编译|冯维维

Nature,  Volume 631 Issue 8020, 11 July 2024

《自然》第631卷8020期，2024年7月11日

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物理学Physics

Observation of Bose–Einstein condensation of dipolar molecules

偶极分子玻色-爱因斯坦凝聚的观察

▲ 作者：Niccolò Bigagli, Weijun Yuan, Siwei Zhang, Boris Bulatovic, Tijs Karman, Ian Stevenson & Sebastian Will

▲链接：

https://www.nature.com/articles/s41586-024-07492-z

▲摘要：

受量子力学定律支配的粒子群表现出有趣的涌现行为。量子材料中的原子量子气体、液氦和电子由于它们的组成和相互作用都表现出不同的性质。超冷偶极分子的量子简并样品有望实现物质的新相，并为量子模拟和量子计算提供新的途径。但即使通过碰撞屏蔽技术减少了快速损失，到目前为止也阻碍了蒸发冷却到玻色-爱因斯坦凝聚体（BEC）。

研究者报道了偶极分子的BEC的实现。通过增强碰撞屏蔽来抑制二体和三体损失，他们蒸发冷却钠-铯分子到量子简并并跨越相变到BEC。当相空间密度超过1时，BEC表现为双峰分布。生成了冷凝分数为60(10)%，温度为6(2)nK的BECs，并发现其稳定寿命接近2秒。这项工作为探索迄今为止无法进入的体制中的偶极量子物质打开了大门，有望在光学晶格中创造奇异的偶极液滴，自组织晶体相和偶极自旋液体。

▲ Abstract：

Ensembles of particles governed by quantum mechanical laws exhibit intriguing emergent behaviour. Atomic quantum gases, liquid helium and electrons in quantum materials all exhibit distinct properties because of their composition and interactions. Quantum degenerate samples of ultracold dipolar molecules promise the realization of new phases of matter and new avenues for quantum simulation and quantum computation. However, rapid losses, even when reduced through collisional shielding techniques, have so far prevented evaporative cooling to a Bose–Einstein condensate (BEC). Here we report on the realization of a BEC of dipolar molecules. By strongly suppressing two- and three-body losses via enhanced collisional shielding, we evaporatively cool sodium–caesium molecules to quantum degeneracy and cross the phase transition to a BEC. The BEC reveals itself by a bimodal distribution when the phase-space density exceeds 1. BECs with a condensate fraction of 60(10)% and a temperature of 6(2) nK are created and found to be stable with a lifetime close to 2 s. This work opens the door to the exploration of dipolar quantum matter in regimes that have been inaccessible so far, promising the creation of exotic dipolar droplets, self-organized crystal phases and dipolar spin liquids in optical lattices.

Tunable entangled photon-pair generation in a liquid crystal

液晶中可调谐纠缠光子对的产生

▲ 作者：Vitaliy Sultanov, Alja? Kav?i?, Emmanouil Kokkinakis, Nerea Sebastián, Maria V. Chekhova & Matja? Humar 

▲链接：

https://www.nature.com/articles/s41586-024-07543-5

▲摘要：

液晶具有自组装、对电场的强响应和可集成到复杂系统中的能力，是光束操纵的关键材料。最近发现的铁电向列液晶也具有相当大的二阶光学非线性，使其成为非线性光学的潜在材料。它们作为量子光源的使用可以大大扩展光子量子技术的边界。然而，自发参数下转换，纠缠光子的基本来源，预示的单光子和压缩光，到目前为止还没有在液晶或任何液体或有机材料中观察到。

研究者在铁电向列液晶中实现了自发参数下转换，并展示了电场可调谐的纠缠光子宽带产生，其效率可与最佳非线性晶体相媲美。施加几伏电压或沿样品方向扭转分子取向，光子对的发射速率和偏振态会发生显著变化。液晶源可以实现一种特殊类型的准相位匹配，这种匹配基于分子扭曲结构，因此可以根据光子对的所需光谱和偏振特性进行重新配置。

