《自然》（20210715出版）一周论文导读

冯维维科学网

编译｜冯维维

Nature, 15 July 2021, VOL 595, Issue 7867

《自然》2021年7月15日，第595卷，7867期

天体和量子物理

Astro- & Quantum Physics

The ¹³CO-rich atmosphere of a young accreting super-Jupiter

年轻木星具有富含¹³CO的大气层

▲ 作者：Yapeng Zhang, Ignas A. G. Snellen, Alexander J. Bohn, Paul Mollière, Christian Ginski, H. Jens Hoeijmakers, Matthew A. Kenworthy, Eric E. Mamajek, Tiffany Meshkat, Maddalena Reggiani & Frans Snik

▲ 链接：

https://www.nature.com/articles/s41586-021-03616-x

▲ 摘要

同位素丰度比在天文学和行星科学中有着重要的作用，为研究太阳系的起源和演化、星际化学和恒星核合成提供了重要依据。与氘/氢的比率相反，碳同位素比率在太阳系中被发现大致不变（约89），但在银河系尺度上有所变化，在当前的本地星际介质中，同位素比率为¹²C/¹³C，约为68。在分子云和原行星盘中，¹²CO/¹³CO比值可以通过冰和气体的分配、低温同位素离子交换反应和同位素选择性光解而改变。

作者报告了在年轻的、正在吸积的超级木星TYC 8998-760- 1b的大气中观测到的¹³CO，其统计意义大于6 西格玛。忽略地球的大气温度结构、化学成分和光谱校准的不确定性表明，¹²CO/¹³CO比率为

（90%的可信度），相对于地球标准和当地星际值而言，¹³C相当丰富。

由于TYC 8998-760-1 b的当前位置大于或等于160个天文单位，远远超出了CO雪线，作者们假设它的碳很大一部分是通过分馏从富含¹³C的冰中积累的。

▲ Abstract

Isotope abundance ratios have an important role in astronomy and planetary sciences, providing insights into the origin and evolution of the Solar System, interstellar chemistry and stellar nucleosynthesis. In contrast to deuterium/hydrogen ratios, carbon isotope ratios are found to be roughly constant (around 89) in the Solar System, but do vary on galactic scales with a ¹²C/¹³C isotopologue ratio of around 68 in the current local interstellar medium. In molecular clouds and protoplanetary disks, ¹²CO/¹³CO ratios can be altered by ice and gas partitioning, low-temperature isotopic ion-exchange reactions and isotope-selective photodissociation9. Here we report observations of ¹³CO in the atmosphere of the young, accreting super-Jupiter TYC 8998-760-1 b, at a statistical significance of more than six sigma. Marginalizing over the planet’s atmospheric temperature structure, chemical composition and spectral calibration uncertainties suggests a ¹²CO/¹³CO ratio of

(90% confidence), a substantial enrichment in ¹³C with respect to the terrestrial standard and the local interstellar value. As the current location of TYC 8998-760-1 b at greater than or equal to 160 astronomical units is far beyond the CO snowline, we postulate that it accreted a substantial fraction of its carbon from ices enriched in ¹³C through fractionation.

Exponential suppression of bit or phase errors with cyclic error correction

迈向纠错量子计算机

▲ 作者：Google Quantum AI

▲ 链接：

https://www.nature.com/articles/s41586-021-03588-y

▲ 摘要

量子计算机和经典计算机一样，很容易出现由底层物理系统“噪声”引起的错误。一种解决办法是在计算机操作中加入一种能在错误出现时发现并纠正这些错误的方法。一种量子纠错方法使用量子纠错码，通过将多个量子比特（量子信息的单位，对应经典计算机的比特）当作一个逻辑量子比特，从而在不破坏逻辑量子比特中存储信息的情况下，发现并纠正错误。为了实现量子计算的潜力，逻辑错误率必须保持在很低的水平。

