《自然》（20200730出版）一周論文導讀

編譯 | 李言

Nature, 30 July 2020，Volume 583 Issue 7818

《自然》2020年7月30日，第583卷，7818期

天文學Astronomy

Night-time measurements of astronomical seeing at Dome A in Antarctica

南極洲冰穹A的夜間天文觀測

▲ 作者：Bin Ma, Zhaohui Shang, Yi Hu, Keliang Hu, Yongjiang Wang, Xu Yang, Michael C. B. Ashley, Paul Hickson & Peng Jiang

▲ 連結：https://www.nature.com/articles/s41586-020-2489-0

▲ 摘要

南極高原上站點的特徵是在一個強而薄的邊界層之上的自由大氣中，湍流相對較弱。在此，我們報告夜晚使用差分圖像運動監視器在冰穹A得到的觀測結果。

在海拔僅8米的地方，它記錄下的觀測數據低至0.13弧秒，提供的觀測數據可與冰穹C的20米高度的觀測數據相媲美。這表明邊界層在8米以下的時間佔31%，平均觀測時間為0.31弧秒，與自由大氣觀測一致。觀測和邊界層厚度與近地表溫度梯度有較強的相關性。

這種相關性證實了從聲波雷達上發現的冰穹A邊界層的中值厚度約為14米。邊界層較薄，使得望遠鏡在其上方定位不那麼困難，因此可以更容易地進入自由大氣層。

▲ Abstract

Sites on the Antarctic plateau are characterized by comparatively weak turbulence in the free atmosphere above a strong but thin boundary layer. Here we report measurements of night-time seeing at Dome A, using a differential image motion monitor. Located at a height of just 8 metres, it recorded seeing as low as 0.13 arcseconds, and provided seeing statistics that are comparable to those at a height of 20 metres at Dome C. This indicates that the boundary layer was below 8 metres for 31 per cent of the time, with median seeing of 0.31 arcseconds, consistent with free-atmosphere seeing. The seeing and boundary-layer thickness are found to be strongly correlated with the near-surface temperature gradient. The correlation confirms a median thickness of approximately 14 metres for the boundary layer at Dome A, as found from a sonic radar. The thinner boundary layer makes it less challenging to locate a telescope above it, thereby giving greater access to the free atmosphere.

The tidal remnant of an unusually metal-poor globular cluster

一個異常缺乏金屬的球狀星團的潮汐殘留物

▲ 作者：Zhen Wan, Geraint F. Lewis, Ting S. Li, Jeffrey D. Simpson, Sarah L. Martell, et al.

▲ 連結：https://www.nature.com/articles/s41586-020-2483-6

▲ 摘要

金屬層的存在可能反映了倖存的球狀星團的最小質量和最大紅移，從而形成了理解宇宙質量積累的關鍵組成部分。

在此，我們報告來自南方恆星流光譜調查對於銀河系的光暈中鳳凰星流的測量結果。鳳凰星流的性質是一致的，它是一個球狀星團的潮汐中斷的殘餘。

然而，其金屬豐度（[Fe/H] =−2.7）大大低於經驗金屬豐度下限。因此，鳳凰星流代表了迄今為止發現的最缺乏金屬的球狀星團的碎片，它的前身與當前宇宙中的球狀星團不同。它的存在意味著金屬豐度底部以下的球狀星團可能曾經存在，但在銀河演化中被摧毀了。

▲ Abstract

The existence of this metallicity floor may reflect a minimum mass and a maximum redshift for surviving globular clusters to form—both critical components for understanding the build-up of mass in the Universe. Here we report measurements from the Southern Stellar Streams Spectroscopic Survey of the spatially thin, dynamically cold Phoenix stellar stream in the halo of the Milky Way. The properties of the Phoenix stream are consistent with it being the tidally disrupted remains of a globular cluster. However, its metal abundance ([Fe/H] = −2.7) is substantially below the empirical metallicity floor. The Phoenix stream thus represents the debris of the most metal-poor globular clusters discovered so far, and its progenitor is distinct from the present-day globular cluster population in the local Universe. Its existence implies that globular clusters below the metallicity floor have probably existed, but were destroyed during Galactic evolution.

