自然（20220217出版）一周论文导读

　　编译 | 冯维维
　　Nature , Volume 602 Issue 7897, 17 February 2022
　　《自然》 2022年2月17日第602卷7897期
　　物理学 Physics
　　Thermal imaging of dust hiding the black hole in NGC 1068
　　活动星系核的统一模型得到确认
　　作者：Violeta Gámez Rosas, Jacob W. Isbell, Gerard Zins, etc.
　　链接：
　　https://www.nature.com/articles/s41586-021-04311-7
　　摘要
　　荷兰莱顿大学的Violeta Gamez Rosas和合作者对临近星系中心黑洞周围浓密遮挡的尘埃进行了观察，确认了活动星系核的统一模型。作者通过对这个星系的高分辨率图像分析得到了这一结论，这些图像提供了研究活动星系核的新机遇。
　　活动星系核是一些星系中心的高能区域，人们认为它由超大质量黑洞驱动。这些核发出的光产生的光谱特征，与光发出的区域有关；不同光谱可用于将活动星系核区分成两类：Ⅰ型和Ⅱ型天体。但统一模型认为，这种区分会产生，是因为观测视线可能有时被围绕和涌入中央黑洞的尘埃环遮挡了。
　　作者观察支持了统一模型。他们获得了原型星系NGC 1068的高分辨率图像，统一理论最初即由此建立。他们使用一个称为MATISSE的仪器，结合了四台欧洲南方天文台的望远镜的光，以获得活动星系核的最佳视图。对这些图像和射电图的分析揭示了环状尘埃云的存在，并帮助确定了其特性，大多与统一模型预测相符。作者还定位了黑洞在尘埃环之下的位置，同样符合模型。
　　Abstract
　　In the widely accepted ‘unified model’ solution of the classification puzzle of active galactic nuclei, the orientation of a dusty accretion torus around the central black hole dominates their appearance. In ‘type-1’ systems, the bright nucleus is visible at the centre of a face-on torus. In ‘type-2’ systems the thick, nearly edge-on torus hides the central engine. Later studies suggested evolutionary effects and added dusty clumps and polar winds but left the basic picture intact. However, recent high-resolution images of the archetypal type-2 galaxy NGC 1068, suggested a more radical revision. The images displayed a ring-like emission feature that was proposed to be hot dust surrounding the black hole at the radius where the radiation from the central engine evaporates the dust. That ring is too thin and too far tilted from edge-on to hide the central engine, and ad hoc foreground extinction is needed to explain the type-2 classification. These images quickly generated reinterpretations of the dichotomy between types 1 and 2. Here we present new multi-band mid-infrared images of NGC 1068 that detail the dust temperature distribution and reaffirm the original model. Combined with radio data (J.F.G. and C.M.V.I., manuscript in preparation), our maps locate the central engine that is below the previously reported ring and obscured by a thick, nearly edge-on disk, as predicted by the unified model. We also identify emission from polar flows and absorbing dust that is mineralogically distinct from that towards the Milky Way centre.
　　图片左侧展示了欧洲南方天文台（ESO）超大型望远镜上的FOcal Reducr和低色散光谱仪2（FRS2）拍摄的活动星系Messier 77的耀眼景色。右侧显示的是这个星系最内部区域的放大图，即其活动星系核，也是由ESO超大型望远镜干涉仪上的MATISSE仪器拍摄的。图片来自：ESO的Jaffe、Gamez-Rosas等人
　　此图展示了Messier 77的核心可能的样子。和其他活动星系核一样，Messier 77的中心区域由一个黑洞驱动，黑洞被一个厚厚的吸积盘包围。在Messier 77的例子中，这个厚厚的环完全遮住了我们对超大质量黑洞的观察。这个活动星系核亦被认为有喷流和尘埃风，它们从黑洞周围的区域流出，垂直于周围的吸积盘。图片来自：欧洲南方天文台M. Kornmesser和L. Calcada
　　Nuclear spin-wave quantum register for a solid-state qubit
　　固态量子位的核自旋波量子寄存器
　　作者：Andrei Ruskuc, Chun-Ju Wu, Jake Rochman, Joonhee Choi & Andrei Faraon
　　链接：
　　https://www.nature.com/articles/s41586-021-04293-6
　　摘要
　　围绕单个光可寻址量子位的固态核自旋是量子网络、计算和模拟的重要资源。虽然具有稀疏核自旋槽的宿主通常被用于减缓量子位退相干，但在核自旋丰富的宿主上开发相干量子系统，可为量子信息应用探索更广泛的材料。这些致密核自旋系综的集体模式为量子存储提供了自然基础，但用它们作为单自旋量子位元的资源迄今仍难以捉摸。
　　作者通过使用一个高相干、光学寻址的 171Yb3+ 量子比特掺杂到一个核自旋丰富的原钒酸钇晶体中，开发了一个鲁棒的量子控制协议来操纵邻近的51V5+ 晶格离子的多级核自旋态。通过动态设计的自旋交换相互作用，他们极化这个核自旋系综，产生集体自旋激发，然后使用其实现一个量子存储器。作者表示，该方法为利用高密度核自旋浴的复杂结构提供了一个框架，为利用单个稀土离子量子位构建大规模量子网络铺平了道路。
　　Abstract
　　Solid-state nuclear spins surrounding inpidual, optically addressable qubits are a crucial resource for quantum networks, computation and simulation. Although hosts with sparse nuclear spin baths are typically chosen to mitigate qubit decoherence, developing coherent quantum systems in nuclear-spin-rich hosts enables exploration of a much broader range of materials for quantum information applications. The collective modes of these dense nuclear spin ensembles provide a natural basis for quantum storage; however, using them as a resource for single-spin qubits has thus far remained elusive. Here, by using a highly coherent, optically addressed  171Yb3+  qubit doped into a nuclear-spin-rich yttrium orthovanadate crystal, we develop a robust quantum control protocol to manipulate the multi-level nuclear spin states of neighbouring 51V5+  lattice ions. Via a dynamically engineered spin-exchange interaction, we polarize this nuclear spin ensemble, generate collective spin excitations, and subsequently use them to implement a quantum memory. Our approach provides a framework for utilizing the complex structure of dense nuclear spin baths, paving the way towards building large-scale quantum networks using single rare-earth ion qubits.
