nature杂志2021年封面

编译 | 未玖

Nature ,6 May 2021,VOL 593,ISSUE 7857

《自然》 2021年5月6日，第593卷，7857期

天文学 Astronomy

Jets from MRC 0600-399 bent by magnetic fields in the cluster Abell 3376

Abell 3376星系团磁场使MRC 0600-399射流弯曲

作者：James O. Chibueze, Haruka Sakemi, Takumi Ohmura, Mami Machida, Hiroki Akamatsu, Takuya Akahori, et al.

链接：

摘要

众所周知，星系团内部有磁场，但磁场的性质至今仍未明确。在冷稠密等离子体进入热环境等离子体形成的密度接触不连续处可以观察到星系团内部磁场，该不连续处存在于合并星系团Abell3376（红移0.0461）中第二亮星系MRC 0600-399附近。

东西方向拉长的X射线发射显示出一个彗星状的结构，达到百万秒差距规模。先前的射电观测发现MRC 0600-399的弯曲喷流，与子星团朝着相同的方向移动，对抗冲压。

研究组报道了MRC 0600-399的射电观测结果，其分辨率和灵敏度分别是先前结果的3.4倍和11倍。与典型射流相比，MRC 0600-399在接触不连续处呈现90度弯曲，准直射流从弯曲点延伸超过100千秒差距。

研究组观察到弥漫的、拉长的发射，将其称之为“双镰刀”结构。光谱指数在弯曲点的下游趋于平缓，表明宇宙射线重新加速。高分辨率数值模拟表明，沿不连续面的有序磁场对射流方向的变化有重要影响。双镰刀射流的形态与模拟结果一致。

该研究结果有助于了解磁场对星系团成员星系和内部介质演化的影响。

Abstract

Galaxy clusters are known to harbour magnetic fields, the nature of which remains unresolved. Intra-cluster magnetic fields can be observed at the density contact discontinuity formed by cool and dense plasma running into hot ambient plasma, and the discontinuity exists near the second-brightest galaxy, MRC 0600-399, in the merging galaxy cluster Abell 3376 (redshift 0.0461). Elongated X-ray emission in the east–west direction shows a comet-like structure that reaches the megaparsec scale. Previous radio observations detected the bent jets from MRC 0600-399, moving in same direction as the sub-cluster, against ram pressure. Here we report radio observations of MRC 0600-399 that have 3.4 and 11 times higher resolution and sensitivity, respectively, than the previous results. In contrast to typical jets, MRC 0600-399 shows a 90-degree bend at the contact discontinuity, and the collimated jets extend over 100 kiloparsecs from the point of the bend. We see diffuse, elongated emission that we name ‘double-scythe’ structures. The spectral index flattens downstream of the bend point, indicating cosmic-ray reacceleration. High-resolution numerical simulations reveal that the ordered magnetic field along the discontinuity has an important role in the change of jet direction. The morphology of the double-scythe jets is consistent with the simulations. Our results provide insights into the effect of magnetic fields on the evolution of the member galaxies and intra-cluster medium of galaxy clusters.

物理学 Physics

Leading hadronic contribution to the muon magnetic moment from lattice QCD

晶格QCD谬子磁矩的领头阶强子贡献

作者：Sz. Borsanyi, Z. Fodor, J. N. Guenther, C. Hoelbling, S. D. Katz, L. Lellouch, et al.

链接：

摘要

粒子物理学的标准模型描述了绝大多数涉及基本粒子的实验和观测。任何偏离它的预测都将是新基础物理学的标志。一个长期存在的偏差与谬子的反常磁矩有关，谬子是一种测量粒子周围磁场的方法。

标准模型预测与紧密分布在3.7个标准差附近的测量结果不一致。当前理论误差和测量误差是可比的；然而，正在进行和计划中的实验旨在将测量误差减少四分之一。理论上，主要误差来源是领头阶强子真空极化（LO-HVP）的贡献。

对于即将进行的测量，必须用独立方法来评估这一贡献的预测，并减少其不确定性。到目前为止，最精确的、与模型无关的测定依赖于色散技术，并结合了电子-正电子湮没为强子的横截面测量。

为了消除对这些实验的依赖，研究组使用从头计算量子色动力学（QCD）和量子电动力学模拟来计算LO-HVP的贡献。

研究组达到了足够的精度来区分谬子反常磁矩的测量和色散方法的预测。该研究结果比用色散关系得到的结果更符合实验测量值。此外，这项工作中使用和开发的方法有望实现精度更高、更强大的计算机。