物理學Physics

Waveguide quantum electrodynamics with superconducting artificial giant atoms

超導人工巨型原子的波導量子電動力學

▲ 作者：Bharath Kannan, Max J. Ruckriegel, Daniel L. Campbell, Anton Frisk Kockum, et al.

▲ 連結：https://www.nature.com/articles/s41586-020-2529-9

▲ 摘要

到目前為止，在巨型原子環境下對固態器件的實驗研究僅限於超導量子位，它與短波表面聲波耦合，以單一頻率探測原子的特性。

在此，我們使用了一種替代的結構，通過將小原子耦合到一個波導上，在多個分離的位置形成一個巨大的原子。該系統實現了可調諧的原子波導耦合，具有較大的通斷比和可由器件設計設計的耦合頻譜。

我們還演示了多個巨型原子之間的無退聚相互作用，這種作用是由波導中模式的準連續譜所介導的——使用小原子是無法實現這一效果的。這些特性使得該體系結構中的量子位在保護和發射結構之間原地切換，同時保留了量子位-量子位相互作用，為高保真量子模擬和非經典流動光子生成提供了可能性。

▲ Abstract

So far, experimental studies with solid-state devices in the giant-atom regime have been limited to superconducting qubits that couple to short-wavelength surface acoustic waves, probing the properties of the atom at only a single frequency. Here we use an alternative architecture that realizes a giant atom by coupling small atoms to a waveguide at multiple, but well separated, discrete locations. This system enables tunable atom–waveguide couplings with large on–off ratios3 and a coupling spectrum that can be engineered by the design of the device. We also demonstrate decoherence-free interactions between multiple giant atoms that are mediated by the quasi-continuous spectrum of modes in the waveguide—an effect that is not achievable using small atoms. These features allow qubits in this architecture to switch between protected and emissive configurations in situ while retaining qubit–qubit interactions, opening up possibilities for high-fidelity quantum simulations and non-classical itinerant photon generation.

Deep strong light–matter coupling in plasmonic nanoparticle crystals

等離子體納米顆粒晶體中強光物質的深耦合

▲ 作者：Niclas S. Mueller, Yu Okamura, Bruno G. M. Vieira, Sabrina Juergensen, Holger Lange, Eduardo B. Barros, Florian Schulz & Stephanie Reich

▲ 連結：https://www.nature.com/articles/s41586-020-2508-1

▲ 摘要

到目前為止，還沒有一種物質的電子激發表現出與自由空間光子的強耦合。在此，我們證明了如果粒子大於粒子間間隙的十倍，等離子體納米顆粒的三維晶體在環境條件下可以實現深度強耦合。

實驗中，直徑在25到60納米的金納米顆粒的面心立方晶體的拉比頻率（1.9到3.3電子伏特）比它們的等離子體元能量高出180%。我們證明了光子和電漿子的連續統一體雜化成偏振子，違反了旋轉波近似。耦合導致珀塞爾效應的擊穿——通過光物質耦合增加輻射阻尼——並增加輻射極化子的壽命。

結果表明，金屬納米粒子和半導體納米粒子可以作為構建一類具有超光物質相互作用的材料的基石，這將在非線性光學、協同效應和基態的研究、極化子化學和量子技術中得到應用。

▲ Abstract

So far, no electronic excitation of a material has shown such strong coupling to free-space photons. Here we show that three-dimensional crystals of plasmonic nanoparticles can realize deep strong coupling under ambient conditions, if the particles are ten times larger than the interparticle gaps. The experimental Rabi frequencies (1.9 to 3.3 electronvolts) of face-centred cubic crystals of gold nanoparticles with diameters between 25 and 60 nanometres exceed their plasmon energy by up to 180 per cent. We show that the continuum of photons and plasmons hybridizes into polaritons that violate the rotating-wave approximation. The coupling leads to a breakdown of the Purcell effect—the increase of radiative damping through light–matter coupling—and increases the radiative polariton lifetime. The results indicate that metallic and semiconducting nanoparticles can be used as building blocks for an entire class of materials with extreme light–matter interaction, which will find application in nonlinear optics, the search for cooperative effects and ground states, polariton chemistry and quantum technology.

