Physics with Trapped Antihydrogen

几个世纪以来，了解我们宇宙的起源和演化一直是科学努力的核心。近几十年来取得了重大进展，物理学和宇宙学结合在一起，形成了一幅连贯的图景。我们的宇宙似乎诞生于一场叫做大爆炸的灾难性事件中，并且在此后的130亿至140亿年间不断演化。虽然可见宇宙的大部分是可以解释的，但仍然有许多深刻的谜团，包括反物质的存在和它的命运。简单地说，反物质是一个谜。虽然物理定律的对称性，特别是量子力学，预测它的存在与物质或多或少是平等的，但宇宙似乎只由后者组成。解决这个难题是基础科学面临的巨大挑战之一。随着宇宙在大爆炸后冷却，似乎所有的反物质都消失了，但留下了微小的过剩物质(十亿分之一)，整个物质宇宙就是从这些物质中产生的。问题是我们不明白这是怎么来的。物质和反物质的行为有不对称性，但它们太小了许多数量级，无法解释宇宙中物质的存在。解决这个问题的一种方法，也是我们选择的方法，是研究反氢--一种宇宙从未有机会制造的原子--并将其性质与氢的性质进行比较。最近，我们取得了很大进展。我们现在能够温和地混合反质子和正电子，创建一些动能足够低的反氢原子，这些原子可以被困在只有0.54K深的磁性最小中性原子陷阱中。这种陷阱是由复杂的线圈排列形成的磁场形成的，该线圈排列在反氢产生区的中心形成最小磁场。反氢，就像氢一样，有一个微小的磁矩--把轨道上的正电子想象成一个微小的电流回路--这样能级就会在外加磁场中移动。那些势能在场中增加的原子将倾向于坐在磁场最小的位置，并可能被俘获。如果需要，我们已经能够限制反原子15分钟或更长时间，所以我们确定它们处于基态。在一个里程碑式的实验中，我们通过用微波辐射轰击捕获的反氢来进行有史以来第一次反原子研究。微波的频率被调谐到共振跃迁，迫使反原子进入无法被囚禁在陷阱中的量子态。结果是，只有在适当设置微波频率的情况下，陷阱中的反氢物质才会被排空。我们能够告诉我们的陷阱已经被清空了，并在反氢击中陷阱墙时发现了湮灭。我们目前正在重建我们的设备，以改进这项实验，并使用激光来处理反氢的光谱。虽然这种能力有很大的机会，但在氢和反氢的性质可以与精密进行比较之前，还有很多工作要做。在这个项目中，我们将从这个方向开始。如果发现了任何不同之处，我们就会发现新的物理学，也许还会在探索早期宇宙中反物质命运的道路上有所作为。

Understanding the origin and evolution of our Universe has been at the heart of scientific endeavour for centuries. Recent decades have seen major advances, as physics and cosmology have combined to produce the beginnings of a coherent picture. Our Universe seems to have been born in a cataclysmic event called the Big Bang, and has continuously evolved over the 13-14 billion years since then. Though much of the visible Universe can be explained, there are still many profound mysteries, including the existence of antimatter, and its fate.Simply put, antimatter is an enigma. Whilst the symmetry of the laws of physics, and in particular quantum mechanics, predict its existence on a more-or-less equal footing to matter, the Universe appears to be composed only of the latter. Addressing this conundrum is one of the great challenges of basic science. As the Universe cooled after the Big Bang it appears that all the antimatter vanished, but leaving a tiny excess (one part in a billion) of matter from which the entire material Universe is created. The problem is we don't understand how this came to be. There are asymmetries in the behaviour of matter and antimatter, but they are too small by many orders of magnitude to account for the existence of matter in the Universe. One way to address this problem, and the way we have chosen, is to study antihydrogen - an atom that the Universe never got the chance to make - and compare its properties with those of hydrogen. Recently, we have made great progress. We are now able to gently mix antiprotons and positrons to create some antihydrogen atoms with low enough kinetic energies to be held in a magnetic minimum neutral atom trap that is only 0.54 K deep. This trap is formed by magnetic fields from a complicated coil arrangement that forms the field minimum in the centre of the antihydrogen production region. Antihydrogen, like hydrogen, has a tiny magnetic moment - think of the orbiting positron as a tiny current loop - such that the energy levels shift in an applied magnetic field. Those atoms whose potential energy increases in the field will prefer to sit at the magnetic field minimum, and may be trapped. We have been able to confine anti-atoms for 15 minutes or more if required, so we are sure that they are in their ground state.In a landmark experiment we have performed the first ever study of an anti-atom by bombarding the trapped antihydrogen with microwave radiation. The frequency of the microwaves was tuned to a resonant transition that forced the anti-atoms into a quantum state that could not be held in the trap. The result was that the trap was emptied of the antihydrogen - but only when the microwave frequency was set appropriately. We were able to tell that our trap had been emptied, and also spot the annihilations as the antihydrogen hit the trap walls. We are currently re-building our apparatus to improve this experiment, and also to use lasers to address the spectrum of antihydrogen. Although this capability has great opportunities, there is much work to be done before the properties of hydrogen and antihydrogen can be compared with precision. In this project we will start in this direction. If any differences are found, we will have discovered new physics, and perhaps come some way along the road to discovering the fate of antimatter in the early Universe.