厉害！氢分子的“电子云”竟然可以“被看到”
据物理学家组织网2018年1月9日报道，德国、美国、西班牙、俄罗斯以及澳大利亚的科学家合作首次创建了氢分子波函数平方图像Ψ2(H2)，相关研究结果已经于2017年12月22日在《自然通讯》（Nature Communications）网站发表——M. Waitz, R. Y. Bello, D. Metz, J. Lower, F. Trinter, C.Schober, M. Keiling, U. Lenz, M. Pitzer, K. Mertens, M. Martins, J. Viefhaus,S. Klumpp, T. Weber, L. Ph. H. Schmidt, J. B. Williams, M. S. Schöffler, V. V. Serov, A. S. Kheifets, L. Argenti, A. Palacios,F. Martín, T. Jahnke, R. Dörner. Imaging thesquare of the correlated two-electron wave function of a hydrogen molecule.Nature Communications, 2018, 8: 2266. DOI: 10.1038/s41467-017-02437-9.
参与此项研究的有来自德国歌德大学（J. W. GoetheUniversität）、德国卡塞尔大学（Universität Kassel）、德国汉堡大学（Universität Hamburg）、德国电子同步加速器中心（Deutsches Elektronen-SynchrotronDESY）、德国FS-FLASH-D；西班牙马德里自治大学（Universidad Autónoma de Madrid）、Instituto Madrileo de EstudiosAvanzados en Nanociencia；美国劳伦斯伯克利国家实验室（Lawrence BerkeleyNational Laboratory）、美国雷诺内华达大学（University of NevadaReno）、美国中佛罗里达大学（University of CentralFlorida）；俄罗斯萨拉托夫州立大学（Saratov StateUniversity）；澳大利亚国立大学（The AustralianNational University）的科研人员。审稿人对此评价是具有里程碑意义的研究成果。它不仅是最前沿的实验结果和非常全面的理论分析的完美结合，而且是氢分子作为一种基本的双电子体系，在相关双电子波函数成像方面的一项原始的、非常有趣的研究，该研究成果结构清晰，同时便于非专业读者阅读。在物理学和化学的广泛领域中，所提出的工作具有很高的影响和激发新思维的潜力。因此，审稿人非常热情地推荐这一具有里程碑意义的作品，在没有任何重大变化的情况下给予发表（Peer Review File）。更多信息请注意浏览原文或者相关报道：
Physicistscreate first direct images of the square of the wave function of a hydrogenmolecule
January9, 2018 by Lisa Zyga
具有两个电子的氢分子的波函数的平方的图像
物理学家第一次开发了一种方法来可视化地描绘电子之间的纠缠。由于这些相关性在决定分子的波函数 - 描述分子的量子态时起着重要的作用，研究人员然后使用新的方法来产生氢（H2）分子的双电子函数的平方。
尽管已经有多种技术用于对原子和分子的单个电子进行成像，但是这是第一种可以直接描绘电子之间相关性的方法，并且允许研究人员探索电子的性质如何相互依赖。
来自德国，西班牙，美国，俄罗斯和澳大利亚各个研究机构的研究人员M. Waitz等人在最近的Nature Communications上发表了一篇关于这种新成像方法的论文。
“还有其他一些方法可以让人们从不同的观察角度重新建立相关性，但据我所知，这是第一次通过查看谱图来获得相关性的指向性，”马德里自治大学的FernandoMartín告诉Phys .ORG。 “与波函数平方不同片段的傅里叶变换（或者等效地表示波动函数在动量空间中的不同片段的表示）是相同的。不需要重建或滤波或变换：频谱直接反射动量空间中的波函数片“。
这种新方法包括结合两种已经广泛使用的成像方法：光电成像和反应片段的重合检测。研究人员同时使用这两种方法，通过使用第一种方法在一个电子上将该电子投射到检测器上，并使用第二种方法在另一个电子上来确定其性质如何响应。
同时使用这两种方法揭示了两个电子如何相关并产生H2相关双电子波函数平方的图像。物理学家强调一个重要的观点：这些是波函数平方的图像，而不是波函数本身。
“波函数在量子物理中是不可观测的，所以不能被观察，”马丁说，“只有波函数的平方是可观测的（如果你有这个工具的话），这是量子物理学基本原理之一，那些声称他们能够观测到波函数的人并没有使用合适的语言，因为这是不可能的：他们所做的是通过做一些近似来从一些测量的光谱中重建它，它不能被间接观测。
研究人员预计，新方法可以用来对两个以上电子的分子进行成像，通过检测多个电子的反应片段。该方法还可以导致对多个分子的波函数之间的相关性进行成像的能力。
马丁说：“显然，要遵循的自然步骤是在更复杂的分子中尝试类似的方法。这种方法很可能适用于小分子，但是如果它能在非常复杂的分子中起作用还不清楚，不是因为基本思想的限制，而是主要因为实验的局限性，因为复杂分子中的重合实验要困难得多由于许多核自由度而进行分析“。
对电子-电子相关性和相应的分子波函数进行可视化的能力对理解物质的基本性质有着深远的影响。例如，称为Hartree-Fock方法的近似波函数最常用的方法之一不考虑前电子 - 电子相关性，因此经常与观测不一致。
此外，电子 - 电子相关性是引人入胜的量子效应的核心，例如电导率（当电阻在极低温度下降至零时）和巨磁电阻（由于附近磁层的磁化的平行对准而导致电阻大大降低时）。电子相关性也在从吸收单光子的分子同时发射两个电子中起作用，这种现象称为“单光子双电离”。
最后，结果也可能导致实际应用，例如能够实现与场电子激光和基于激光的X射线源的相关成像。
进一步探索：用于光物质相互作用的空间时间传感器
Image of the square of thewave function of a hydrogen molecule with two electrons. Credit: Waitz et al.Published in Nature Communications
For the first time, physicistshave developed a method to visually image the entanglement between electrons.As these correlations play a prominent role in determining a molecule's wavefunction—which describes the molecule's quantum state—the researchers then usedthe new method to produce the first images of the square of the two-electronwave function of a hydrogen (H2) molecule.
Although numerous techniquesalready exist for imaging the individual electrons of atoms and molecules, this is the first method that can directly image thecorrelations between electrons and allow researchers to explore how theproperties of electrons depend on one another.
The researchers, M. Waitz etal., from various institutes in Germany, Spain, the US, Russia, and Australia,have published a paper on the new imaging method in a recent issue of NatureCommunications.
