，这里观察到的相干时间足够长，以至于在晶体中以 9 公里/小时的速度运动的核自旋的退相干距离将比光纤中的光长。这使得一些非常早期的纠缠分配方法得以重新审视，特别是那些传输自旋而不是光的方法。
