核钟登上《自然》封面，物理学将迎来突破

On September 4, 2024, the research results of academician Ye Jun and Zhang Chuankun's team: the atomic clock project, once again appeared on the cover of Nature.

2024年9月4日，叶军院士、张传坤团队的研究成果：原子钟项目，再次登Nature封面。

This article, entitled "Frequency ratio of the ²²⁹ᵐ Th nuclear isomeric transition and the ⁸⁷Sr atomic clock", was published in Nature and was sexted as the cover of that issue.

这篇题为《钍-229m核异构跃迁和锶-87原子钟的频率比》的文章，发表在Nature 上，并被选为当期封面。

It took only 27 days from submission to acceptance of this paper, and it was reported by Nature's nature podcast series, news & views series, news series, editorials series and Science's News series.

这篇论文从投稿到接收仅仅27天，并同时被《Nature》的nature podcast系列、news & views系列、news系列、editorials系列以及《Science》的News系列报道。

The achievement launches a new human endeavor: a new type of clock timer developed by the team based on tiny changes in energy in atomic nuclei - a nuclear clock, may surpass today's most advanced atomic clocks in accuracy and stability, and is less sensitive to interference, which makes scientists excited to use them as detectors of dark matter and other fundamental physics problems, using this discovery to observe whether the laws of physics change over time.

这项成果发起了一项人类新的努力：团队开发的一种基于原子核中能量微小变化的新型时钟计时器——核钟，在准确性和稳定性方面可能超越当今最先进的原子钟，且对干扰的敏感度更低，这让科学家们很高兴将它们用作暗物质和其他基本物理问题的探测器，利用这一发现来观察物理定律是否随着时间的推移而变化。

This research by Ye Jun's team also marks the beginning of nuclear-based solid-state optical clocks, laying the foundation for the future use of this new type of clock in practical situations.

叶军团队的这项研究也标志着核基固态光学钟的开端，给这种新型时钟以后用在实际情况中打下了基础。

The "Ultimate Timer" is launched for the first time worldwide!

「终极计时器」全球首次启动！

The technology for measuring time has a long history of innovation: from calculating the phases of the moon to the invention of the pendulum and quartz oscillator.

测量时间的技术经历了悠久的创新历史：从计算月相到钟摆和石英振盪器的发明。
原创翻译：龙腾网 https://www.ltaaa.cn 转载请注明出处

The current global timekeeping standard is an atomic clock based on the microwave frequency transitions of cesium atoms. These precision devices can be precisely synchronized around the world with an accuracy of up to 16 decimal places, enough to support space missions and help people use the global GPS positioning system to achieve navigation with an accuracy of less than 1 meter.

目前全球计时标准是一种基于铯原子微波频率跃迁的原子钟，这些精密设备可以在全球范围内精确同步，精度可达小数点后16位数，足以支持太空任务，并帮助人们使用全球GPS定位系统实现精度在1米以内的导航。

A different type of atomic clock uses transitions that emit light in the optical (rather than microwave) range. By precisely amplifying specific energy transitions in atomic nuclei, researchers are closer than ever to building a new type of timekeeper: a nuclear clock.

另一种不同类型的原子钟使用在光学（而不是微波）范围内发射光的跃迁。通过精确放大原子核中的特定能量转变，研究人员比以往任何时候都更接近于构建一种新型计时器：核钟。

Until now, the most accurate clocks were optical clocks made of strontium atoms, which are largely unaffected by external disturbances and are about 100 times more accurate than standard cesium clocks.

在此之前，精度最高的钟是由锶原子构成的光钟，基本不受外界扰动的影响。它比标准铯钟的精度高出约100倍。

Specifically, the team used a vacuum ultraviolet (VUV) frequency comb to directly excite narrow thorium-229 (²²⁹Th) nuclear clock transitions in a solid-state CaF₂ host material and determine the absolute transition frequencies.

具体而言，团队成员使用真空紫外（VUV）频率梳直接激发固态 CaF₂ 主体材料中的窄钍-229（²²⁹Th）核钟跃迁，并确定绝对跃迁频率。

They stabilized the fundamental frequency comb to a strontium-87 (⁸⁷Sr) clock at JILA and coherently upconverted the fundamental frequency to its seventh harmonic in the VUV range by using a femtosecond enhancement cavity.

他们将基带梳稳定至JILA的锶-87（⁸⁷Sr）时钟，并通过使用飞秒增强腔将基频相干上变频到VUV范围内的第七谐波。

This VUV frequency comb established a frequency lix between the energy levels of the nucleus and the energy levels of the electrons, allowing them to directly measure the ratio of the frequency of the thorium-229 nuclear clock transition to that of the strontium-87 atomic clock. The results showed that the thorium-229 nuclear transition frequency is 2,020,407,384.335(2) kHz, and the ratio to the frequency of the strontium-87 atomic clock is 4.707072615078(5).