Abstract

The standard model of particle physics describes the vast majority of experiments and observations involving elementary particles. Any deviation from its predictions would be a sign of new, fundamental physics. One long-standing discrepancy concerns the anomalous magnetic moment of the muon, a measure of the magnetic field surrounding that particle. Standard-model predictions exhibit disagreement with measurements that is tightly scattered around 3.7 standard deviations. Today, theoretical and measurement errors are comparable; however, ongoing and planned experiments aim to reduce the measurement error by a factor of four. Theoretically, the dominant source of error is the leading-order hadronic vacuum polarization (LO-HVP) contribution. For the upcoming measurements, it is essential to evaluate the prediction for this contribution with independent methods and to reduce its uncertainties. The most precise, model-independent determinations so far rely on dispersive techniques, combined with measurements of the cross-section of electron–positron annihilation into hadrons. To eliminate our reliance on these experiments, here we use ab initio quantum chromodynamics (QCD) and quantum electrodynamics simulations to compute the LO-HVP contribution. We reach sufficient precision to discriminate between the measurement of the anomalous magnetic moment of the muon and the predictions of dispersive methods. Our result favours the experimentally measured value over those obtained using the dispersion relation. Moreover, the methods used and developed in this work will enable further increased precision as more powerful computers become available.

Rashba valleys and quantum Hall states in few-layer black arsenic

薄层黑砷中的拉什巴谷和量子霍尔态

作者：Feng Sheng, Chenqiang Hua, Man Cheng, Jie Hu, Xikang Sun, Qian Tao, et al.

链接：

摘要

在非中心对称的二维电子系统中，自旋-轨道耦合（SOC）与库仑相互作用、能带拓扑和外部调制力动态相互作用时，可能会出现令人兴奋的现象。

研究组报道了中心对称的薄层黑砷中SOC和斯塔克效应之间的协同效应，表现为粒子-空穴不对称的拉什巴谷形成和由静电门可逆控制的新奇量子霍尔态。

这些不寻常的发现源于黑砷的皱褶方形晶格，其中重4p轨道形成了一个以 pz 对称的布里渊区为中心的Γ谷，并在X点的时间反转不变动量附近与 px 起源双重简并D谷共存。

当垂直电场破坏结构反转对称性时，强拉什巴SOC被 px 带激活，产生时间反转对称的自旋谷“味”的 D 谷，而Γ谷的拉什巴劈裂受到 pz 对称性的约束。有趣的是，巨大的斯塔克效应显示了相同的 px 轨道选择性，共同移动 D 拉什巴谷的能带最大值，使其超过Γ拉什巴顶部。

这种协调效应使研究组能够实现二维空穴气体的门-可调拉什巴谷操控，其特点是由于形成了依赖于“味”的朗道能级谱，量子霍尔态发生了非常规的偶-奇跃迁。对于二维电子气体，Γ拉什巴谷的量子化特征是从平凡的抛物型腔到螺旋狄拉克费米子的能带拓扑结构中的特殊密度依赖跃迁。

Abstract

Exciting phenomena may emerge in non-centrosymmetric two-dimensional electronic systems when spin–orbit coupling (SOC) interplays dynamically with Coulomb interactions, band topology and external modulating forces. Here we report synergetic effects between SOC and the Stark effect in centrosymmetric few-layer black arsenic, which manifest as particle–hole asymmetric Rashba valley formation and exotic quantum Hall states that are reversibly controlled by electrostatic gating. The unusual findings are rooted in the puckering square lattice of black arsenic, in which heavy 4p orbitals form a Brillouin zone-centred Γ valley with pz symmetry, coexisting with doubly degenerate D valleys of px origin near the time-reversal-invariant momenta of the X points. When a perpendicular electric field breaks the structure inversion symmetry, strong Rashba SOC is activated for the px bands, which produces spin–valley-flavoured D valleys paired by time-reversal symmetry, whereas Rashba splitting of the Γ valley is constrained by the pz symmetry. Intriguingly, the giant Stark effect shows the same px-orbital selectiveness, collectively shifting the valence band maximum of the D Rashba valleys to exceed the Γ Rashba top. Such an orchestrating effect allows us to realize gate-tunable Rashba valley manipulations for two-dimensional hole gases, hallmarked by unconventional even-to-odd transitions in quantum Hall states due to the formation of a flavour-dependent Landau level spectrum. For two-dimensional electron gases, the quantization of the Γ Rashba valley is characterized by peculiar density-dependent transitions in the band topology from trivial parabolic pockets to helical Dirac fermions.

