Department of Physics, College of Science, Swansea University, Swansea, UK.
School of Physics and Astronomy, University of Manchester, Manchester, UK.
Nature. 2021 Apr;592(7852):35-42. doi: 10.1038/s41586-021-03289-6. Epub 2021 Mar 31.
The photon-the quantum excitation of the electromagnetic field-is massless but carries momentum. A photon can therefore exert a force on an object upon collision. Slowing the translational motion of atoms and ions by application of such a force, known as laser cooling, was first demonstrated 40 years ago. It revolutionized atomic physics over the following decades, and it is now a workhorse in many fields, including studies on quantum degenerate gases, quantum information, atomic clocks and tests of fundamental physics. However, this technique has not yet been applied to antimatter. Here we demonstrate laser cooling of antihydrogen, the antimatter atom consisting of an antiproton and a positron. By exciting the 1S-2P transition in antihydrogen with pulsed, narrow-linewidth, Lyman-α laser radiation, we Doppler-cool a sample of magnetically trapped antihydrogen. Although we apply laser cooling in only one dimension, the trap couples the longitudinal and transverse motions of the anti-atoms, leading to cooling in all three dimensions. We observe a reduction in the median transverse energy by more than an order of magnitude-with a substantial fraction of the anti-atoms attaining submicroelectronvolt transverse kinetic energies. We also report the observation of the laser-driven 1S-2S transition in samples of laser-cooled antihydrogen atoms. The observed spectral line is approximately four times narrower than that obtained without laser cooling. The demonstration of laser cooling and its immediate application has far-reaching implications for antimatter studies. A more localized, denser and colder sample of antihydrogen will drastically improve spectroscopic and gravitational studies of antihydrogen in ongoing experiments. Furthermore, the demonstrated ability to manipulate the motion of antimatter atoms by laser light will potentially provide ground-breaking opportunities for future experiments, such as anti-atomic fountains, anti-atom interferometry and the creation of antimatter molecules.
光子——电磁场的量子激发——是无质量的，但带有动量。因此，光子在碰撞时可以对物体施加力。通过施加这种力来减缓原子和离子的平移运动，即所谓的激光冷却，在 40 年前首次得到证明。在随后的几十年里，它彻底改变了原子物理学，现在它是许多领域的常用工具，包括研究量子简并气体、量子信息、原子钟和基础物理检验。然而，这种技术尚未应用于反物质。在这里，我们展示了由反质子和正电子组成的反物质原子——反氢的激光冷却。通过用脉冲、窄线宽的莱曼-α激光辐射激发反氢的 1S-2P 跃迁，我们对磁囚禁的反氢样本进行多普勒冷却。虽然我们仅在一个维度上应用激光冷却，但该陷阱将反原子的纵向和横向运动耦合起来，从而导致在所有三个维度上冷却。我们观察到横向动能的中位数降低了一个数量级以上——其中相当一部分反原子达到了亚微电子伏特的横向动能。我们还报告了在激光冷却的反氢原子样本中观察到的激光驱动的 1S-2S 跃迁。所观察到的光谱线比没有激光冷却时获得的光谱线窄约四倍。激光冷却的演示及其直接应用对反物质研究具有深远的意义。更局部、更密集和更冷的反氢样本将极大地改善正在进行的实验中对反氢的光谱和引力研究。此外，通过激光光操纵反物质原子运动的能力为未来的实验提供了潜在的突破性机会，例如反原子喷泉、反原子干涉测量和反物质分子的产生。