　　Magnetic control of tokamak plasmas through deep reinforcement learning
　　基于深度强化学习的托卡马克等离子体磁控制
　　作者：Jonas Degrave, Federico Felici, Martin Riedmiller, etc.
　　链接：
　　https://www.nature.com/articles/s41586-021-04301-9
　　摘要
　　磁约束核聚变，特别是托卡马克结构核聚变是一种有前景的可持续能源。一个核心挑战是塑造和维持托卡马克容器内的高温等离子体。这需要使用磁致动器线圈进行高维、高频、闭环控制，更复杂的是等离子体结构的不同要求。
　　作者介绍了一个此前尚未描述的托卡马克磁控制器设计的架构，它可以自主学习命令全套控制线圈。该体系结构满足在高层指定的控制目标，同时满足物理和操作约束。这种方法在问题规范方面具有前所未有的灵活性和通用性，并显著减少了生产新等离子体配置的设计工作。
　　Abstract
　　Nuclear fusion using magnetic confinement, in particular in the tokamak configuration, is a promising path towards sustainable energy. A core challenge is to shape and maintain a high-temperature plasma within the tokamak vessel. This requires high-dimensional, high-frequency, closed-loop control using magnetic actuator coils, further complicated by the perse requirements across a wide range of plasma configurations. In this work, we introduce a previously undescribed architecture for tokamak magnetic controller design that autonomously learns to command the full set of control coils. This architecture meets control objectives specified at a high level, at the same time satisfying physical and operational constraints. This approach has unprecedented flexibility and generality in problem specification and yields a notable reduction in design effort to produce new plasma configurations.
　　Resolving the gravitational redshift across a millimetre-scale atomic sample
　　解析毫米尺度原子样本的引力红移
　　作者：Tobias Bothwell, Colin J. Kennedy, Alexander Aeppli, Dhruv Kedar, John M. Robinson, Eric Oelker, Alexander Staron & Jun Ye
　　链接：
　　https://www.nature.com/articles/s41586-021-04349-7
　　摘要
　　爱因斯坦的广义相对论指出，时钟在不同的引力势下，相对于实验室坐标的速度是不同的——这一效应被称为引力红移。作为空间和时间的基本探测器，原子钟长期以来被用于在30厘米到数千公里的距离尺度上检验这一预测。
　　一旦时钟对弯曲时空中振荡的量子物体的有限波函数变得敏感，将使广义相对论和量子力学的结合研究成为可能。
　　作者在一个毫米尺度的超冷锶样品中测量了与引力红移一致的线性频率梯度。通过将分数频率测量的不确定度提高10倍以上，达到7.6   1 0 21 ，研究结果得以实现。这预示着一种新的时钟操作方式，需要对引力扰动进行样品内校正。
　　Abstract
　　Einstein’s theory of general relativity states that clocks at different gravitational potentials tick at different rates relative to lab coordinates—an effect known as the gravitational redshift. As fundamental probes of space and time, atomic clocks have long served to test this prediction at distance scales from 30 centimetres to thousands of kilometres. Ultimately, clocks will enable the study of the union of general relativity and quantum mechanics once they become sensitive to the finite wavefunction of quantum objects oscillating in curved space-time. Towards this regime, we measure a linear frequency gradient consistent with the gravitational redshift within a single millimetre-scale sample of ultracold strontium. Our result is enabled by improving the fractional frequency measurement uncertainty by more than a factor of 10, now reaching 7.6   1 0 21 . This heralds a new regime of clock operation necessitating intra-sample corrections for gravitational perturbations.