Coherent many-body exciton in van der Waals antiferromagnet NiPS3

範德華磁性材料中發現的多體激子

▲ 作者：Soonmin Kang, Kangwon Kim, Beom Hyun Kim, Jonghyeon Kim, Kyung Ik Sim, et al.

▲ 連結：https://www.nature.com/articles/s41586-020-2520-5

▲ 摘要

激子是受庫侖相互作用約束的電子-空穴對玻色子準粒子。激子態的玻色-愛因斯坦凝聚長期以來一直是各種模型系統推測的主題，最近在光學晶格和二維材料中發現了更多的例子。

在此，我們展示了一個自旋軌道糾纏激子態出現在一種磁性範德華材料NiPS3，當它處於低於奈耳溫度150開的時候。它本質上起源於張-萊斯單重態的原型多體態，並在反鐵磁序的輔助下達到相干態。利用組態相互作用理論，我們確定了它是由張-萊斯三重態向張-萊斯單重態的轉變產生的。

我們結合了三種光譜工具——共振非彈性x射線散射、光致發光和光學吸收——來表徵激子，並證明激子線寬在50開以下的極窄。

▲ Abstract

An exciton is the bosonic quasiparticle of electron–hole pairs bound by the Coulomb interaction. Bose–Einstein condensation of this exciton state has long been the subject of speculation in various model systems, and examples have been found more recently in optical lattices and two-dimensional materials. Here we show that a spin–orbit-entangled exciton state appears below the Néel temperature of 150 kelvin in NiPS3, an antiferromagnetic van der Waals material. It arises intrinsically from the archetypal many-body states of the Zhang–Rice singlet, and reaches a coherent state assisted by the antiferromagnetic order. Using configuration-interaction theory, we determine the origin of the coherent excitonic excitation to be a transition from a Zhang–Rice triplet to a Zhang–Rice singlet. We combine three spectroscopic tools—resonant inelastic X-ray scattering, photoluminescence and optical absorption—to characterize the exciton and to demonstrate an extremely narrow excitonic linewidth below 50 kelvin.

氣候學Climatology

North Atlantic climate far more predictable than models imply

北大西洋氣候遠比模型所暗示的更可預測

▲ 作者：D. M. Smith, A. A. Scaife, R. Eade, P. Athanasiadis, A. Bellucci, I. Bethke, et al.

▲ 連結：https://www.nature.com/articles/s41586-020-2525-0

▲ 摘要

對氣候模型中的信號和不確定性進行量化，對於氣候變化的檢測、歸因、預測和預測至關重要。在這裡，我們評估了過去60年的氣候模型預測，表明北大西洋冬季氣候的年代際變化是高度可預測的，儘管單個模型模擬之間缺乏一致性，而且原始模型輸出的預測能力較差。

至關重要的是，目前的模型低估了北大西洋振蕩（北大西洋大氣環流中主要的變化模式）的可預測信號（總變化率的可預測部分）一個數量級。因此，與完美模型相比，現有模型需要100倍的集合成員來提取該信號，相對於其他因素，該信號對氣候的影響被低估了。

為了解決這些限制，我們實現了兩階段的後處理技術。我們首先調整北大西洋振蕩的整體平均預報的方差，以匹配可預測信號的觀測方差。然後，我們選擇和使用北大西洋振蕩充分接近方差調整的集合平均數預測北大西洋振蕩的集合成員。