这种光源有望在功能、亮度和所产生量子态的可调性方面优于标准非线性光学材料。这一概念可以扩展到复杂的拓扑结构，宏观器件和多像素可调量子光源。

▲ Abstract：

Liquid crystals, with their ability to self-assemble, strong response to an electric field and integrability into complex systems, are key materials in light-beam manipulation1. The recently discovered ferroelectric nematic liquid crystals also have considerable second-order optical nonlinearity, making them a potential material for nonlinear optics. Their use as sources of quantum light could considerably extend the boundaries of photonic quantum technologies6. However, spontaneous parametric down-conversion, the basic source of entangled photons, heralded single photons and squeezed light, has so far not been observed in liquid crystals—or in any liquids or organic materials. Here we implement spontaneous parametric down-conversion in a ferroelectric nematic liquid crystal and demonstrate electric-field tunable broadband generation of entangled photons, with an efficiency comparable to that of the best nonlinear crystals. The emission rate and polarization state of photon pairs is markedly varied by applying a few volts or twisting the molecular orientation along the sample. A liquid-crystal source enables a special type of quasi-phase matching, which is based on the molecular twist structure and is therefore reconfigurable for the desired spectral and polarization properties of photon pairs. Such sources promise to outperform standard nonlinear optical materials in terms of functionality, brightness and the tunability of the generated quantum state. The concepts developed here can be extended to complex topological structures, macroscopic devices and multi-pixel tunable quantum light sources.

Fast-moving stars around an intermediate-mass black hole in ω Centauri

围绕半人马ω球状星团内一个中等质量黑洞的快速移动恒星

▲ 作者：Maximilian H?berle, Nadine Neumayer, Anil Seth, Andrea Bellini, Mattia Libralato, Holger Baumgardt, Matthew Whitaker, Antoine Dumont, Mayte Alfaro-Cuello, Jay Anderson, Callie Clontz, Nikolay Kacharov, Sebastian Kamann, Anja Feldmeier-Krause, Antonino Milone, Maria Selina Nitschai, Renuka Pechetti & Glenn van de Ven

▲链接：

https://www.nature.com/articles/s41586-024-07511-z

▲摘要：

科学家在银河系半人马座欧米茄星团（半人马ω球状星团）内观测到的快速移动恒星为一个中等质量黑洞的存在提供了证据。该研究基于哈勃空间望远镜20多年的观测结果，或有助于今后研究人员搜索这类难以发现的黑洞。

科学家已发现各种质量的黑洞，从5~150倍太阳质量（M⊙）的恒星质量黑洞到星系中央的超大质量黑洞（超过10万倍太阳质量；105M⊙）。不过，目前只发现了少数存在争议的候选中等质量黑洞，这类黑洞的质量在太阳质量的150倍到10万倍不等。

鉴于半人马ω球状星团高质量和复杂星族的特性，它是寻找中等质量黑洞的一个很有希望的目标，但之前对该区域发现了一个黑洞的报道一直面临争议。德国马普学会天文研究所的MaximilianHaberle和同事利用哈勃空间望远镜20年的影像，监测了该星团中央附近恒星的运动。他们提出有7个恒星在中央区域快速移动，并认为这说明半人马ω球状星团的中央有一个中等质量黑洞。他们推测该黑洞质量的下限为8200M⊙左右。

“这些新发现的恒星是半人马ω球状星团中存在一个中等质量黑洞的最佳证据。”加拿大麦吉尔大学的DarylHaggard和美国旧金山州立大学的AdrienneCool在一篇同期发表的新闻观点文章中写道，研究结果或表明在其他球状星团中搜寻中等质量黑洞是合理的。

欧空局哈勃拍摄的半人马座欧米茄星团新彩色图像。图片来自：ESA哈勃&NASA，M.Haberle

▲ Abstract：

Black holes have been found over a wide range of masses, from stellar remnants with masses of 5–150 solar masses (M☉), to those found at the centres of galaxies with M>105M☉. However, only a few debated candidate black holes exist between 150M☉ and 105M☉. Determining the population of these intermediate-mass black holes is an important step towards understanding supermassive black hole formation in the early universe. Several studies have claimed the detection of a central black hole in ω Centauri, the most massive globular cluster of the Milky Way. However, these studies have been questioned because of the possible mass contribution of stellar mass black holes, their sensitivity to the cluster centre and the lack of fast-moving stars above the escape velocity. Here we report the observations of seven fast-moving stars in the central 3 arcsec (0.08 pc) of ω Centauri. The velocities of the fast-moving stars are significantly higher than the expected central escape velocity of the star cluster, so their presence can be explained only by being bound to a massive black hole. From the velocities alone, we can infer a firm lower limit of the black hole mass of about 8,200M☉, making this a good case for an intermediate-mass black hole in the local universe.

