编译 | 未玖
Nature, 28 July 2022, Volume 607 Issue 7920
《自然》2022年7月28日，第607卷，7920期
物理学Physics
Evaporation of microwave-shielded polar molecules to quantum degeneracy
微波屏蔽极性分子通过蒸发实现量子简并
▲ 作者：Andreas Schindewolf, Roman Bause, Xing-Yan Chen, Marcel Duda, Tijs Karman, Immanuel Bloch, et al.
▲ 链接：
https://www.nature.com/articles/s41586-022-04900-0
▲ 摘要：
超冷极性分子具有强大的电偶极矩和丰富的内部结构，这使它们成为探索奇异量子物质、实现量子信息方案和测试自然界基本对称性的理想构建块。实现其全部潜能需要将相互作用的分子气体深度冷却到量子简并态。
然而，到目前为止，分子之间在短程内的本征不稳定碰撞阻止了其通过弹性碰撞直接冷却到三维量子简并。
研究组使用微波屏蔽技术，演示了将费米子钠-钾分子的三维气体蒸发冷却至远低于费米温度。通过将旋转态与蓝色失谐圆偏振微波耦合而设计的排斥势垒，来阻止分子达到短程。
微波处理诱导分子之间的强可调谐偶极相互作用，导致高弹性碰撞率，可超过非弹性碰撞率至少460倍。如此巨大的弹性与非弹性碰撞比允许研究组将分子气体冷却到21 纳开尔文，相当于费米温度的0.36倍。
这种寒冷而致密的极性分子样品为探索具有强偶极相互作用的多体现象奠定了基础。
▲ Abstract：
Ultracold polar molecules offer strong electric dipole moments and rich internal structure, which makes them ideal building blocks to explore exotic quantum matter, implement quantum information schemes and test the fundamental symmetries of nature. Realizing their full potential requires cooling interacting molecular gases deeply into the quantum-degenerate regime. However, the intrinsically unstable collisions between molecules at short range have so far prevented direct cooling through elastic collisions to quantum degeneracy in three dimensions. Here we demonstrate evaporative cooling of a three-dimensional gas of fermionic sodium–potassium molecules to well below the Fermi temperature using microwave shielding. The molecules are protected from reaching short range with a repulsive barrier engineered by coupling rotational states with a blue-detuned circularly polarized microwave. The microwave dressing induces strong tunable dipolar interactions between the molecules, leading to high elastic collision rates that can exceed the inelastic ones by at least a factor of 460. This large elastic-to-inelastic collision ratio allows us to cool the molecular gas to 21 nanokelvin, corresponding to 0.36 times the Fermi temperature. Such cold and dense samples of polar molecules open the path to the exploration of many-body phenomena with strong dipolar interactions.
Exceptional-point-based accelerometers with enhanced signal-to-noise ratio
具有增强信噪比的异常点加速度计
▲ 作者：Rodion Kononchuk, Jizhe Cai, Fred Ellis, Ramathasan Thevamaran & Tsampikos Kottos
▲ 链接：
https://www.nature.com/articles/s41586-022-04904-w
▲ 摘要：
异常点（EP）是特征值与其对应的特征向量合并的非厄米简并。近年来，EPs作为一种提高传感器响应度的手段引起了人们的关注，通过在传感器附近发生突变共振失谐来实现。
然而，在许多情况下，EP实施伴随着噪声增强，导致传感器性能下降。过量噪声可以是基本性质（由于特征基崩溃），也可以是与用于实现EPs的放大机制相关的技术性质。
研究组使用基于EP的宇称时间对称机电加速度计表明，增强的技术噪声可以通过增强的应用加速度响应来超越。利用传输峰值简并度（TPD）的失谐量（传感器与传输线弱耦合时形成）作为灵敏度的度量，可以减轻特征基崩溃引发的噪声。
这些TPD发生在双正交特征基仍然完整的频率和控制参数下，与宇称时间对称传感器的EPs不同。与系统远离TPD运行的配置相比，该设备显示出三倍的信噪比增强。
▲ Abstract：
Exceptional points (EP) are non-Hermitian degeneracies where eigenvalues and their corresponding eigenvectors coalesce. Recently, EPs have attracted attention as a means to enhance the responsivity of sensors, through the abrupt resonant detuning occurring in their proximity. In many cases, however, the EP implementation is accompanied by noise enhancement, leading to the degradation of the sensor’s performance. The excess noise can be of fundamental nature (owing to the eigenbasis collapse) or of technical nature associated with the amplification mechanisms utilized for the realization of EPs. Here we show, using an EP-based parity–time symmetric electromechanical accelerometer, that the enhanced technical noise can be surpassed by the enhanced responsivity to applied accelerations. The noise owing to eigenbasis collapse is mitigated by exploiting the detuning from a transmission peak degeneracy (TPD) — which forms when the sensor is weakly coupled to transmission lines — as a measure of the sensitivity. These TPDs occur at a frequency and control parameters for which the biorthogonal eigenbasis is still complete and are distinct from the EPs of the parity–time symmetric sensor. Our device shows a threefold signal-to-noise-ratio enhancement compared with configurations for which the system operates away from the TPD.
