《自然》（20200827出版）一周論文導讀

編譯 | 未玖

nature,27 August 2020,VOL 584,ISSUE 7822

《自然》2020年8月27日，第584卷，7822期

物理學Physics

Impact of ionizing radiation on superconducting qubit coherence

電離輻射對超導量子位相干性的影響

▲ 作者：Antti P. Vepsäläinen, Amir H. Karamlou, John L. Orrell, Akshunna S. Dogra, Ben Loer, Francisca Vasconcelos, et al.

▲ 連結：

https://www.nature.com/articles/s41586-020-2619-8

▲ 摘要

依賴量子位的技術需要較長的相干時間和高保真運算。超導量子位是實現這些目標的主要平臺之一。但超導量子位的相干性受分裂庫珀電子對的影響。

實驗觀測到的分裂庫珀對（稱為準粒子）的密度比巴丁-庫珀-施裡弗超導理論預測的平衡值高出幾個數量級。

先前的工作表明，紅外光子會大大增加準粒子密度，但即使在最佳隔離的系統中，它仍然遠高於預期，這表明存在另一種生成機制。

研究組提供的證據表明，來自環境放射性物質和宇宙射線的電離輻射會導致這種觀測差異。電離輻射的影響導致準粒子密度升高，研究組推測這最終把該處測量的超導量子位相干時間限制在毫秒級別。

研究組進一步證明，屏蔽輻射可減少電離輻射的通量，從而增加能量弛豫時間。儘管對現今量子位的影響很小，但減少或減輕電離輻射的影響對於實現容錯超導量子計算機至關重要。

▲ Abstract

Technologies that rely on quantum bits (qubits) require long coherence times and high-fidelity operations. Superconducting qubits are one of the leading platforms for achieving these objectives. However, the coherence of superconducting qubits is affected by the breaking of Cooper pairs of electrons. The experimentally observed density of the broken Cooper pairs, referred to as quasiparticles, is orders of magnitude higher than the value predicted at equilibrium by the Bardeen-Cooper-Schrieffer theory of superconductivity. Previous work has shown that infrared photons considerably increase the quasiparticle density, yet even in the best-isolated systems, it remains much higher than expected, suggesting that another generation mechanism exists. Here we provide evidence that ionizing radiation from environmental radioactive materials and cosmic rays contributes to this observed difference. The effect of ionizing radiation leads to an elevated quasiparticle density, which we predict would ultimately limit the coherence times of superconducting qubits of the type measured here to milliseconds. We further demonstrate that radiation shielding reduces the flux of ionizing radiation and thereby increases the energy-relaxation time. Albeit a small effect for today’s qubits, reducing or mitigating the impact of ionizing radiation will be critical for realizing fault-tolerant superconducting quantum computers.

Electronically integrated, mass-manufactured, microscopic robots

電子集成、批量生產的微型機器人

▲ 作者：Marc Z. Miskin, Alejandro J. Cortese, Kyle Dorsey, Edward P. Esposito, Michael F. Reynolds, Qingkun Liu, et al.