Tunable superconductivity in electron- and hole-doped Bernal bilayer graphene

电子和空穴掺杂Bernal双层石墨烯的可调谐超导性

▲ 作者：Chushan Li, Fan Xu, Bohao Li, Jiayi Li, Guoan Li, Kenji Watanabe, Takashi Taniguchi, Bingbing Tong, Jie Shen, Li Lu, Jinfeng Jia, Fengcheng Wu, Xiaoxue Liu & Tingxin Li

▲链接：

https://www.nature.com/articles/s41586-024-07584-w

▲摘要：

基于石墨烯的高质量二维电子系统已经成为研究超导性的高度可调平台。具体来说，在电子和空穴掺杂的扭曲石墨烯涡流体系中都观察到了超导性，而在晶体石墨烯体系中，迄今为止只在空穴掺杂的菱形三层石墨烯(RTG)和空穴掺杂的Bernal双层石墨烯(BBG)中观察到超导性。最近，由于接近单层WSe2, BBG中的超导性得到了增强。

研究者报道了通过静电掺杂在电子和空穴掺杂的BBG/WSe2器件中观察到的超导性和一系列风味对称性破缺相。观察到的超导性的强度可以通过施加垂直电场来调节。电子掺杂和空穴掺杂超导的最大berezinski - kosterlitz - Thouless转变温度分别约为210 mK和400 mK。只有当外加电场驱动BBG电子或空穴波函数向WSe2层移动时，超导性才会出现，这强调了WSe2层在观察到的超导性中的重要性。空穴掺杂的超导违反了泡利顺磁极限，符合伊辛类超导体。相比之下，电子掺杂的超导性服从泡利极限，尽管在导带中邻近诱导的伊辛自旋轨道耦合也很明显。该发现突出了与BBG中传导带相关的丰富物理特性，为进一步研究晶体石墨烯的超导机制和基于BBG的超导体器件的开发铺平了道路。

▲ Abstract：

Graphene-based, high-quality, two-dimensional electronic systems have emerged as a highly tunable platform for studying superconductivity. Specifically, superconductivity has been observed in both electron- and hole-doped twisted graphene moiré systems, whereas in crystalline graphene systems, superconductivity has so far been observed only in hole-doped rhombohedral trilayer graphene (RTG) and hole-doped Bernal bilayer graphene (BBG). Recently, enhanced superconductivity has been demonstrated in BBG because of the proximity to a monolayer WSe2. Here we report the observation of superconductivity and a series of flavour-symmetry-breaking phases in electron- and hole-doped BBG/WSe2 devices by electrostatic doping. The strength of the observed superconductivity is tunable by applied vertical electric fields. The maximum Berezinskii–Kosterlitz Thouless transition temperature for the electron- and hole-doped superconductivity is about 210mK and 400mK, respectively. Superconductivities emerge only when the applied electric fields drive the BBG electron or hole wavefunctions towards the WSe2 layer, underscoring the importance of the WSe2 layer in the observed superconductivity. The hole-doped superconductivity violates the Pauli paramagnetic limit, consistent with an Ising-like superconductor. By contrast, the electron-doped superconductivity obeys the Pauli limit, although the proximity-induced Ising spin–orbit coupling is also notable in the conduction band. Our findings highlight the rich physics associated with the conduction band in BBG, paving the way for further studies into the superconducting mechanisms of crystalline graphene and the development of superconductor devices based on BBG.

化学Chemistry

Glassy gels toughened by solvent

用溶剂增韧的玻璃状凝胶

▲ 作者：Meixiang Wang, Xun Xiao, Salma Siddika, Mohammad Shamsi, Ethan Frey, Wen Qian, Wubin Bai, Brendan T. O’Connor & Michael D. Dickey

▲链接：

https://www.nature.com/articles/s41586-024-07564-0

▲摘要：

玻璃状聚合物通常坚硬而坚固，但延展性有限。通过溶胀，玻璃状聚合物可以变成柔软而脆弱的凝胶，但具有增强的延展性。性能的显著变化是由于溶剂增加了链间的自由体积，同时减弱了聚合物与聚合物之间的相互作用。研究者展示了极性聚合物与离子液体——离子凝胶，在适当浓度下的溶剂化可以产生一种独特的材料，称为玻璃凝胶，具有玻璃和凝胶的理想性能。