气候学 Climatology

Projected land ice contributions to twenty-first-century sea level rise

陆地冰对21世纪海平面上升的贡献预测

作者：Tamsin L. Edwards, Sophie Nowicki, Ben Marzeion, Regine Hock, Heiko Goelzer, Hélène Seroussi, et al.

链接：

摘要

陆地冰对全球平均海平面上升的贡献尚未通过最新一组 社会 经济情景的冰盖和冰川模型预测，也未通过由各种计算机模型所产生的不确定性进行的协调 探索 预测。最近的两个国际项目使用多种模型进行了大量预测，但主要使用上一代情景和气候模型，无法充分 探索 已知的不确定性。

研究组使用冰盖和冰川模型的统计模拟来估计新情景下这些预测的概率分布。他们发现，与目前的排放承诺相比，将全球升温限制在1.5 以内，陆地冰融化造成的21世纪海平面上升将减少一半。到2100年，平均海平面当量（SLE）从25 cm降至13 cm，冰川占海平面贡献的一半。

由于在气候变暖过程中冰损失增加和降雪积累的竞争过程存在不确定性，预测的南极贡献并未对排放情景作出明确反应。然而，在风险规避（悲观）的假设下，南极冰损失可能高出5倍，在当前政策和承诺下，陆地冰对SLE的贡献中位数增加到42 cm，即使在1.5 升温的情况下，第95个百分位的预测值也超过半米。

这几乎不可能缓解未来沿海洪水的威胁。考虑到这一大范围（13 cm至42cm SLE），在气候政策和南极应对措施进一步受到限制之前，21世纪海平面上升的适应规划必须考虑到陆地冰贡献的三个不确定性因素。

Abstract

The land ice contribution to global mean sea level rise has not yet been predicted using ice sheet and glacier models for the latest set of socio-economic scenarios, nor using coordinated exploration of uncertainties arising from the various computer models involved. Two recent international projects generated a large suite of projections using multiple models, but primarily used previous-generation scenarios and climate models, and could not fully explore known uncertainties. Here we estimate probability distributions for these projections under the new scenarios using statistical emulation of the ice sheet and glacier models. We find that limiting global warming to 1.5 degrees Celsius would halve the land ice contribution to twenty-first-century sea level rise, relative to current emissions pledges. The median decreases from 25 to 13 centimetres sea level equivalent (SLE) by 2100, with glaciers responsible for half the sea level contribution. The projected Antarctic contribution does not show a clear response to the emissions scenario, owing to uncertainties in the competing processes of increasing ice loss and snowfall accumulation in a warming climate. However, under risk-averse (pessimistic) assumptions, Antarctic ice loss could be five times higher, increasing the median land ice contribution to 42 centimetres SLE under current policies and pledges, with the 95th percentile projection exceeding half a metre even under 1.5 degrees Celsius warming. This would severely limit the possibility of mitigating future coastal flooding. Given this large range (between 13 centimetres SLE using the main projections under 1.5 degrees Celsius warming and 42 centimetres SLE using risk-averse projections under current pledges), adaptation planning for twenty-first-century sea level rise must account for a factor-of-three uncertainty in the land ice contribution until climate policies and the Antarctic response are further constrained.

The Paris Climate Agreement and future sea-level rise from Antarctica

巴黎气候协定和未来南极洲海平面上升

作者：Robert M. DeConto, David Pollard, Richard B. Alley, Isabella Velicogna, Edward Gasson, Natalya Gomez, et al.

链接：

摘要

《巴黎协定》旨在将21世纪全球平均升温控制在比工业化前水平高2 以内，并进一步努力控制在1.5 以内。由于海洋热膨胀和陆地冰损失的综合作用，未来几十年的温室气体排放量将对全球平均海平面（GMSL）造成一个世纪甚至更长时间的影响。

南极冰原（AIS）是地球上最大的陆地冰库，其冰损失正在加剧。AIS的广大区域位于海平面以下，容易受到动态不稳定性的影响，这种不稳定性能够产生非常迅速的冰盖消退。然而，实施《巴黎协定》温控目标以减缓或阻止这些不稳定发生的可能性，尚未通过基于物理学的模型进行直接测试。

研究组使用一个经过观测校准的冰盖-冰架模型来证明，当全球升温被限制在2 以内时，整个21世纪南极冰层的流失速度将与今天类似。然而，与现行政策更为一致的情景（允许升温3 ）将在2060年左右使南极冰层融化的速度突然加快，到2100年将导致GMSL每年上升约0.5厘米，比今天快一个数量级。