信息科学Information Science
Experimental quantum key distribution certified by Bell's theorem
贝尔定理证明的量子密钥分布实验
▲ 作者：D. P. Nadlinger, P. Drmota, B. C. Nichol, G. Araneda, D. Main, R. Srinivas, et al.
▲ 链接：
https://www.nature.com/articles/s41586-022-04941-5
▲ 摘要：
加密密钥交换协议传统上依赖于计算猜想，如质因数分解的困难程度，以提供抵御窃听攻击的安全性。值得注意的是，量子密钥分发协议（例如Bennett-Brassard方案）提供了针对此类攻击的信息论安全性，这是经典方法无法实现的一种更强大的安全形式。
然而，迄今为止实现的量子协议受制于一类新型攻击，这种攻击利用了所实现的量子状态或测量与其理论建模之间的不匹配，如诸多实验所示。
研究组介绍了一个完整的、不受这些漏洞影响的量子密钥分发协议的实验实现，遵循埃克特的开创性建议，即根据贝尔定理使用量子纠缠来绑定对方信息。
通过将理论发展与改进的光纤链路相结合，在两个捕获离子量子位之间产生纠缠，研究组在8小时运行时间内创建的150万贝尔对中，获得了95628个具有设备无关安全性的密钥位。他们采取措施确保窃听者无法获取有关测量结果的信息。
这些测量在没有空间状分离的情况下进行。研究结果表明，在一般假设下，可证明安全的密码技术在实际设备中可行，并为基于设备独立性原则的下一步量子信息应用铺平了道路。
▲ Abstract：
Cryptographic key exchange protocols traditionally rely on computational conjectures such as the hardness of prime factorization to provide security against eavesdropping attacks. Remarkably, quantum key distribution protocols such as the Bennett–Brassard scheme provide information-theoretic security against such attacks, a much stronger form of security unreachable by classical means. However, quantum protocols realized so far are subject to a new class of attacks exploiting a mismatch between the quantum states or measurements implemented and their theoretical modelling, as demonstrated in numerous experiments. Here we present the experimental realization of a complete quantum key distribution protocol immune to these vulnerabilities, following Ekert’s pioneering proposal to use entanglement to bound an adversary’s information from Bell’s theorem. By combining theoretical developments with an improved optical fibre link generating entanglement between two trapped-ion qubits, we obtain 95,628 key bits with device-independent security from 1.5 million Bell pairs created during eight hours of run time. We take steps to ensure that information on the measurement results is inaccessible to an eavesdropper. These measurements are performed without space-like separation. Our result shows that provably secure cryptography under general assumptions is possible with real-world devices, and paves the way for further quantum information applications based on the device-independence principle.
材料科学Materials Science
Magnetic memory and spontaneous vortices in a van der Waals superconductor
范德华超导体中的磁记忆和自发涡旋
▲ 作者：Eylon Persky, Anders V. Bjørlig, Irena Feldman, Avior Almoalem, Ehud Altman, Erez Berg, et al.
▲ 链接：
https://www.nature.com/articles/s41586-022-04855-2
▲ 摘要：
掺杂莫特绝缘体展示了一些最有趣的物质量子相，包括量子自旋液体、非常规超导体和非费米液体金属。当巡游电子接近莫特绝缘态时，通常会出现这种相位，因此具有强烈的空间相关性。
范德瓦尔斯异质结构不同层之间的接近性顺理成章地为实验研究局域、关联电子和巡游电子之间的关系提供了一个平台。
课题组通过研究二硫化钽4Hb-TaS2的磁景观来探索这种关系，它实现了候选自旋液体和超导体的交替堆叠。他们报道了一个自发涡旋相，其涡旋密度可在常规状态下修整。
研究组表明，时间反演对称性在常规状态下被破坏，表明存在一个独立于超导体的磁相。值得注意的是，该相位不会产生使用传统技术可检测到的铁磁信号。
研究组使用扫描超导量子干涉装置显微镜来证明它与铁磁有序不兼容。这种不寻常磁相的发现表明，将超导性与强关联系统结合起来可产生意想不到的物理现象。
▲ Abstract：
Doped Mott insulators exhibit some of the most intriguing quantum phases of matter, including quantum spin liquids, unconventional superconductors and non-Fermi liquid metals. Such phases often arise when itinerant electrons are close to a Mott insulating state, and thus experience strong spatial correlations. Proximity between different layers of van der Waals heterostructures naturally realizes a platform for experimentally studying the relationship between localized, correlated electrons and itinerant electrons. Here we explore this relationship by studying the magnetic landscape of tantalum disulfide 4Hb-TaS2, which realizes an alternating stacking of a candidate spin liquid and a superconductor. We report on a spontaneous vortex phase whose vortex density can be trained in the normal state. We show that time-reversal symmetry is broken in the normal state, indicating the presence of a magnetic phase independent of the superconductor. Notably, this phase does not generate ferromagnetic signals that are detectable using conventional techniques. We use scanning superconducting quantum interference device microscopy to show that it is incompatible with ferromagnetic ordering. The discovery of this unusual magnetic phase illustrates how combining superconductivity with a strongly correlated system can lead to unexpected physics.