▲ 連結：

https://www.nature.com/articles/s41586-020-2626-9

▲ 摘要

五十年來微電子領域摩爾定律標度為微型機器人領域的迅速發展帶來了巨大機遇。

現今電、磁和光學系統提供了前所未有的複雜性、小尺寸和低成本的組合，可用於製造小於人類視覺解析度極限（小於100微米）的機器人。

但存在一個主要障礙：尚未有可以與半導體加工無縫集成並響應標準電子控制信號的微米級致動器系統。

研究組通過開發一種新型電壓可控電化學致動器來克服這一障礙，它可以在低電壓（200微伏）、低功率（10毫瓦）下工作，並且與矽加工完全兼容。

為了展示其潛力，研究組開發了小於100微米的步行機器人原型的光刻製作和發布協議。過程的每一步都並行進行，可在每4英寸矽晶片上生產100多萬個機器人。

該研究結果是向大規模製造、矽基、功能性機器人的重大進步，這些機器人非常小，以至於肉眼無法分辨。

▲ Abstract

Fifty years of Moore’s law scaling in microelectronics have brought remarkable opportunities for the rapidly evolving field of microscopic robotics. Electronic, magnetic and optical systems now offer an unprecedented combination of complexity, small size and low cost, and could be readily appropriated for robots that are smaller than the resolution limit of human vision (less than a hundred micrometres). However, a major roadblock exists: there is no micrometre-scale actuator system that seamlessly integrates with semiconductor processing and responds to standard electronic control signals. Here we overcome this barrier by developing a new class of voltage-controllable electrochemical actuators that operate at low voltages (200 microvolts), low power (10 nanowatts) and are completely compatible with silicon processing. To demonstrate their potential, we develop lithographic fabrication-and-release protocols to prototype sub-hundred-micrometre walking robots. Every step in this process is performed in parallel, allowing us to produce over one million robots per four-inch wafer. These results are an important advance towards mass-manufactured, silicon-based, functional robots that are too small to be resolved by the naked eye.

化學Chemistry

Tying different knots in a molecular strand

在一條分子鏈上打不同的結

▲ 作者：David A. Leigh, Fredrik Schaufelberger, Lucian Pirvu, Joakim Halldin Stenlid, David P. August & Julien Segard

▲ 連結：

https://www.nature.com/articles/s41586-020-2614-0

▲ 摘要

從鞋帶到用於攀登、釣魚和航行的繩結，結的特性使其獲得廣泛應用。儘管結存在於DNA、蛋白質及其他長鏈聚合物中，但在一條合成納米鏈中打出不同類型結的方法鮮有報導。

高對稱性的分子結此前通過非共價相互作用組裝和纏結分子鏈來合成，但在這種情況下，模板和/或鏈結構本質上決定了拓撲結構，這意味著通常可能只有一種類型的結。

研究組展示了在人工分子鏈中不同金屬離子的分散配位使其能夠打成多個結。利用過渡金屬和鑭系離子以類似蛋白質伴侶的方式誘導鏈摺疊，在同一分子鏈中選擇性地製備了三種拓撲異構體：一個大環結（01）、一個三葉結（31）和一個三扭結（52）。

研究組發現金屬離子誘導的摺疊可通過立體引導進行：在單結的情況下，鑭系元素（III）配位的正交圖案僅由特定手性的銅（I）配位正交形成。

研究組還出乎意料地發現，金屬離子配位將分子結結構從一個區域轉移至另一個區域，導致新打結的構象扭曲（拓撲應變）增加。

結的拓撲結構會影響鏈的化學性質：較緊實的52結可以同時結合兩種不同的金屬離子，而較鬆散的31異構體只能結合一個銅（I）離子或一個鑥（III）離子。

在納米鏈上打不同類型結的能力，為探索合成低聚物、聚合物和超分子的結構修飾和性質提供了機遇。

▲ Abstract

The properties of knots are exploited in a range of applications, from shoelaces to the knots used for climbing, fishing and sailing. Although knots are found in DNA and proteins, and form randomly in other long polymer chains, methods for tying different sorts of knots in a synthetic nanoscale strand are lacking. Molecular knots of high symmetry have previously been synthesized by using non-covalent interactions to assemble and entangle molecular chains, but in such instances the template and/or strand structure intrinsically determines topology, which means that only one type of knot is usually possible. Here we show that interspersing coordination sites for different metal ions within an artificial molecular strand enables it to be tied into multiple knots. Three topoisomers—an unknot (01) macrocycle, a trefoil (31) knot, and a three-twist (52) knot—were each selectively prepared from the same molecular strand by using transition-metal and lanthanide ions to guide chain folding in a manner reminiscent of the action of protein chaperones. We find that the metal-ion-induced folding can proceed with stereoinduction: in the case of one knot, a lanthanide(III)-coordinated crossing pattern formed only with a copper(I)-coordinated crossing of particular handedness. In an unanticipated finding, metal-ion coordination was also found to translocate an entanglement from one region of a knotted molecular structure to another, resulting in an increase in writhe (topological strain) in the new knotted conformation. The knot topology affects the chemical properties of the strand: whereas the tighter 52 knot can bind two different metal ions simultaneously, the looser 31 isomer can bind only either one copper(I) ion or one lutetium(III) ion. The ability to tie nanoscale chains into different knots offers opportunities to explore the modification of the structure and properties of synthetic oligomers, polymers and supramolecules.