作者报道称，谷歌AI设计的量子处理器“悬铃木”（Sycamore）实现了错误抑制的指数增长。该实验演示或为可扩展容错量子计算机的开发铺平了道路。

他们研究了悬铃木处理器的量子纠错能力，悬铃木包含一个54超导量子比特的二维阵列。他们运行了两种量子纠错码：一种是最多由21个量子比特组成的一维链重复码，用来测试错误抑制能力；另一种是由7个量子比特组成的二维表面码，作为与更大码的设置相容性的原理验证实验。作者的研究表明，将重复码基于的量子比特数量从5个提高到21个，对逻辑错误的抑制实现了最多100倍的指数增长。这种错误抑制能力在50次纠错实验中均表现稳定。

这些结果之所以令人振奋，是因为它们表明量子纠错可以成功将错误率控制在一定范围内。虽然这个错误率还没达到实现量子计算机潜力的阈值，但本文的结果表明，悬铃木架构或已逼近这一阈值。

▲ Abstract

Realizing the potential of quantum computing requires sufficiently low logical error rates1. Many applications call for error rates as low as 10⁻¹⁵, but state-of-the-art quantum platforms typically have physical error rates near 10−3. Quantum error correction promises to bridge this divide by distributing quantum logical information across many physical qubits in such a way that errors can be detected and corrected. Errors on the encoded logical qubit state can be exponentially suppressed as the number of physical qubits grows, provided that the physical error rates are below a certain threshold and stable over the course of a computation. Here we implement one-dimensional repetition codes embedded in a two-dimensional grid of superconducting qubits that demonstrate exponential suppression of bit-flip or phase-flip errors, reducing logical error per round more than 100-fold when increasing the number of qubits from 5 to 21. Crucially, this error suppression is stable over 50 rounds of error correction. We also introduce a method for analysing error correlations with high precision, allowing us to characterize error locality while performing quantum error correction. Finally, we perform error detection with a small logical qubit using the 2D surface code on the same device and show that the results from both one- and two-dimensional codes agree with numerical simulations that use a simple depolarizing error model. These experimental demonstrations provide a foundation for building a scalable fault-tolerant quantum computer with superconducting qubits.

Real-time optimal quantum control of mechanical motion at room temperature

室温下机械运动的实时最优量子控制

▲ 作者：Lorenzo Magrini, Philipp Rosenzweig, Constanze Bach, Andreas Deutschmann-Olek, Sebastian G. Hofer, Sungkun Hong, Nikolai Kiesel, Andreas Kugi & Markus Aspelmeyer

▲ 链接：

https://www.nature.com/articles/s41586-021-03602-3

▲ 摘要

通过测量和反馈精确控制物理系统动力学的能力是现代工程的支柱。今天，对应用量子技术日益增长的需求要求对单个量子系统进行这种级别的控制。

以最优的方式实现这一目标是一项具有挑战性的任务，它依赖于量子有限的测量和专门定制的状态估计和反馈算法。迄今为止，成功的实施包括光学和原子系统水平的实验。

作者演示了光学捕获纳米粒子量子轨迹的实时最优控制。他们将接近海森堡极限的共焦位置传感与卡尔曼滤波的最优状态估计相结合，在位置不确定性为零点波动1.3倍的情况下实时跟踪粒子在相空间中的运动。优化反馈使量子谐振子稳定在平均占用0.56±0.02个量子，实现了室温下的量子基态冷却。

作者表示，该研究建立了量子卡尔曼滤波作为一种方法来实现量子控制的机械运动，具有潜在的意义上的传感在所有尺度。结合悬浮，这为全面控制固态宏观量子物体在线性和非线性系统中的波包动力学铺平了道路。

▲ Abstract

The ability to accurately control the dynamics of physical systems by measurement and feedback is a pillar of modern engineering. Today, the increasing demand for applied quantum technologies requires adaptation of this level of control to individual quantum systems. Achieving this in an optimal way is a challenging task that relies on both quantum-limited measurements and specifically tailored algorithms for state estimation and feedback. Successful implementations thus far include experiments on the level of optical and atomic systems. Here we demonstrate real-time optimal control of the quantum trajectory8 of an optically trapped nanoparticle. We combine confocal position sensing close to the Heisenberg limit with optimal state estimation via Kalman filtering to track the particle motion in phase space in real time with a position uncertainty of 1.3 times the zero-point fluctuation. Optimal feedback allows us to stabilize the quantum harmonic oscillator to a mean occupation of 0.56 ± 0.02 quanta, realizing quantum ground-state cooling from room temperature. Our work establishes quantum Kalman filtering as a method to achieve quantum control of mechanical motion, with potential implications for sensing on all scales. In combination with levitation, this paves the way to full-scale control over the wavepacket dynamics of solid-state macroscopic quantum objects in linear and nonlinear systems.

