编译 | 冯维维
Nature, 25 January 2024, Volume 625 Issue 7996
《自然》，第625卷，7996期，2024年1月25日
物理学Physics
Testing quantum electrodynamics in extreme fields using helium-like uranium
使用类氦铀在极端磁场中测试量子电动力学
▲ 作者：R. Loetzsch, H. F. Beyer, L. Duval, U. Spillmann et al.
▲ 链接：
https://www.nature.com/articles/s41586-023-06910-y
▲ 摘要：
量子电动力学（QED），即描述光与物质之间相互作用的量子场论，通常被认为是现代物理学中最成熟的量子理论。但这主要是基于在相对低场强和光原子和离子领域进行的极其精确的研究。
在非常强的电磁场领域，例如在最重的高电荷离子中，QED计算进入了一个性质不同的非微扰状态。但相应的实验研究非常具有挑战性，理论预测仅得到部分验证。研究者提出了一个对高阶QED效应和高Z区电子-电子相互作用敏感的实验。
这是通过使用基于多普勒调谐X射线发射的多参考方法来实现的，这些发射来自储存的具有不同电荷态的相对论性铀离子。实验结果可以区分几种最先进的理论方法，并为强场领域的计算提供了重要的基准。
▲ Abstract：
Quantum electrodynamics (QED), the quantum field theory that describes the interaction between light and matter, is commonly regarded as the best-tested quantum theory in modern physics. However, this claim is mostly based on extremely precise studies performed in the domain of relatively low field strengths and light atoms and ions. In the realm of very strong electromagnetic fields such as in the heaviest highly charged ions, QED calculations enter a qualitatively different, non-perturbative regime. Yet, the corresponding experimental studies are very challenging, and theoretical predictions are only partially tested. Here we present an experiment sensitive to higher-order QED effects and electron–electron interactions in the high-Z regime. This is achieved by using a multi-reference method based on Doppler-tuned X-ray emission from stored relativistic uranium ions with different charge states. Moreover, our experimental result can discriminate between several state-of-the-art theoretical approaches and provides an important benchmark for calculations in the strong-field domain.
Observing dynamical phases of BCS superconductors in a cavity QED simulator
在腔电动力学模拟器中观察BCS超导体的动态相位
▲ 作者：Dylan J. Young, Anjun Chu, Eric Yilun Song et al.
▲ 链接：
https://www.nature.com/articles/s41586-023-06911-x
▲ 摘要：
在传统的巴丁-库珀-史瑞佛（BCS）超导体中，由于材料中声子介导的吸引相互作用，具有相反动量的电子结合成库珀对。虽然超导性在热平衡时自然产生，但当系统参数突然改变时，超导性也可能出现在平衡外。由此产生的非平衡相预计会发生在真实材料和超冷费米子原子中，但并非所有的都被直接观察到。
研究者实现了一种利用腔量子电动力学（QED）产生所提出的动态相的替代方法。该系统编码了88Sr原子与光学腔耦合的长寿命电子跃迁中库珀对的存在或不存在，并将电子之间的相互作用表示为通过腔的光子介导的相互作用。
这为量子模拟开辟了令人兴奋的前景，包括设计非常规超导体和探索超越平均场效应的潜力，如光谱形状因子，以及增加量子传感的相干时间。
▲ Abstract：
In conventional Bardeen–Cooper–Schrieffer superconductors, electrons with opposite momenta bind into Cooper pairs due to an attractive interaction mediated by phonons in the material. Although superconductivity naturally emerges at thermal equilibrium, it can also emerge out of equilibrium when the system parameters are abruptly changed. The resulting out-of-equilibrium phases are predicted to occur in real materials and ultracold fermionic atoms, but not all have yet been directly observed. Here we realize an alternative way to generate the proposed dynamical phases using cavity quantum electrodynamics (QED). Our system encodes the presence or absence of a Cooper pair in a long-lived electronic transition in 88Sr atoms coupled to an optical cavity and represents interactions between electrons as photon-mediated interactions through the cavity. This opens up exciting prospects for quantum simulation, including the potential to engineer unconventional superconductors and to probe beyond mean-field effects like the spectral form factor, and for increasing the coherence time for quantum sensing.
Nozaki–Bekki solitons in semiconductor lasers
半导体激光器中的野崎-贝克孤子
▲ 作者：Nikola Opačak, Dmitry Kazakov, Lorenzo L. Columbo et al.
