Nature, 25 January 2024, Volume 625 Issue 7996
《自然》，第625卷，7996期，2024年1月25日
物理學Physics
Testing quantum electrodynamics in extreme fields using helium-like uranium
使用類氦鈾在極端磁場中測試量子電動力學
▲ 作者：R. Loetzsch, H. F. Beyer, L. Duval, U. Spillmann et al.
▲ 鏈接：
https://www.nature.com/articles/s41586-023-06910-y
▲ 摘要：
量子電動力學（QED），即描述光與物質之間相互作用的量子場論，通常被認為是現代物理學中最成熟的量子理論。但這主要是基于在相對低場強和光原子和離子領域進行的極其精確的研究。
在非常強的電磁場領域，例如在最重的高電荷離子中，QED計算進入了一個性質不同的非微擾狀態。但相應的實驗研究非常具有挑戰性，理論預測僅得到部分驗證。研究者提出了一個對高階QED效應和高Z區電子-電子相互作用敏感的實驗。
這是通過使用基于多普勒調諧X射線發射的多參考方法來實現的，這些發射來自儲存的具有不同電荷態的相對論性鈾離子。實驗結果可以區分幾種最先進的理論方法，并為強場領域的計算提供了重要的基準。
▲ 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
在腔電動力學模擬器中觀察BCS超導體的動態相位
▲ 作者：Dylan J. Young, Anjun Chu, Eric Yilun Song et al.
▲ 鏈接：
https://www.nature.com/articles/s41586-023-06911-x
▲ 摘要：
在傳統的巴丁-庫珀-史瑞佛（BCS）超導體中，由于材料中聲子介導的吸引相互作用，具有相反動量的電子結合成庫珀對。雖然超導性在熱平衡時自然產生，但當系統參數突然改變時，超導性也可能出現在平衡外。由此產生的非平衡相預計會發生在真實材料和超冷費米子原子中，但并非所有的都被直接觀察到。
研究者實現了一種利用腔量子電動力學（QED）產生所提出的動態相的替代方法。該系統編碼了88Sr原子與光學腔耦合的長壽命電子躍遷中庫珀對的存在或不存在，并將電子之間的相互作用表示為通過腔的光子介導的相互作用。
這為量子模擬開辟了令人興奮的前景，包括設計非常規超導體和探索超越平均場效應的潛力，如光譜形狀因子，以及增加量子傳感的相干時間。
▲ 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
半導體激光器中的野崎-貝克孤子
▲ 作者：Nikola Opa?ak, Dmitry Kazakov, Lorenzo L. Columbo et al.
▲ 鏈接：
https://www.nature.com/articles/s41586-023-06915-7
▲ 摘要：
光學頻率梳光源，發射出完美的周期和相干光波形，最近迅速發展到芯片級集成解決方案。其中，有兩類特別重要——半導體法布里-珀羅激光器和無源環形克爾微諧振器。研究者將這兩種技術融合在環形半導體激光器中，并展示了自由運行孤子的形成范例，稱為野崎-貝克奇孤子。
研究者證明了野崎-貝克孤子在環形激光器中結構穩定，并且隨著激光偏壓的調整而自發形成，從而消除了對外部光泵的需要。通過結合結論性的實驗結果和一個互補的詳細理論模型，他們揭示了這些孤子的顯著特征，并為其產生提供了指導。
除了在環形激光器內循環的基本孤子之外，研究者還展示了多孤子狀態，驗證了其局域性，并提供了對孤子晶體形成的見解。研究結果鞏固了一個單片電驅動平臺，可用于直接產生孤子，并為激光多模動力學和克爾參數過程的交叉研究領域打開了大門。
▲ 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
非線性光學研究離子傳導中記憶的持久性
▲ 作者：Andrey D. Poletayev, Matthias C. Hoffmann et al.
▲ 鏈接：
https://www.nature.com/articles/s41586-023-06827-6
▲ 摘要：
預測凝聚態的實際傳輸速率使材料、設備和工藝的合理設計成為可能。這對于開發可充電電池等低碳能源技術尤其重要。研究者使用單周期太赫茲泵來脈沖觸發電池固體電解質中的離子跳躍。
這是通過誘導瞬態雙折射可視化的，可以在皮秒時間尺度上直接探測離子跳變的各向異性。瞬態信號的弛豫測量了定向記憶的衰減和擴散熵的產生。研究者擴展了用硅瞬態雙折射的實驗結果來確定離子跳躍的振動嘗試頻率。
利用非線性光學方法，他們探索了離子在其最快極限下的輸運，在原子尺度上區分了相關的傳導機制和真正的隨機游走，并證明了激活輸運與信息熱力學之間的聯系。
▲ 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.
生態學Ecology
▲ 作者：Scott Jasechko, Hansj?rg Seybold et al.
▲ 鏈接：
https://www.nature.com/articles/s41586-023-06879-8
▲ 摘要：
地下水資源對生態系統和生計至關重要。過度抽取地下水會導致地下水位下降，以及海水入侵、地面下沉、河流枯竭和水井干涸。然而，當前地下水下降的全球速度和普遍程度受到的約束很差，因為在全球范圍內尚未合成原位地下水水位。
研究者分析了170,000口監測井和1,693個含水層系統的地下水位趨勢，這些國家占全球地下水提取量的75%左右。研究表明，在21世紀，地下水位的快速下降（> 50 m /年）是普遍存在的，特別是在擁有大片農田的干旱地區。
至關重要的是，研究還表明，在過去40年里，世界上30%的區域含水層的地下水位下降速度加快。地下水位普遍加速加深，這突出說明迫切需要采取更有效的措施來解決地下水枯竭問題。該分析還揭示了一些具體案例，在這些案例中，隨著政策變化、含水層補給管理和地表水轉移，枯竭趨勢已經逆轉，表明枯竭的含水層系統有恢復的潛力。
▲ 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
熱帶樹木群落中常見物種的一致模式
▲ 作者：Declan L. M. Cooper, Simon L. Lewis et al.
▲ 鏈接：
https://www.nature.com/articles/s41586-023-06820-z
▲ 摘要：
樹木構成了地球上最具生物多樣性的生態系統——熱帶森林。由于對大多數熱帶樹種知之甚少，大量的樹種給了解這些森林帶來了巨大的挑戰，包括它們對環境變化的反應。對普通物種的關注可能會規避這一挑戰。
作者研究了非洲、亞馬遜和東南亞熱帶原始森林中1,003,805棵樹干直徑至少為10厘米的樹木的盤查數據，研究了常見樹種的豐度模式。這些樹木分布在1568個地點，覆蓋了封閉樹冠、結構完整的熱帶森林。研究者估計，2.2%、2.2%和2.3%的物種分別占這些地區熱帶樹木的50%。
通過對所有封閉冠層熱帶森林的外推，研究估計，在地球上8000億棵樹干直徑至少為10厘米的熱帶樹木中，只有1053種占了一半。 盡管存在不同的生物地理、氣候和人為歷史，研究者發現在各大洲的常見物種和物種豐度分布模式明顯一致。這表明樹木群落組合的基本機制可能適用于所有熱帶森林。
重新采樣分析表明，最常見的物種可能屬于一個可控的已知物種列表，這使得有針對性的努力能夠了解它們的生態。雖然它們沒有降低稀有物種的重要性，該結果為了解世界上最多樣化的森林提供了新的機會，包括通過關注構成大多數樹木的常見物種來模擬它們對環境變化的反應。
▲ 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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