這種方法大大改進了歐洲和北美東部冬季氣候的十年預測。對大西洋多年代際變化的預測也得到了改進，這表明北大西洋振蕩並不僅僅是由大西洋多年代際變化驅動的。

▲ Abstract

Quantifying signals and uncertainties in climate models is essential for the detection, attribution, prediction and projection of climate change. Here we assess retrospective climate model predictions of the past six decades and show that decadal variations in North Atlantic winter climate are highly predictable, despite a lack of agreement between individual model simulations and the poor predictive ability of raw model outputs. Crucially, current models underestimate the predictable signal (the predictable fraction of the total variability) of the North Atlantic Oscillation (the leading mode of variability in North Atlantic atmospheric circulation) by an order of magnitude. Consequently, compared to perfect models, 100 times as many ensemble members are needed in current models to extract this signal, and its effects on the climate are underestimated relative to other factors. To address these limitations, we implement a two-stage post-processing technique. We first adjust the variance of the ensemble-mean North Atlantic Oscillation forecast to match the observed variance of the predictable signal. We then select and use only the ensemble members with a North Atlantic Oscillation sufficiently close to the variance-adjusted ensemble-mean forecast North Atlantic Oscillation. This approach greatly improves decadal predictions of winter climate for Europe and eastern North America. Predictions of Atlantic multidecadal variability are also improved, suggesting that the North Atlantic Oscillation is not driven solely by Atlantic multidecadal variability.

動物學Zoology

Global status and conservation potential of reef sharks

珊瑚礁鯊的全球狀態和保護潛力

▲ 作者：M. Aaron MacNeil, Demian D. Chapman, Michelle Heupel, Colin A. Simpfendorfer, et al.

▲ 連結：https://www.nature.com/articles/s41586-020-2519-y

▲ 摘要

幾十年的過度開發已經摧毀了鯊魚的數量，也給它們的生態狀況留下了相當大的疑問。然而，我們對鯊魚的了解大多是從工業漁業的捕撈記錄中推斷出來的，而關於生活在沿海棲息地的鯊魚的信息則少得多。

在這裡，我們利用來自超過15000個標準化帶餌的遠程水下攝像頭的數據來解決這一資訊缺口，這些攝像頭被部署在58個國家的371個珊瑚礁上，以估計全球礁鯊的保護狀況。我們的研究結果揭示了捕魚對礁鯊數量的深遠影響：在我們調查的中，幾乎20%的珊瑚礁群裡沒有發現鯊魚。

在一些國家，礁鯊幾乎完全沒有出現在珊瑚礁中，鯊魚的減少與社會經濟條件密切相關，例如最近的市場的大小和鄰近程度、管理不善和人口密度。

然而，保護礁鯊的機會依然存在：鯊魚保護區、封閉區域、捕撈限制以及沒有刺網和延繩釣，都與礁鯊數量相對高得多有關。這些結果揭示了礁鯊種群恢復和管理的若干政策途徑，從自上而下的捕魚管理到間接改善治理條件。礁鯊只有通過參與熱帶漁業的關鍵社會經濟方面，才可能有機會恢復種群數量。

▲ Abstract

Decades of overexploitation have devastated shark populations, leaving considerable doubt as to their ecological status. Yet much of what is known about sharks has been inferred from catch records in industrial fisheries, whereas far less information is available about sharks that live in coastal habitats. Here we address this knowledge gap using data from more than 15,000 standardized baited remote underwater video stations that were deployed on 371 reefs in 58 nations to estimate the conservation status of reef sharks globally. Our results reveal the profound impact that fishing has had on reef shark populations: we observed no sharks on almost 20% of the surveyed reefs. Reef sharks were almost completely absent from reefs in several nations, and shark depletion was strongly related to socio-economic conditions such as the size and proximity of the nearest market, poor governance and the density of the human population. However, opportunities for the conservation of reef sharks remain: shark sanctuaries, closed areas, catch limits and an absence of gillnets and longlines were associated with a substantially higher relative abundance of reef sharks. These results reveal several policy pathways for the restoration and management of reef shark populations, from direct top-down management of fishing to indirect improvement of governance conditions. Reef shark populations will only have a high chance of recovery by engaging key socio-economic aspects of tropical fisheries.