▲ 链接：
https://www.nature.com/articles/s41586-023-06915-7
▲ 摘要：
光学频率梳光源，发射出完美的周期和相干光波形，最近迅速发展到芯片级集成解决方案。其中，有两类特别重要——半导体法布里-珀罗激光器和无源环形克尔微谐振器。研究者将这两种技术融合在环形半导体激光器中，并展示了自由运行孤子的形成范例，称为野崎-贝克奇孤子。
研究者证明了野崎-贝克孤子在环形激光器中结构稳定，并且随着激光偏压的调整而自发形成，从而消除了对外部光泵的需要。通过结合结论性的实验结果和一个互补的详细理论模型，他们揭示了这些孤子的显著特征，并为其产生提供了指导。
除了在环形激光器内循环的基本孤子之外，研究者还展示了多孤子状态，验证了其局域性，并提供了对孤子晶体形成的见解。研究结果巩固了一个单片电驱动平台，可用于直接产生孤子，并为激光多模动力学和克尔参数过程的交叉研究领域打开了大门。
▲ Abstract：
Optical frequency-comb sources, which emit perfectly periodic and coherent waveforms of light, have recently rapidly progressed towards chip-scale integrated solutions. Among them, two classes are particularly significant—semiconductor Fabry–Perót lasers and passive ring Kerr microresonators. Here we merge the two technologies in a ring semiconductor laser and demonstrate a paradigm for the formation of free-running solitons, called Nozaki–Bekki solitons. We show that Nozaki–Bekki solitons are structurally stable in a ring laser and form spontaneously with tuning of the laser bias, eliminating the need for an external optical pump. By combining conclusive experimental findings and a complementary elaborate theoretical model, we reveal the salient characteristics of these solitons and provide guidelines for their generation. Beyond the fundamental soliton circulating inside the ring laser, we demonstrate multisoliton states as well, verifying their localized nature and offering an insight into formation of soliton crystals. Our results consolidate a monolithic electrically driven platform for direct soliton generation and open the door for a research field at the junction of laser multimode dynamics and Kerr parametric processes.
The persistence of memory in ionic conduction probed by nonlinear optics
非线性光学研究离子传导中记忆的持久性
▲ 作者：Andrey D. Poletayev, Matthias C. Hoffmann et al.
▲ 链接：
https://www.nature.com/articles/s41586-023-06827-6
▲ 摘要：
预测凝聚态的实际传输速率使材料、设备和工艺的合理设计成为可能。这对于开发可充电电池等低碳能源技术尤其重要。研究者使用单周期太赫兹泵来脉冲触发电池固体电解质中的离子跳跃。
这是通过诱导瞬态双折射可视化的，可以在皮秒时间尺度上直接探测离子跳变的各向异性。瞬态信号的弛豫测量了定向记忆的衰减和扩散熵的产生。研究者扩展了用硅瞬态双折射的实验结果来确定离子跳跃的振动尝试频率。
利用非线性光学方法，他们探索了离子在其最快极限下的输运，在原子尺度上区分了相关的传导机制和真正的随机游走，并证明了激活输运与信息热力学之间的联系。
▲ Abstract：
Predicting practical rates of transport in condensed phases enables the rational design of materials, devices and processes. This is especially critical to developing low-carbon energy technologies such as rechargeable batteries. Here we use single-cycle terahertz pumps to impulsively trigger ionic hopping in battery solid electrolytes. This is visualized by an induced transient birefringence, enabling direct probing of anisotropy in ionic hopping on the picosecond timescale. The relaxation of the transient signal measures the decay of orientational memory, and the production of entropy in diffusion. We extend experimental results using in silico transient birefringence to identify vibrational attempt frequencies for ion hopping. Using nonlinear optical methods, we probe ion transport at its fastest limit, distinguish correlated conduction mechanisms from a true random walk at the atomic scale, and demonstrate the connection between activated transport and the thermodynamics of information.
生态学Ecology
Rapid groundwater decline and some cases of recovery in aquifers globally
全球地下水快速下降和含水层恢复情况
▲ 作者：Scott Jasechko, Hansjörg Seybold et al.