这种VUV频率梳在原子核能级与电子能级之间建立了频率联系，使他们能够直接测量钍-229核钟跃迁和锶-87原子钟的频率比。结果显示，钍-229的核跃迁频率为2,020,407,384.335（2） kHz，并且与锶-87原子钟的频率比为4.707072615078（5）。

This result greatly advances the research on thorium-229 nuclear clocks, improving the measurement accuracy by six orders of magnitude compared to previous measurements. They precisely measured nuclear quadrupole splitting and extracted the intrinsic properties of the isomers.

这一结果极大推进了钍-229核时钟的研究，相比以往的测量精度提高了六个数量级。他们精确测量了核四极劈裂并提取异构体的内禀性质。

The authors also precisely measured the nuclear quadrupole splitting and extracted the intrinsic properties of the isomers. These results mark the beginning of nuclear-based solid-state optical clocks. This work represents a fusion of precision metrology, ultrafast strong-field physics, nuclear physics, and fundamental physics.

作者还精确测量核四极分裂并提取异构体的内在特性，这些结果标志着基于核的固态光学钟的开始，这项工作代表了精密计量学、超快强场物理、核物理和基础物理的融合。

Once operational, the nuclear clock will be able to reveal one of the secrets of the universe: whether the nuclear and electromagnetic forces are always constant, or whether some as-yet-unidentified mechanism causes them to drift slowly over time.

一旦核钟投入使用，它将能够揭示宇宙的一个秘密：核力与电磁力是否总是恒定的，抑或某种尚未明确的机制令它们随时间缓慢漂移。

However, nuclear clocks are difficult to make, and it took scientists nearly 50 years to get close to a working model.

但是，核钟很难制造，科学家花了将近50年的时间才接近工作模型。

Early measurements suggest that the energy required is 3.5 eV, which means that the transition could be driven by conventional lasers.

早期测量数据表明，所需能量为3.5 eV，这意味着这种跃迁可以通过常规激光器来驱动。

However, subsequent measurements overturned this result, indicating that the actual transition energy is closer to 7.8 eV, which is in the VUV spectrum and may trigger the atoms to release electrons rather than radiate - a process that is faster and less desirable here.

然而，后续的测量推翻了这一结果，表明实际的跃迁能量接近7.8 eV，这一能量位于VUV频谱范围内，并且可能会触发原子释放电子而不是辐射——电子释放的过程更加快速，并且在这里是不希望发生的。

Furthermore, achieving these energies with lasers is also extremely challenging.

此外，利用激光器达到这样的能量同样极具挑战性。

For most atoms, a huge amount of energy is required to lift the nucleus out of its lowest energy state, far beyond what can be achieved using precise, stable probing lasers.

对于大多数原子来说，需要大量的能量才能将原子核从其最低能量状态中抬离，这远远超出了使用精确、稳定的探测激光器所能达到的效果。

The researchers used a laser device called a frequency comb to detect thorium-229 nuclei embedded in the crystal.

研究人员使用一种称为频率梳的激光设备探测了嵌入晶体中的钍-229 原子核

Physicists proposed a thorium-based clock in 2003. But theoretical models of atomic nuclei couldn't predict the energy of this transition with the necessary accuracy, so finding it meant searching through a huge number of possible values.

物理学家在2003年提出了一种基于钍的时钟。但是原子核的理论模型无法以必要的精度预测这种跃迁的能量，因此找到这个跃迁意味着要搜索大量可能的值。

Furthermore, thorium-229 decays slowly from its first energy state. Thus, with a half-life of about 30 minutes, the probability of observing one decay per second is low.

此外，钍-229从其第一个能态缓慢衰变。因此，半衰期约为30分钟，观察到每秒衰变的概率很低。

But scientists' exploration has not stopped. In 2023, rare isotope experts at CERN, the European particle physics laboratory near Geneva, Switzerland, produced thorium-229 using an innovative method and observed low-energy transitions for the first time.

但科学家的探索并不曾止步。 2023年，瑞士日内瓦附近的欧洲粒子物理实验室CERN的稀有同位素专家通过使用创新方法制造了钍-229，并首次观察到了低能跃迁。

The team implanted enough excited thorium-229 ions into calcium fluoride crystals to detect the emitted photons directly using a VUV spectrometer. This research improved the accuracy of the photon energy estimate to 8.3 eV, accelerating the development of lasers capable of stimulating this transition.