更多化石燃料密集使用会导致更大的加速。虽然有基岩和海平面反馈机制，以及地球工程二氧化碳减排，但因支撑冰架的变薄和消失而引发的冰盖消退将持续好几个世纪。这些结果表明，如果超出《巴黎协定》的目标，南极洲海平面将有可能迅速而不可阻挡地上升。

Abstract

The Paris Agreement aims to limit global mean warming in the twenty-first century to less than 2 degrees Celsius above preindustrial levels, and to promote further efforts to limit warming to 1.5 degrees Celsius. The amount of greenhouse gas emissions in coming decades will be consequential for global mean sea level (GMSL) on century and longer timescales through a combination of ocean thermal expansion and loss of land ice. The Antarctic Ice Sheet (AIS) is Earth’s largest land ice reservoir, and its ice loss is accelerating. Extensive regions of the AIS are grounded below sea level and susceptible to dynamical instabilities that are capable of producing very rapid retreat. Yet the potential for the implementation of the Paris Agreement temperature targets to slow or stop the onset of these instabilities has not been directly tested with physics-based models. Here we use an observationally calibrated ice sheet–shelf model to show that with global warming limited to 2 degrees Celsius or less, Antarctic ice loss will continue at a pace similar to today’s throughout the twenty-first century. However, scenarios more consistent with current policies (allowing 3 degrees Celsius of warming) give an abrupt jump in the pace of Antarctic ice loss after around 2060, contributing about 0.5 centimetres GMSL rise per year by 2100—an order of magnitude faster than today. More fossil-fuel-intensive scenarios result in even greater acceleration. Ice-sheet retreat initiated by the thinning and loss of buttressing ice shelves continues for centuries, regardless of bedrock and sea-level feedback mechanisms or geoengineered carbon dioxide reduction. These results demonstrate the possibility that rapid and unstoppable sea-level rise from Antarctica will be triggered if Paris Agreement targets are exceeded.

考古学 Archaeology

Earliest known human burial in Africa

非洲最早的人类墓葬

作者：María Martinón-Torres, Francesco d’Errico, Elena Santos, Ana Álvaro Gallo, Noel Amano, William Archer, et al.

链接：

摘要

人类殡葬习俗的起源和演变是人们感兴趣和喜欢争论的话题。中石器时代（MSA）的人类墓葬在非洲很罕见，在东非也不为人知。

研究组在肯尼亚热带高地海岸的Panga ya Saidi（PYS）洞穴遗址MSA层中发现了一具距今7.83万年前的2.5-3.0岁幼儿的部分骨骼。最新挖掘揭示了这个坑的特征，里面有一个蜷曲的幼儿。

对坑中内容物和包裹考古层的地球化学、粒度和微观形态的分析表明，该坑是人为挖掘的。埋藏学证据，如骨骼部分的严密连接或良好的解剖关系以及腐烂的组织学证据，均支持新鲜尸体的就地分解。

在分解过程中，不稳定的关节几乎没有移位，这表明其埋葬在一个填满的空间（墓地）中，至此，人们在PYS发现了非洲已知最古老的人类墓葬。

部分骨骼的形态评估与智人分类一致，尽管在牙齿中保留了一些原始特征，但越来越多的证据表明，在我们物种出现的过程中，现代特征的非渐进组装。PYS墓葬揭示了MSA人群如何与死者互动。

Abstract

The origin and evolution of hominin mortuary practices are topics of intense interest and debate. Human burials dated to the Middle Stone Age (MSA) are exceedingly rare in Africa and unknown in East Africa. Here we describe the partial skeleton of a roughly 2.5- to 3.0-year-old child dating to 78.3 4.1 thousand years ago, which was recovered in the MSA layers of Panga ya Saidi (PYS), a cave site in the tropical upland coast of Kenya. Recent excavations have revealed a pit feature containing a child in a flexed position. Geochemical, granulometric and micromorphological analyses of the burial pit content and encasing archaeological layers indicate that the pit was deliberately excavated. Taphonomical evidence, such as the strict articulation or good anatomical association of the skeletal elements and histological evidence of putrefaction, support the in-place decomposition of the fresh body. The presence of little or no displacement of the unstable joints during decomposition points to an interment in a filled space (grave earth), making the PYS finding the oldest known human burial in Africa. The morphological assessment of the partial skeleton is consistent with its assignment to Homo sapiens, although the preservation of some primitive features in the dentition supports increasing evidence for non-gradual assembly of modern traits during the emergence of our species. The PYS burial sheds light on how MSA populations interacted with the dead.