Quantum control of a nanoparticle optically levitated in cryogenic free space

低温自由空间光学悬浮纳米粒子的量子控制

▲ 作者：Felix Tebbenjohanns, M. Luisa Mattana, Massimiliano Rossi, Martin Frimmer & Lukas Novotny

▲ 链接：

https://www.nature.com/articles/s41586-021-03617-w

▲ 摘要

宏观尺度上的量子力学测试要求对机械运动及其退相干进行极端控制。通过设计谐振腔内微机械振荡器与电磁场之间的辐射-压力耦合，机械运动的量子控制已经实现。此外，以测量为基础的、依赖腔增强探测方案的反馈控制，已被用于冷却微机械振荡器到他们的量子基态。

与机械拴系系统相比，光悬浮纳米粒子尤其有希望用于大质量物体的物质波实验，因为它们的捕获潜力完全可控。

作者在低温自由空间中光学悬浮一个飞克（10⁻¹⁵克）介电粒子，这足以抑制热效应，使测量反向作用成为主要的退相干机制。通过有效的量子测量，他们对粒子的动力学进行量子控制。他们通过基于测量的反馈将其质心运动冷却为平均占据0.65个运动量子，对应的状态纯度为0.43。光学谐振器的缺乏及其带宽限制使人们有望将电磁场的全部量子控制转移到机械系统中。作者表示，该实验平台为研究宏观尺度下的量子力学提供了一条途径。

▲ Abstract

Tests of quantum mechanics on a macroscopic scale require extreme control over mechanical motion and its decoherence. Quantum control of mechanical motion has been achieved by engineering the radiation–pressure coupling between a micromechanical oscillator and the electromagnetic field in a resonator. Furthermore, measurement-based feedback control relying on cavity-enhanced detection schemes has been used to cool micromechanical oscillators to their quantum ground states.

In contrast to mechanically tethered systems, optically levitated nanoparticles are particularly promising candidates for matter-wave experiments with massive objects, since their trapping potential is fully controllable. Here we optically levitate a femtogram (10⁻¹⁵ grams) dielectric particle in cryogenic free space, which suppresses thermal effects sufficiently to make the measurement backaction the dominant decoherence mechanism. With an efficient quantum measurement, we exert quantum control over the dynamics of the particle. We cool its centre-of-mass motion by measurement-based feedback to an average occupancy of 0.65 motional quanta, corresponding to a state purity of 0.43. The absence of an optical resonator and its bandwidth limitations holds promise to transfer the full quantum control available for electromagnetic fields to a mechanical system. Together with the fact that the optical trapping potential is highly controllable, our experimental platform offers a route to investigating quantum mechanics at macroscopic scales.

生态与气候

Ecology & Climate

Amazonia as a carbon source linked to deforestation and climate change

亚马孙流域碳平衡面临威胁

▲ 作者：Luciana V. Gatti, Luana S. Basso, John B. Miller, Manuel Gloor, Lucas Gatti Domingues, Henrique L. G. Cassol, Graciela Tejada, Luiz E. O. C. Aragão, Carlos Nobre, Wouter Peters, Luciano Marani, Egidio Arai, Alber H. Sanches, Sergio M. Corrêa, Liana Anderson, Celso Von Randow, Caio S. C. Correia, Stephane P. Crispim & Raiane A. L. Neves

▲ 链接：

https://www.nature.com/articles/s41586-021-03629-6

▲ 摘要

亚马孙雨林，或称亚马孙流域，是全球面积最大的热带雨林。因此，亚马孙雨林对于大气中碳的累积和储存具有关键作用。人为森林砍伐和气候变化这类因素被认为引起了碳汇能力下降，改变了当地含碳气体的平衡，而这种平衡是衡量生态系统健康的指标。

新研究发现森林砍伐和区域气候变化可能威胁到亚马孙雨林大气中碳的缓冲潜力。一些地区的碳排放超过了碳吸收。研究结果帮助我们进一步了解了气候变化和人为干扰的相互作用，以及这种相互作用对全球最大热带雨林碳平衡的长期影响。