"There are other methodsthat allow one to reconstruct correlations from different observations;however, to my knowledge, this is the first time that one gets a directimage of correlations by just looking at a spectrum," coauthor FernandoMartín at the Universidad Autónoma de Madrid told Phys.org. "Therecorded spectra are identical to the Fourier transforms of the differentpieces of the square of the wave function (or equivalently, to therepresentation of the different pieces of the wave function in momentum space).No reconstruction or filtering or transformation is needed: the spectrumdirectly reflects pieces of the wave function in momentum space."
The new method involvescombining two imaging methods that are already widely used: photoelectronimaging and the coincident detection of reaction fragments. The researcherssimultaneously employed both methods by using the first method on one electronto project that electron onto a detector, and using the second method on theother electron to determine how its properties change in response.
The simultaneous use of bothmethods reveals how the two electrons are correlated and produces an image ofthe square of the H2correlated two-electron wave function. Thephysicists emphasize one important point: that these are images of the squareof the wave function, and not the wave function itself.
"The wave function is notan observable in quantum physics, so it cannot be observed," Martín said."Only the square of the wave function is an observable (if you have thetools to do it). This is one of the basic principles of quantum physics. Thosewho claim that they are able to observe the wave function are not using theproper language because this is not possible: what they do is to reconstruct itfrom some measured spectra by making some approximations. It can never be adirect observation."
The researchers expect thatthe new approach can be used to image molecules with more than two electrons aswell, by detecting the reaction fragments of multiple electrons. The methodcould also lead to the ability to image correlations between the wave functionsof multiple molecules.
"Obviously, the naturalstep to follow is to try a similar method in more complicated molecules,"Martín said. "Most likely, the method will work for small molecules, butit is not clear if it will work in very complex molecules. Not because oflimitations in the basic idea, but mainly because of experimental limitations,since coincidence experiments in complex molecules are much more difficult toanalyze due to the many nuclear degrees of freedom."
The ability to visualizeelectron-electron correlations and the corresponding molecular wave functions hasfar-reaching implications for understanding the basic properties of matter. Forinstance, one of the most commonly used methods for approximating a wavefunction, called the Hartree-Fock method, does not account forelectron-electron correlations and, as a result, often disagrees withobservations.
In addition, electron-electroncorrelations lie at the heart of fascinating quantum effects, such assuperconductivity (when electrical resistance drops to zero at very coldtemperatures) and giant magnetoresistance (when electrical resistance greatlydecreases due to the parallel alignment of the magnetization of nearby magneticlayers). Electron correlations also play a role in the simultaneous emission oftwo electrons from a molecule that has absorbed a single photon, a phenomenoncalled "single-photon double ionization."
And finally, the results mayalso lead to practical applications, such as the ability to realize correlation imaging withfield-electron lasers and with laser-based X-ray sources.
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