量子通信是当下热门的科研话题之一。

但由于光子的衰减，量子通信会被距离所限制， 这些光子就正如神话故事里的“牛郎”和“织女”一样，被分隔在光纤两端 。

如果说牛郎织女可以靠着“鹊桥”每隔一年相会一次。

那么在量子世界里，能让“光子牛郎”和“光子织女”相遇的“鹊桥”就是 “量子中继” 。

中国科学技术大学郭光灿院士团队 李传锋、周宗权研究组 利用固态量子存储器和外置纠缠光源，首次实现两个 吸收型量子存储器之间的量子纠缠 ，并演示了多模式量子中继。

该研究成果登上国际著名学术期刊《Nature》新一期封面，这也是中国量子存储和量子中继领域的重大进展。

受限于光子数在光纤中的指数衰减，远程量子纠缠的传输距离被限制在百公里水平。

中科大科研团队是这么描绘远程量子纠缠传输难题：“通过光纤向距离一千公里外的地方每秒发射一百亿个光子，要花三百年才能接收到一个光子。”

距离问题，就成了当下量子网络建设亟待解决的问题之一 。

为此，科学家们提出量子中继的思想，即将远距离传输划分为若干短距离基本链路，先在基本链路的两个临近节点间建立可预报的量子纠缠，然后通过纠缠交换技术进行级联，从而逐步扩大量子纠缠的距离。

通俗易懂来讲： 如果直接发送光子很困难，那么可以像短跑接力一样，将光子分段传输，从而实现远距离通信。

这其中的技术核心是量子存储技术，作为量子中继的核心器件， 量子存储器对光子比特进行缓存，并用于储存光子纠缠态 。

而当前业内一直研究的课题，就是 提升纠缠连接效率 。

目前，国际上的研究者已在冷原子气体和单量子系统中实现量子中继的基本链路，但均基于 发射型量子存储器 构建，其纠缠光子是由存储器本身发射出来的。此前，李传锋教授在接受新华社采访时表示：发射型量子存储器，要么一次只能传输1个量子，效率低；要么一次传输多个量子，但精确率低。

这种架构难以同时支持确定性光子发射和多模式复用存储，限制了纠缠分发的速率。

而此次李传锋、周宗权研究组的 吸收型量子存储器 的量子中继架构，把量子存储器和量子光源分离开来，故能同时兼容确定性光子源和多模式复用， 是目前理论上通信速率最优的量子中继方案 。

简单来说， 存储和发射分离开来，可以保证传输时的精确度。

此前，李传锋、周宗权研究组长期从事基于稀土离子掺杂晶体的固态量子存储器的研究。

早在2015年，该团队就首次利用光子的空间自由度实现复用量子存储，存储维度数达到51维，至今保持固态量子存储维度数最高水平。

此后，研究组还进一步证明他们的存储器可以在时间和频率自由度实现任意脉冲操作，代表性的操作包括脉冲排序、分束、分频、异频光子合束和窄带滤波等。

在本次实验里，研究组研究的是 基于稀土离子掺杂晶体 的固态量子存储，这种存储器利用两块1.4毫米厚的掺钕钒酸钇晶体，分别处理光的两种正交偏振态。

基于独创的“三明治”结构，每对纠缠光子中的一个光子被三明治型量子存储器所存储，然后，每对纠缠光子中的另一个光子会同时传输至中间站点，这就实现了量子中继。

《Nature》杂志审稿人对此次的研究成功给予高度评价并表示：“这是在地面上实现远距离量子网络的一项重大成就。”

总结来讲，这次研究成果就是如何让单个微弱的纠缠态光子在光纤中尽可能远的传输，传输的其实依然是量子密钥，距离真正的“量子+通讯”还有一定距离。

但这依然是中科大这些年量子通讯领域一系列进步的延续，也延续着该领域的国际领先地位。

目前来看， 中国已经走在了国际量子通信领域的前列。

一方面，中国拥有世界上最好的光纤资源，这为其大规模实施量子通信提供了土壤。另一方面，行业头部企业在产品上的突破和通信巨头的入局，国内外相关标准制定的推进，也为量子通信低成本产业化带来可能。

在未来， 信息安全本身是一个刚需、庞大的市场，而量子通信作为信息安全中的重要战略产业 ，也将一直保持行业技术突破，直到我国实现“量子霸权”。

对于今后的研究方向，李传锋教授表示：下一步，研究组将继续提高量子存储器的各项指标，并采用确定性纠缠光源，从而大幅提高纠缠分发的速率，努力实现超越光纤直接传输的实用化量子中继器。

我们也期待中科大科研团队能在未来有更大的突破。