　　化学 Chemistry
　　Hydrogen trapping and embrittlement in high-strength Al alloys
　　高强度铝合金的脆化
　　作者：Huan Zhao, Poulami Chakraborty, Dirk Ponge, Tilmann Hickel, Binhan Sun, Chun-Hung Wu, Baptiste Gault & Dierk Raabe
　　链接：
　　https://www.nature.com/articles/s41586-021-04343-z
　　摘要
　　交通运输业对温室气体排放的限制越来越严格，这促使人们重新审视汽车材料。高强度铝合金常用于飞机，有助于减轻汽车的重量，但容易受到环境退化的影响。氢＂脆化＂常被认为是主要原因；但其失效的确切机制尚不清楚。因为对合金内部H的原子尺度分析仍然是一个挑战，这阻碍了采用合金设计策略来提高材料的耐久性。
　　作者对高强度7xxx铝合金中第二相颗粒和晶界处的H进行了近原子尺度的分析。他们利用这些观察结果来指导原子从头计算，结果表明合金元素与H的共偏析有利于晶界脱聚，H强分块进入第二相粒子，将溶质H从基体中去除，从而防止H脆断。该见解进一步推进了对铝合金中H辅助脆化机理的理解，强调了H陷阱在减少开裂和指导新合金设计方面的作用。
　　Abstract
　　Ever more stringent regulations on greenhouse gas emissions from transportation motivate efforts to revisit materials used for vehicles. High-strength aluminium alloys often used in aircrafts could help reduce the weight of automobiles, but are susceptible to environmental degradation. Hydrogen ‘embrittlement’ is often indicated as the main culprit; however, the exact mechanisms underpinning failure are not precisely known: atomic-scale analysis of H inside an alloy remains a challenge, and this prevents deploying alloy design strategies to enhance the durability of the materials. Here we performed near-atomic-scale analysis of H trapped in second-phase particles and at grain boundaries in a high-strength 7xxx Al alloy. We used these observations to guide atomistic ab initio calculations, which show that the co-segregation of alloying elements and H favours grain boundary decohesion, and the strong partitioning of H into the second-phase particles removes solute H from the matrix, hence preventing H embrittlement. Our insights further advance the mechanistic understanding of H-assisted embrittlement in Al alloys, emphasizing the role of H traps in minimizing cracking and guiding new alloy design.
　　气候 Climate
　　Warming weakens the night-time barrier to global fire
　　气候变暖削弱夜间抵御全球火灾的屏障
　　作者：Jennifer K. Balch, John T. Abatzoglou, Maxwell B. Joseph, Michael J. Koontz, Adam L. Mahood, Joseph McGlinchy, Megan E. Cattau & A. Park Williams
　　链接：
　　https://www.nature.com/articles/s41586-021-04325-1
　　摘要
　　因为夜间温度和水汽压差（VPD）较低，这一时间为减缓或扑灭火灾提供了一个关键窗口。然而，火灾危险通常是根据白天的情况来评估的，所以要抓住促进火灾蔓延的因素，而非阻止火灾的因素。尽管白天天气条件的变化会加剧火灾，但人们对夜间天气条件的潜在变化及其与火灾减少相关的作用还知之甚少。
　　作者展示了夜间火灾强度的增加，与更热、更干燥的夜晚有关。基于全球卫星对日间和夜间火灾探测的观测以及相应的每小时气候数据，他们确定了与陆地覆盖相关的VPD阈值，低于这个阈值的火灾探测非常罕见。从1979年到2020年，全球每日最低VPD增加了25%。在可燃土地上，每年夜间可燃时间增加了110个小时，这使得可燃性永不停止的夜晚增加了5个小时。在这段时间内，近五分之一的可燃土地上，易燃的夜晚至少增加了一周。根据卫星记录，从全球来看，2003年至2020年，夜间火灾的强度增加了7.2%。这些结果进一步表明，近年来扑灭野火的队伍在夜间缺乏救援。作者预计，人为气候变化导致的夜间持续变暖将导致更强烈、更持久和更大的火灾。
　　Abstract
　　Night-time provides a critical window for slowing or extinguishing fires owing to the lower temperature and the lower vapour pressure deficit (VPD). However, fire danger is most often assessed based on daytime conditions, capturing what promotes fire spread rather than what impedes fire. Although it is well appreciated that changing daytime weather conditions are exacerbating fire, potential changes in night-time conditions—and their associated role as fire reducers—are less understood. Here we show that night-time fire intensity has increased, which is linked to hotter and drier nights. Our findings are based on global satellite observations of daytime and night-time fire detections and corresponding hourly climate data, from which we determine landcover-specific thresholds of VPD (VPDt), below which fire detections are very rare (less than 95 per cent modelled chance). Globally, daily minimum VPD increased by 25 per cent from 1979 to 2020. Across burnable lands, the annual number of flammable night-time hours—when VPD exceeds VPDt—increased by 110 hours, allowing five additional nights when flammability never ceases. Across nearly one-fifth of burnable lands, flammable nights increased by at least one week across this period. Globally, night fires have become 7.2 per cent more intense from 2003 to 2020, measured via a satellite record. These results reinforce the lack of night-time relief that wildfire suppression teams have experienced in recent years. We expect that continued night-time warming owing to anthropogenic climate change will promote more intense, longer-lasting and larger fires.