▲ 链接：
https://www.nature.com/articles/s41586-023-06879-8
▲ 摘要：
地下水资源对生态系统和生计至关重要。过度抽取地下水会导致地下水位下降，以及海水入侵、地面下沉、河流枯竭和水井干涸。然而，当前地下水下降的全球速度和普遍程度受到的约束很差，因为在全球范围内尚未合成原位地下水水位。
研究者分析了170,000口监测井和1,693个含水层系统的地下水位趋势，这些国家占全球地下水提取量的75%左右。研究表明，在21世纪，地下水位的快速下降（> 50 m /年）是普遍存在的，特别是在拥有大片农田的干旱地区。
至关重要的是，研究还表明，在过去40年里，世界上30%的区域含水层的地下水位下降速度加快。地下水位普遍加速加深，这突出说明迫切需要采取更有效的措施来解决地下水枯竭问题。该分析还揭示了一些具体案例，在这些案例中，随着政策变化、含水层补给管理和地表水转移，枯竭趋势已经逆转，表明枯竭的含水层系统有恢复的潜力。
▲ Abstract：
Groundwater resources are vital to ecosystems and livelihoods. Excessive groundwater withdrawals can cause groundwater levels to decline, resulting in seawater intrusion, land subsidence, streamflow depletion and wells running dry. However, the global pace and prevalence of local groundwater declines are poorly constrained, because in situ groundwater levels have not been synthesized at the global scale. Here we analyse in situ groundwater-level trends for 170,000 monitoring wells and 1,693 aquifer systems in countries that encompass approximately 75% of global groundwater withdrawals. We show that rapid groundwater-level declines (>0.5 m year−1) are widespread in the twenty-first century, especially in dry regions with extensive croplands. Critically, we also show that groundwater-level declines have accelerated over the past four decades in 30% of the world’s regional aquifers. This widespread acceleration in groundwater-level deepening highlights an urgent need for more effective measures to address groundwater depletion. Our analysis also reveals specific cases in which depletion trends have reversed following policy changes, managed aquifer recharge and surface-water diversions, demonstrating the potential for depleted aquifer systems to recover.
Consistent patterns of common species across tropical tree communities
热带树木群落中常见物种的一致模式
▲ 作者：Declan L. M. Cooper, Simon L. Lewis et al.
▲ 链接：
https://www.nature.com/articles/s41586-023-06820-z
▲ 摘要：
树木构成了地球上最具生物多样性的生态系统——热带森林。由于对大多数热带树种知之甚少，大量的树种给了解这些森林带来了巨大的挑战，包括它们对环境变化的反应。对普通物种的关注可能会规避这一挑战。
作者研究了非洲、亚马逊和东南亚热带原始森林中1,003,805棵树干直径至少为10厘米的树木的盘查数据，研究了常见树种的丰度模式。这些树木分布在1568个地点，覆盖了封闭树冠、结构完整的热带森林。研究者估计，2.2%、2.2%和2.3%的物种分别占这些地区热带树木的50%。
通过对所有封闭冠层热带森林的外推，研究估计，在地球上8000亿棵树干直径至少为10厘米的热带树木中，只有1053种占了一半。 尽管存在不同的生物地理、气候和人为历史，研究者发现在各大洲的常见物种和物种丰度分布模式明显一致。这表明树木群落组合的基本机制可能适用于所有热带森林。
重新采样分析表明，最常见的物种可能属于一个可控的已知物种列表，这使得有针对性的努力能够了解它们的生态。虽然它们没有降低稀有物种的重要性，该结果为了解世界上最多样化的森林提供了新的机会，包括通过关注构成大多数树木的常见物种来模拟它们对环境变化的反应。
▲ Abstract：
Trees structure the Earth’s most biodiverse ecosystem, tropical forests. The vast number of tree species presents a formidable challenge to understanding these forests, including their response to environmental change, as very little is known about most tropical tree species. A focus on the common species may circumvent this challenge. Here we investigate abundance patterns of common tree species using inventory data on 1,003,805 trees with trunk diameters of at least 10 cm across 1,568 locations in closed-canopy, structurally intact old-growth tropical forests in Africa, Amazonia and Southeast Asia. We estimate that 2.2%, 2.2% and 2.3% of species comprise 50% of the tropical trees in these regions, respectively. Extrapolating across all closed-canopy tropical forests, we estimate that just 1,053 species comprise half of Earth’s 800 billion tropical trees with trunk diameters of at least 10 cm. Despite differing biogeographic, climatic and anthropogenic histories, we find notably consistent patterns of common species and species abundance distributions across the continents. This suggests that fundamental mechanisms of tree community assembly may apply to all tropical forests. Resampling analyses show that the most common species are likely to belong to a manageable list of known species, enabling targeted efforts to understand their ecology. Although they do not detract from the importance of rare species, our results open new opportunities to understand the world’s most diverse forests, including modelling their response to environmental change, by focusing on the common species that constitute the majority of their trees.
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