地理學Geography

Vulnerability of Antarctica’s ice shelves to meltwater-driven fracture

南極洲冰架易受融水驅動而崩塌

▲ 作者：Ching-Yao Lai, Jonathan Kingslake, Martin G. Wearing, Po-Hsuan Cameron Chen, Pierre Gentine, Harold Li, et al.

▲ 連結：

https://www.nature.com/articles/s41586-020-2627-8

▲ 摘要

大氣變暖可能加速南極冰蓋的後退，因為它加劇了表面融化，並促進了「水力壓裂」，即融水流入並擴大裂縫，可能引發冰架崩塌。

支撐冰蓋的冰架崩塌加速了冰流和海平面上升。但人們不知道在被水淹沒時，南極洲冰架的支撐區域是否以及有多少容易發生水力壓裂。

研究組提供了兩條證據，證明許多支撐區域很脆弱。

首先，研究組訓練了一個深度卷積神經網絡（DCNN）來繪製所有南極冰架衛星圖像中裂縫的表面情況；

其次，研究組基於線性彈性斷裂力學建立了裂縫穩定性圖，以預測在當前應力條件下基底和乾燥表面裂縫的形成位置。儘管在衛星圖像中檢測裂縫存在局限性，但研究組發現理論預測和DCNN繪製的裂縫之間密切一致；

最後，研究組使用線性彈性斷裂力學理論來預測表面裂縫在充滿水後會變得不穩定的地方。

如今，許多經常被融水淹沒的地區對水力壓裂具有一定的復原性，應力很低，使得所有充滿水的裂縫都很穩定。但60±10%的冰架（按面積）都支撐著上遊的冰層，如果被水淹沒，很容易發生水力壓裂。

DCNN圖證實了這些支撐區域存在裂縫。如果地表融化加劇，導致水淹沒研究組所確定的廣泛脆弱區域，則可能引發水力壓裂。這些地區是大氣變暖對冰蓋質量平衡影響最大的地區。

▲ Abstract

Atmospheric warming threatens to accelerate the retreat of the Antarctic Ice Sheet by increasing surface melting and facilitating 『hydrofracturing』, where meltwater flows into and enlarges fractures, potentially triggering ice-shelf collapse. The collapse of ice shelves that buttress the ice sheet accelerates ice flow and sea-level rise. However, we do not know if and how much of the buttressing regions of Antarctica’s ice shelves are vulnerable to hydrofracture if inundated with water. Here we provide two lines of evidence suggesting that many buttressing regions are vulnerable. First, we trained a deep convolutional neural network (DCNN) to map the surface expressions of fractures in satellite imagery across all Antarctic ice shelves. Second, we developed a stability diagram of fractures based on linear elastic fracture mechanics to predict where basal and dry surface fractures form under current stress conditions. We find close agreement between the theoretical prediction and the DCNN-mapped fractures, despite limitations associated with detecting fractures in satellite imagery. Finally, we used linear elastic fracture mechanics theory to predict where surface fractures would become unstable if filled with water. Many regions regularly inundated with meltwater today are resilient to hydrofracture—stresses are low enough that all water-filled fractures are stable. Conversely, 60 ± 10 per cent of ice shelves (by area) both buttress upstream ice and are vulnerable to hydrofracture if inundated with water. The DCNN map confirms the presence of fractures in these buttressing regions. Increased surface melting could trigger hydrofracturing if it leads to water inundating the widespread vulnerable regions we identify. These regions are where atmospheric warming may have the largest impact on ice-sheet mass balance.

Glacial cooling and climate sensitivity revisited

氣候敏感性與冰川降溫

▲ 作者：Jessica E. Tierney, Jiang Zhu, Jonathan King, Steven B. Malevich, Gregory J. Hakim & Christopher J. Poulsen

▲ 連結：

https://www.nature.com/articles/s41586-020-2617-x

▲ 摘要

末次冰盛期（LGM）是研究古氣候變化最好的時期之一，為研究氣候系統如何響應溫室氣體和冰層變化提供了絕佳機會。

先前研究試圖通過古溫度計來限制冰川冷卻的幅度和模式，但這些指標分布不均，並具有不確定性，較難構建全場的LGM氣候狀態。

研究組結合了大量海面溫度的地球化學指標與使用同位素的氣候模型集合，使用數據同化來進行LGM溫度的場重建。

重建的有效性通過保留指標以及獨立冰芯和洞穴碳酸鹽d18O測量來驗證。研究組的同化數據對全球平均LGM冷卻的限制為-6.1℃。

考慮到溫室氣體、冰原和礦物粉塵氣溶膠的輻射力量，這種冷卻轉化為3.4℃（2.4-4.5℃）的平衡氣候敏感性，該數值高於先前基於LGM的估計值，但與傳統的2-4.5℃範圍一致。

