《科学》（20210416出版）一周论文导读
编译 | 李言
Science , 16 APRIL 2021, VOL 372, ISSUE 6539
《科学》 2021年4月16日，第372卷，6539期
古生物学 Paleobiology
Absolute abundance and preservation rate of Tyrannosaurus rex
霸王龙的绝对丰度和化石保存率
作者：Charles R. Marshall, Daniel V. Latorre, Connor J. Wilson, Tanner M. Frank, et al.
链接：
https://science.sciencemag.org/content/372/6539/284
摘要
虽然仅从化石就可以推断出许多信息，但要估计灭绝物种的数量和化石生存率，还需要现有物种的数据。在此，我们利用现存物种之间的种群密度和体重之间的关系，结合我们对霸王龙的了解，来计算种群变量和成年霸王龙数量。
我们估算，在个体数量为 20000左右的时候，它们能延续12.7万代，而曾经存在的霸王龙总数约为25亿只，每8000万只霸王龙能存有一块化石。
这些值的不确定性超过两个数量级，主要是由于密度—体质量关系的变化，而不是由于古生物输入变量的变化。
Abstract
Although much can be deduced from fossils alone, estimating abundance and preservation rates of extinct species requires data from living species. Here, we use the relationship between population density and body mass among living species combined with our substantial knowledge of Tyrannosaurus rex to calculate population variables and preservation rates for postjuvenile T. rex . We estimate that its abundance at any one time was ~20,000 inpiduals, that it persisted for ~127,000 generations, and that the total number of T. rex that ever lived was ~2.5 billion inpiduals, with a fossil recovery rate of 1 per ~80 million inpiduals or 1 per 16,000 inpiduals where its fossils are most abundant. The uncertainties in these values span more than two orders of magnitude, largely because of the variance in the density–body mass relationship rather than variance in the paleobiological input variables.
物理学 Physics
Realization of a multinode quantum network of remote solid-state qubits
远程固态量子比特的多态量子网络的实现
作者：M. Pompili, S. L. N. Hermans, S. Baier, H. K. C. Beukers, et al.
链接：
https://science.sciencemag.org/content/372/6539/259
摘要
纠缠态在未来量子互联网节点上的分布将开启根本性的新技术。在此，我们报告一个基于三节点纠缠的量子网络的实现。我们将基于金刚石通信量子位的远程量子节点组合成一个可扩展的相位稳定架构，并辅以健壮的存储量子位和局部量子逻辑。
此外，我们还实现了网络上的实时通讯和前馈门运作。我们证明了两个没有后选择的量子网络协议：真正的多部纠缠态在三个节点上的分布和纠缠态通过一个中间节点进行交换。我们的工作为 探索 、测试和开发多态量子网络协议和量子网络控制栈奠定了关键平台。
Abstract
The distribution of entangled states across the nodes of a future quantum internet will unlock fundamentally new technologies. Here, we report on the realization of a three-node entanglement-based quantum network. We combine remote quantum nodes based on diamond communication qubits into a scalable phase-stabilized architecture, supplemented with a robust memory qubit and local quantum logic. In addition, we achieve real-time communication and feed-forward gate operations across the network. We demonstrate two quantum network protocols without postselection: the distribution of genuine multipartite entangled states across the three nodes and entanglement swapping through an intermediary node. Our work establishes a key platform for exploring, testing, and developing multinode quantum network protocols and a quantum network control stack.
Gapped magnetic ground state in quantum spin liquid candidate κ-(BEDT-TTF)2Cu2(CN)3
量子自旋液体候选κ-(BEDT-TTF)2Cu2(CN)3的缝隙磁基态
作者：Björn Miksch, Andrej Pustogow, Mojtaba Javaheri Rahim, Andrey A. Bardin, et al.
链接：
https://science.sciencemag.org/content/372/6539/276
摘要
几何挫折、量子纠缠和无序可能会阻止具有强交换作用的局域自旋的长程有序，从而导致物质的奇异状态。κ-(BEDT-TTF)2Cu2(CN)3被认为是这种难以捉摸的量子自旋液态的主要候选者，但它的基态性质仍然令人费解。
我们提出一个毫开尔文温度以下的多频率电子自旋共振（ESR）的研究，揭示了自旋敏感性在6 k时的迅速下降。这种自旋间隙的打开，伴随着结构的改变，与固态价键基态的形成是一致的。
我们发现当本征自旋形成单线态时，杂质对ESR响应的贡献占主导地位。直接探测电子表明了缺陷对于无磁序量子自旋系统的低能量特性的关键作用。
Abstract
Geometrical frustration, quantum entanglement, and disorder may prevent long-range ordering of localized spins with strong exchange interactions, resulting in an exotic state of matter. κ-(BEDT-TTF)2Cu2(CN)3 is considered the prime candidate for this elusive quantum spin liquid state, but its ground-state properties remain puzzling. We present a multifrequency electron spin resonance (ESR) study down to millikelvin temperatures, revealing a rapid drop of the spin susceptibility at 6 kelvin. This opening of a spin gap, accompanied by structural modifications, is consistent with the formation of a valence bond solid ground state. We identify an impurity contribution to the ESR response that becomes dominant when the intrinsic spins form singlets. Probing the electrons directly manifests the pivotal role of defects for the low-energy properties of quantum spin systems without magnetic order.