离子液体增加了自由体积，因此，尽管缺乏传统的溶剂(例如，水)，可扩展性。但在室温下，离子液体在聚合物链之间形成强而丰富的非共价交联，形成坚硬、坚韧、玻璃状和均匀的网络(即没有相分离)。尽管液体含量超过54wt %，但玻璃凝胶具有巨大的断裂强度(42 MPa)、韧性(110 MJ m?3)、屈服强度(73 MPa)和杨氏模量(1 GPa)。这些值与热塑性塑料(如聚乙烯)相似，但与热塑性塑料不同的是，玻璃状凝胶可以变形到670%的应变，并且在加热时可以完全快速恢复。这些透明材料通过一步聚合形成，具有令人印象深刻的粘合、自愈和形状记忆特性。

▲ Abstract：

Glassy polymers are generally stiff and strong yet have limited extensibility. By swelling with solvent, glassy polymers can become gels that are soft and weak yet have enhanced extensibility. The marked changes in properties arise from the solvent increasing free volume between chains while weakening polymer–polymer interactions. Here we show that solvating polar polymers with ionic liquids (that is, ionogels) at appropriate concentrations can produce a unique class of materials called glassy gels with desirable properties of both glasses and gels. The ionic liquid increases free volume and therefore extensibility despite the absence of conventional solvent (for example, water). Yet, the ionic liquid forms strong and abundant non-covalent crosslinks between polymer chains to render a stiff, tough, glassy, and homogeneous network (that is, no phase separation), at room temperature. Despite being more than 54wt% liquid, the glassy gels exhibit enormous fracture strength (42 MPa), toughness (110 MJ m?3), yield strength (73 MPa) and Young’s modulus (1 GPa). These values are similar to those of thermoplastics such as polyethylene, yet unlike thermoplastics, the glassy gels can be deformed up to 670% strain with full and rapid recovery on heating. These transparent materials form by a one-step polymerization and have impressive adhesive, self-healing and shape-memory properties.

Direct radical functionalization of native sugars

天然糖的直接自由基功能化

▲ 作者：Yi Jiang, Yi Wei, Qian-Yi Zhou, Guo-Quan Sun, Xia-Ping Fu, Nikita Levin, Yijun Zhang, Wen-Qiang Liu, NingXi Song, Shabaz Mohammed, Benjamin G. Davis & Ming Joo Koh

▲链接：

https://www.nature.com/articles/s41586-024-07548-0

▲摘要：

天然存在的糖和碳水化合物含有许多具有相似反应活性的羟基。因此，化学家通常依靠费力的、多步骤的保护基团策略，将这些可再生原料转化为制造聚糖的试剂(糖基供体)。

将天然糖直接转化为复合糖仍然是一个显著的挑战。研究者描述了一种光诱导方法，通过均溶(单电子)化学绕过不必要的羟基掩膜和操纵，从广泛可用的天然糖构建块实现位点和立体选择性化学糖基化。这一过程使人联想到自然界在区域控制下产生瞬时糖基供体，在光激活下与亲电试剂进行基于自由基的交叉偶联。

通过选择性的单糖和寡糖的头聚糖功能化，这种无保护基团的“帽和糖基酸”方法提供了一种直接的途径，可以获得大量代谢旺盛的糖基化合物。由于其生物相容性，该方法被扩展到蛋白质的直接翻译后糖基化。

▲ Abstract：

Naturally occurring (native) sugars and carbohydrates contain numerous hydroxyl groups of similar reactivity. Chemists, therefore, rely typically on laborious, multi-step protecting-group strategies3 to convert these renewable feedstocks into reagents (glycosyl donors) to make glycans. The direct transformation of native sugars to complex saccharides remains a notable challenge. Here we describe a photoinduced approach to achieve site- and stereoselective chemical glycosylation from widely available native sugar building blocks, which through homolytic (one-electron) chemistry bypasses unnecessary hydroxyl group masking and manipulation. This process is reminiscent of nature in its regiocontrolled generation of a transient glycosyl donor, followed by radical-based cross-coupling with electrophiles on activation with light. Through selective anomeric functionalization of mono- and oligosaccharides, this protecting-group-free ‘cap and glycosylate’ approach offers straightforward access to a wide array of metabolically robust glycosyl compounds. Owing to its biocompatibility, the method was extended to the direct post-translational glycosylation of proteins.