▲ Abstract

The Last Glacial Maximum (LGM), one of the best studied palaeoclimatic intervals, offers an excellent opportunity to investigate how the climate system responds to changes in greenhouse gases and the cryosphere. Previous work has sought to constrain the magnitude and pattern of glacial cooling from palaeothermometers, but the uneven distribution of the proxies, as well as their uncertainties, has challenged the construction of a full-field view of the LGM climate state. Here we combine a large collection of geochemical proxies for sea surface temperature with an isotope-enabled climate model ensemble to produce a field reconstruction of LGM temperatures using data assimilation. The reconstruction is validated with withheld proxies as well as independent ice core and speleothem d18Omeasurements. Our assimilated product provides a constraint on global mean LGM cooling of −6.1 degrees Celsius. Given assumptions concerning the radiative forcing of greenhouse gases, ice sheets and mineral dust aerosols, this cooling translates to an equilibrium climate sensitivity of 3.4 degrees Celsius (2.4-4.5 degrees Celsius), a value that is higher than previous LGM-based estimates but consistent with the traditional consensus range of 2-4.5 degrees Celsius.

天文學Astronomy

A measure of the size of the magnetospheric accretion region in TW Hydrae

TW-Hydrae磁層吸積區尺寸的測量

▲ 作者：GRAVITY Collaboration

▲ 連結：

https://www.nature.com/articles/s41586-020-2613-1

▲ 摘要

對年輕恆星體的光譜幹涉研究表明，氫輻射（眾所周知的吸積活動示蹤劑）主要來自幾毫秒跨度的區域，通常位於塵埃升華半徑之內。氫輻射的起源可能是恆星磁層、旋轉的星風或吸積盤。

對於中等質量的赫比格Ae/Be星，布拉開γ（Brγ）輻射在空間上可分辨，這排除了大部分輻射來自磁層的可能性，因為該來源中檢測到的弱磁場會形成非常緊密的磁層。

金牛T型星源中較大的磁層使其更易於分辨。磁層的角度很小（十分之幾毫秒），但加上星風的存在，觀測結果很難解釋。

研究組報導了光學長基線幹涉觀測，在空間上解析了金牛T型星TW Hydrae的內盤。他們發現近紅外氫輻射來自大約3.5恆星半徑的區域。

該區域在連續的塵埃盤發射區域內（橫跨7個恆星半徑），且在共轉半徑之內，是前者的兩倍大。這表明氫輻射起源於吸積柱，正如磁層吸積模型所預期的那樣，而不是源於更大距離（超過一個天文單位）的星風。

▲ Abstract

Spectro-interferometric studies in young stellar objects show that hydrogen emission (a well known tracer of accretion activity) mostly comes from a region a few milliarcseconds across, usually located within the dust sublimation radius. The origin of the hydrogen emission could be the stellar magnetosphere, a rotating wind or a disk. In the case of intermediate-mass Herbig AeBe stars, the fact that Brackett γ(Brγ) emission is spatially resolved rules out the possibility that most of the emission comes from the magnetosphere because the weak magnetic fields detected in these sources result in very compact magnetospheres. In the case of T Tauri sources, their larger magnetospheres should make them easier to resolve. The small angular size of the magnetosphere (a few tenths of a milliarcsecond), however, along with the presence of winds make the interpretation of the observations challenging. Here we report optical long-baseline interferometric observations that spatially resolve the inner disk of the T Tauri star TW Hydrae. We find that the near-infrared hydrogen emission comes from a region approximately 3.5 stellar radii across. This region is within the continuum dusty disk emitting region (7 stellar radii across) and also within the corotation radius, which is twice as big. This indicates that the hydrogen emission originates in the accretion columns, as expected in magnetospheric accretion models, rather than in a wind emitted at much larger distance (more than one astronomical unit).