材料科学 Materials Sciences
Nematicity and competing orders in superconducting magic-angle graphene
超导魔角石墨烯的线形和竞争有序性
作者：Yuan Cao, Daniel Rodan-Legrain, Jeong Min Park, Noah F. Q. Yuan, et al.
链接：
https://science.sciencemag.org/content/372/6539/264
摘要
在固体系统中，强相互作用的电子在基态往往表现出多重破坏的对称性。不同序参量之间的相互作用可以产生丰富的相图。我们报告在魔角扭曲双层石墨烯（TBG）中识别具有破碎旋转对称性的缠绕相。
利用横向电阻测量，我们发现一个强各向异性相位位于超导圆顶欠掺杂区域上方的“楔”内。当它与超导圆顶交叉时，我们观察到临界温度的降低。
此外，超导态对方向相关的平面内磁场表现出各向异性的响应，揭示了整个超导穹丘的向列有序。这些结果表明，向列起伏可能在TBG的低温相中起重要作用。
Abstract
Strongly interacting electrons in solid-state systems often display multiple broken symmetries in the ground state. The interplay between different order parameters can give rise to a rich phase diagram. We report on the identification of intertwined phases with broken rotational symmetry in magic-angle twisted bilayer graphene (TBG). Using transverse resistance measurements, we find a strongly anisotropic phase located in a “wedge” above the underdoped region of the superconducting dome. Upon its crossing with the superconducting dome, a reduction of the critical temperature is observed. Furthermore, the superconducting state exhibits an anisotropic response to a direction-dependent in-plane magnetic field, revealing nematic ordering across the entire superconducting dome. These results indicate that nematic fluctuations might play an important role in the low-temperature phases of magic-angle TBG.
Realization of an ideal Weyl semimetal band in a quantum gas with 3D spin-orbit coupling
基于三维自旋轨道耦合的量子气体中理想外尔半金属带的实现
作者：Zong-Yao Wang, Xiang-Can Cheng, Bao-Zong Wang, Jin-Yi Zhang, et al.
链接：
https://science.sciencemag.org/content/372/6539/271
摘要
外尔半金属是三维无间隙拓扑相，在体带中存在外尔锥。根据格点论，外尔锥必须成对出现，锥的最小数目为两个。只有两个外尔锥的半金属是理想外尔半金属（IWSM）。
在此，我们报道了通过工程的三维自旋轨道耦合在超冷原子上实现IWSM带的实验结果。在平衡态下，通过虚拟切片成像技术可以清晰地测量拓扑外尔点，并在动力学中得到进一步的解决。
IWSM波段的实现为研究固体中难以接触的各种奇异现象开辟了一条新途径。
Abstract
Weyl semimetals are three-dimensional (3D) gapless topological phases with Weyl cones in the bulk band. According to lattice theory, Weyl cones must come in pairs, with the minimum number of cones being two. A semimetal with only two Weyl cones is an ideal Weyl semimetal (IWSM). Here we report the experimental realization of an IWSM band by engineering 3D spin-orbit coupling for ultracold atoms. The topological Weyl points are clearly measured via the virtual slicing imaging technique in equilibrium and are further resolved in the quench dynamics. The realization of an IWSM band opens an avenue to investigate various exotic phenomena that are difficult to access in solids.
环境科学 Environmental Sciences
Carbon loss from boreal forest wildfires offset by increased dominance of deciduous trees
北方森林野火造成的碳损失被落叶树木的增加所抵消
作者：Michelle C. Mack, Xanthe J. Walker, Jill F. Johnstone, Heather D. Alexander, et al.
链接：
https://science.sciencemag.org/content/372/6539/280
摘要
在北方森林中，气候变暖正在将野火干扰机制转变为更频繁的火灾，这些火灾会更深入地燃烧到有机土壤中，将封存的碳释放到大气中。为了理解碳储量的不稳定，有必要在长期生态变化的背景下考虑这些影响。
在阿拉斯加的北方针叶林中，我们发现，在严重火灾的催化下，优势植物物种的变化弥补了土壤碳在10年时间尺度上的更大燃烧。有机土壤的严重燃烧使树木的优势从生长缓慢的黑云杉转移到生长迅速的落叶阔叶树，导致土壤碳储量在扰动周期内净增加5倍。
减少未来以落叶为主的北方针叶林的火灾活动可以增加这种碳在景观上的保有时间，从而减轻对气候变暖的反馈。
Abstract
In boreal forests, climate warming is shifting the wildfire disturbance regime to more frequent fires that burn more deeply into organic soils, releasing sequestered carbon to the atmosphere. To understand the destabilization of carbon storage, it is necessary to consider these effects in the context of long-term ecological change. In Alaskan boreal forests, we found that shifts in dominant plant species catalyzed by severe fire compensated for greater combustion of soil carbon over decadal time scales. Severe burning of organic soils shifted tree dominance from slow-growing black spruce to fast-growing deciduous broadleaf trees, resulting in a net increase in carbon storage by a factor of 5 over the disturbance cycle. Reduced fire activity in future deciduous-dominated boreal forests could increase the tenure of this carbon on the landscape, thereby mitigating the feedback to climate warming.
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