作者整理了2010年至2018年巴西亚马孙流域上空对流层二氧化碳和一氧化碳浓度的飞机观测结果，对流层是地球大气的最下层。研究团队对四个地点的逾600例垂直分布（从地表到海平面以上约4.5千米）数据进行了分析，结果显示亚马孙流域东部的总碳排高于西部。具体而言，亚马孙流域东南部被锁定为一个净碳排放源，在研究期间从碳汇直接变成了碳源。作者认为，旱季和森林砍伐的加剧对当地生态系统构成了压力，导致火灾事故增多，这些可能是造成东部碳排放增加的原因。

通过揭示亚马孙流域的森林砍伐和气候变化之间的联系，该研究表明，这种相互作用可能对亚马孙地区的碳平衡及其生态系统的脆弱性具有长期负面影响。

▲ Abstract

Amazonia hosts the Earth’s largest tropical forests and has been shown to be an important carbon sink over recent decades1,2,3. This carbon sink seems to be in decline, however, as a result of factors such as deforestation and climate change. Here we investigate Amazonia’s carbon budget and the main drivers responsible for its change into a carbon source. We performed 590 aircraft vertical profiling measurements of lower-tropospheric concentrations of carbon dioxide and carbon monoxide at four sites in Amazonia from 2010 to 20184. We find that total carbon emissions are greater in eastern Amazonia than in the western part, mostly as a result of spatial differences in carbon-monoxide-derived fire emissions. Southeastern Amazonia, in particular, acts as a net carbon source (total carbon flux minus fire emissions) to the atmosphere. Over the past 40 years, eastern Amazonia has been subjected to more deforestation, warming and moisture stress than the western part, especially during the dry season, with the southeast experiencing the strongest trends. We explore the effect of climate change and deforestation trends on carbon emissions at our study sites, and find that the intensification of the dry season and an increase in deforestation seem to promote ecosystem stress, increase in fire occurrence, and higher carbon emissions in the eastern Amazon. This is in line with recent studies that indicate an increase in tree mortality and a reduction in photosynthesis as a result of climatic changes across Amazonia.

A lithium-isotope perspective on the evolution of carbon and silicon cycles

锂同位素对碳和硅循环演化的展望

▲ 作者：Boriana Kalderon-Asael, Joachim A. R. Katchinoff, Noah J. Planavsky, Ashleigh v. S. Hood, Mathieu Dellinger, Eric J. Bellefroid, David S. Jones, Axel Hofmann, Frantz Ossa Ossa, Francis A. Macdonald, Chunjiang Wang, Terry T. Isson, Jack G. Murphy, John A. Higgins, A. Joshua West, Malcolm W. Wallace, Dan Asael & Philip A. E. Pogge von Strandmann

▲ 链接：

https://www.nature.com/articles/s41586-021-03612-1

▲ 摘要

全球碳和硅循环的演化被认为是地球气候长期稳定的重要因素。然而，关于其发挥作用的反馈机制仍存在许多问题，且这些元素在地球表面环境中的来源和吸收受到的数量限制有限。

作者认为锂同位素记录可以用来跟踪控制长期碳和硅循环的过程。通过分析来自100多个地层单元的600多个浅水海相碳酸盐岩样品，他们构建了一个跨越过去30亿年的新的碳酸盐岩锂同位素记录。这些数据表明，碳酸锂同位素值随着时间的推移而增加，这是由海水的锂同位素条件的长期变化所驱动的，而不是由较老样品的沉积改变所驱动的。

利用质量平衡模拟方法，作者认为锂同位素的变化趋势反映了前寒武纪碳硅旋回向现代碳硅旋回的转变。他们推测这种转变与生物控制的海洋硅循环的逐渐转变和陆地植物的进化辐射有关。