Dislocation-induced stop-and-go kinetics of interfacial transformations
位错诱导的界面相变间歇式动力学
▲ 作者：Xianhu Sun, Dongxiang Wu, Lianfeng Zou, Stephen D. House, Xiaobo Chen, Meng Li, et al.
▲ 链接：
https://www.nature.com/articles/s41586-022-04880-1
▲ 摘要：
大多数工程材料都基于多相微结构，要么通过控制相平衡产生，要么像制备薄膜那样通过制造不同材料而产生。
在这两种过程中，通过跨异质界面的错配位错（或几何错配位错），微观结构产生弛豫以达到平衡态。尽管错配位错无处不在，但因其埋藏在材料内部，无法直接探测其动态行为。
研究组以氧化铜到铜的界面相变为例，证明了错配位错在以间歇方式调整氧化物到金属界面相变中的作用，通过这种方式，界面台阶的横向流动固定在错配位错的核心，直到位错攀移至新的氧化物/金属界面位置。
结合原子计算，研究组发现钉扎效应与金属原子填充位错核处空位的非局域传输有关。该结果为固体-固体界面相变提供了机理上的见解，并对利用埋藏界面的结构缺陷来调节质量传输和相变动力学具有重要意义。
▲ Abstract：
Most engineering materials are based on multiphase microstructures produced either through the control of phase equilibria or by the fabrication of different materials as in thin-film processing. In both processes, the microstructure relaxes towards equilibrium by mismatch dislocations (or geometric misfit dislocations) across the heterophase interfaces. Despite their ubiquitous presence, directly probing the dynamic action of mismatch dislocations has been unachievable owing to their buried nature. Here, using the interfacial transformation of copper oxide to copper as an example, we demonstrate the role of mismatch dislocations in modulating oxide-to-metal interfacial transformations in an intermittent manner, by which the lateral flow of interfacial ledges is pinned at the core of mismatch dislocations until the dislocation climbs to the new oxide/metal interface location. Together with atomistic calculations, we identify that the pinning effect is associated with the non-local transport of metal atoms to fill vacancies at the dislocation core. These results provide mechanistic insight into solid–solid interfacial transformations and have substantial implications for utilizing structural defects at buried interfaces to modulate mass transport and transformation kinetics.
地球科学Earth Science
Threshold response to melt drives large-scale bed weakening in Greenland
融化阈值响应导致格陵兰大规模床层减弱
▲ 作者：Nathan Maier, Florent Gimbert & Fabien Gillet-Chaulet
▲ 链接：
https://www.nature.com/articles/s41586-022-04927-3
▲ 摘要：
格陵兰岛的冰速很大程度上由基底运动决定，由融水输送到冰层底部来调节。有证据表明，融化速率的增加增强了冰川下排水网络排出基底水的能力，增加了河床摩擦力，导致冰速变慢。这限制了融化营力随温度升高而增加质量损失的可能性。
研究组表明融化营力对动力学有显著影响，但融化速率以外的因素主要控制其影响。通过某种方法来检查整个格陵兰西部的摩擦变异性，他们表明融化营力的主要影响是床层强度从北到南的突然变化，这无法用融水产生的变化来解释。与没有消融的地区相比，南部消融区减弱了20%-40%，而在格陵兰岛北部，消融区却增强了。
研究组表明，这种减弱与持续的基础蓄水量一致，阈值与滑动和水势梯度的差异有关，使床层脱水的排水通道内压力起主要控制作用。这些特征主要取决于边缘是陆地还是海洋，这表明随着温度升高，格陵兰岛北部可能会发生增加质量损失的动态变化。
该结果指出了这些发现的物理表征，这将改善百年尺度上的模拟冰盖演变。
▲ Abstract：
Ice speeds in Greenland are largely set by basal motion, which is modulated by meltwater delivery to the ice base. Evidence suggests that increasing melt rates enhance the subglacial drainage network’s capacity to evacuate basal water, increasing bed friction and causing the ice to slow. This limits the potential of melt forcing to increase mass loss as temperatures increase. Here we show that melt forcing has a pronounced influence on dynamics, but factors besides melt rates primarily control its impact. Using a method to examine friction variability across the entirety of western Greenland, we show that the main impact of melt forcing is an abrupt north-to-south change in bed strength that cannot be explained by changes in melt production. The southern ablation zone is weakened by 20–40 per cent compared with regions with no melt, whereas in northern Greenland the ablation zone is strengthened. We show that the weakening is consistent with persistent basal water storage and that the threshold is linked to differences in sliding and hydropotential gradients, which exert primary control on the pressures within drainage pathways that dewater the bed. These characteristics are mainly set by whether a margin is land or marine terminating, suggesting that dynamic changes that increase mass loss are likely to occur in northern Greenland as temperatures increase. Our results point to physical representations of these findings that will improve simulated ice-sheet evolution at centennial scales.
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