该团队将足够数量的激发态钍-229离子注入氟化钙晶体中，以便使用VUV光谱仪直接检测到发射的光子。这项研究提高了光子能量估计的准确性，将其达到8.3 eV，加速了能够激发这种跃迁的激光器的开发。

In parallel, other authors involved in the current work developed a VUV frequency comb that can excite nuclear transitions and simultaneously synchronize them with a nearby optical clock regulated by strontium atoms.

与此同时，参与当前工作的其他作者开发了一种VUV频率梳，可以激发核跃迁，并同时与附近由锶原子调节的光学时钟同步。

Zhang Chuankun et al. brought the Austrian crystal device to the United States, where they made history by driving excitation with a VUV frequency comb.

张传坤等人将奥地利晶体装置带到了美国，在那里他们用VUV频率梳驱动激发，创造了历史。
原创翻译：龙腾网 https://www.ltaaa.cn 转载请注明出处

Repeated experiments showed that this frequency comb could excite nuclear transitions and read out their frequencies in relation to those of strontium transitions.

通过反复实验表明，这种频率梳可以激发核跃迁，并读出其频率与锶跃迁频率的关系。

The lifetime of the nuclear excited states they observed was about 10 minutes, meaning that the system could be used to generate a 2 petahertz (1PHz is 10¹⁵Hz) clock with an uncertainty in the microhertz range.

他们观察到的核激发态的寿命约为10分钟，这意味着该系统可用于产生2拍赫兹（1PHz为10¹⁵Hz）的时钟，且不确定性在微赫兹范围内。

Zhang and his colleagues found a total of seven transitions, five of which were expected due to the energy level splitting caused by the interaction between the charge distribution of the atomic nuclei and the strong intrinsic electric field of the crystal.

张传坤等人总共发现了七个跃迁，其中五个跃迁是预期的，这是由于原子核的电荷分布与晶体的强固有电场相互作用而产生的能级分裂。

Currently, any hopes for greater precision are complicated by the width of the comb teeth, which are widened by the process by which they are generated. Further improvements are needed to shrink these teeth for metrology purposes, perhaps by transferring existing technology from the optical to the VUV frequency range.

目前，任何更高精确度的希望都因梳齿的宽度而变得复杂，梳齿的宽度因其生成过程而变宽。出于计量目的，需要进一步改进以缩小这些齿，可能通过将现有技术从光学频率范围转移到VUV频率范围。

One exciting prospect involves monitoring how the frequency of transitions in nuclear clocks changes over time. This could reveal hypothesized tiny changes in the fine structure constant (which quantifies the strength of electromagnetic interactions between charged particles) and in the coupling between nuclear particles, all of which would inspire the search for new physics.

一个令人兴奋的前景涉及监测核钟的跃迁频率如何随时间变化。这可以揭示精细结构常数（量化带电粒子之间电磁相互作用的强度）以及核粒子之间耦合的假设微小变化，所有这些都将激发对新物理学的探索。
原创翻译：龙腾网 https://www.ltaaa.cn 转载请注明出处

Another tantalizing physics application of nuclear clocks is the search for candidate particles for dark matter, the invisible stuff thought to make up 85 percent of the mass of the universe.

核钟的另一个诱人的物理应用是寻找暗物质的候选粒子，这种看不见的物质被认为占宇宙质量的85%。

Many models propose that ultralight dark matter particles will interact directly with the strong nuclear force, which binds protons and neutrons together in atomic nuclei. If these particles interact with thorium nuclei, they would disrupt the transition frequencies, disrupting the clock in a detectable way. This work has revealed nuclear behavior in unprecedented detail. The JILA measurements provide evidence that nuclei such as thorium expand and contract unexpectedly as they move between excited and ground states.

许多模型提出超轻暗物质粒子将与强核力直接相互作用，强核力将原子核中的质子和中子结合在一起。如果这些粒子与钍原子核相互作用，它们就会扰乱跃迁频率，从而以可检测的方式扰乱时钟。这项工作已经以前所未有的细节揭示了核行为。 JILA测量提供的证据表明，钍等原子核在激发态和基态之间移动时会意外地膨胀和收缩。

The astonishing achievement of Zhang and others foreshadows many fascinating future discoveries and caps off three decades of brilliant research.

张传坤等人的惊人成就预示着许多令人着迷的未来发现，并为三十年来的精彩研究画上句号。

Quantum mechanics has become an important frxwork for understanding and explaining the microscopic world, and the revolution in quantum science is changing our world at an astonishing speed. "Clocks" are one of them, and nuclear clocks are opening a new chapter in human quantum technology.

量子力学已成为了解和解释微观世界的重要框架，而量子科学的革命正在以惊人的速度改变我们的世界。「时钟」就是其中一种，核钟正在开启人类量子科技的新篇章。