▲ Abstract

The evolution of the global carbon and silicon cycles is thought to have contributed to the long-term stability of Earth’s climate. Many questions remain, however, regarding the feedback mechanisms at play, and there are limited quantitative constraints on the sources and sinks of these elements in Earth’s surface environments. Here we argue that the lithium-isotope record can be used to track the processes controlling the long-term carbon and silicon cycles. By analysing more than 600 shallow-water marine carbonate samples from more than 100 stratigraphic units, we construct a new carbonate-based lithium-isotope record spanning the past 3 billion years. The data suggest an increase in the carbonate lithium-isotope values over time, which we propose was driven by long-term changes in the lithium-isotopic conditions of sea water, rather than by changes in the sedimentary alterations of older samples. Using a mass-balance modelling approach, we propose that the observed trend in lithium-isotope values reflects a transition from Precambrian carbon and silicon cycles to those characteristic of the modern. We speculate that this transition was linked to a gradual shift to a biologically controlled marine silicon cycle and the evolutionary radiation of land plants.

古生物学

Paleontology

Pleistocene sediment DNA reveals hominin and faunal turnovers at Denisova Cave

沉积物DNA揭示丹尼索瓦洞穴人类和动物群更替

▲ 作者：Elena I. Zavala, Zenobia Jacobs, Benjamin Vernot, Michael V. Shunkov, Maxim B. Kozlikin, Anatoly P. Derevianko, Elena Essel, Cesare de Fillipo, Sarah Nagel, Julia Richter, Frédéric Romagné, Anna Schmidt, Bo Li, Kieran O’Gorman, Viviane Slon, Janet Kelso, Svante Pääbo, Richard G. Roberts & Matthias Meyer

▲ 链接：

https://www.nature.com/articles/s41586-021-03675-0

▲ 摘要

西伯利亚南部的丹尼索瓦洞穴是尼安德特人亲缘关系的古人类类群丹尼索瓦人的类型地点。从这些沉积物中发现的十几具古人类遗骸还包括尼安德特人、尼安德特人和丹尼索瓦人的后代，这表明丹尼索瓦洞穴是这些古人类之间的接触带。

然而，这些类群在遗址上出现的顺序、古人类居住的时间和环境背景，以及特定的古人类类群与考古组合之间的联系仍不确定。

作者报告了728个沉积物样本的DNA分析，这些样本以网格状方式从更新世地层中收集。他们分别从685和175个样本中提取了古代动物群和古人类线粒体mtDNA。

人类mtDNA最早的证据来自丹尼索瓦人，与旧石器时代中期早期的石器有关，这些石器大约存放于25万到17万年前；尼安德特人的mtDNA最早出现在这一时期的末期。

研究者发现丹尼索瓦人的线粒体DNA发生了变化，这与动物区系mtDNA组成的变化相一致。他们还发现丹尼索瓦人和尼安德特人重复地占据着这个地方——可能一直到或在至少4.5万年前的最初旧石器时代晚期开始，当时现代人类的mtDNA首次被记录在沉积物中。

▲ Abstract

Denisova Cave in southern Siberia is the type locality of the Denisovans, an archaic hominin group who were related to Neanderthals. The dozen hominin remains recovered from the deposits also include Neanderthals and the child of a Neanderthal and a Denisovan7, which suggests that Denisova Cave was a contact zone between these archaic hominins. However, uncertainties persist about the order in which these groups appeared at the site, the timing and environmental context of hominin occupation, and the association of particular hominin groups with archaeological assemblages. Here we report the analysis of DNA from 728 sediment samples that were collected in a grid-like manner from layers dating to the Pleistocene epoch. We retrieved ancient faunal and hominin mitochondrial (mt)DNA from 685 and 175 samples, respectively. The earliest evidence for hominin mtDNA is of Denisovans, and is associated with early Middle Palaeolithic stone tools that were deposited approximately 250,000 to 170,000 years ago; Neanderthal mtDNA first appears towards the end of this period. We detect a turnover in the mtDNA of Denisovans that coincides with changes in the composition of faunal mtDNA, and evidence that Denisovans and Neanderthals occupied the site repeatedly—possibly until, or after, the onset of the Initial Upper Palaeolithic at least 45,000 years ago, when modern human mtDNA is first recorded in the sediments.

编辑｜宗华

排版｜华园

合作事宜：hezuo@stimes.cn

投稿事宜：tougao@stimes.cn

喜欢本文？点赞 + 在看支持一下！

继续滑动看下一个

​《自然》（20210715出版）一周论文导读

《自然》（20210715出版）一周论文导读