Understanding Collisions of Ultracold Polar Molecules

在绝对零度以上约百万分之一度的温度下，原子和分子进入一个新的状态，在这个状态下，它们的所有运动都受量子力学定律的支配。在过去的20年里，原子物理学在这一领域出现的新现象的基础上出现了复兴。原子玻色-爱因斯坦凝聚体的实验（获得2001年诺贝尔物理学奖）已经探索了广泛的主题，包括非线性原子光学，量子涡旋和光学晶格中的相变。与此同时，简并费米气体也被用来提高我们对费米超流性和极化子物理的理解。所有这些进展的核心是对超冷原子碰撞的深入和详细的理解，这是在过去20年中通过理论和实验的密切相互作用而发展起来的。该领域的注意力现在转向超冷分子提供的新可能性。与原子不同，分子可以具有电偶极矩，一端带正电，另一端带负电。这些偶极子意味着分子之间的相互作用比原子之间的相互作用更强，而且关键是在更远的距离上。分子也有比原子更复杂的内部结构：它们有多个旋转的原子核，它们可以旋转并被外部场定向。正因为如此，超冷分子为新物理现象的研究和新量子技术的发展提供了许多新的可能性。例子包括研究量子磁性的奇异形式，以及利用所谓的“量子模拟器”设计新材料特性的潜力，这种模拟器基于限制在光学晶格中的分子阵列。我们最近成为世界上第三个成功形成超冷极性分子样品的团队。在我们的例子中，这是通过将铷和铯原子的气体冷却到超冷温度，然后将原子配对形成分子来完成的。我们形成的分子最初是非极性的，只有非常弱的结合，但我们成功地将它们转移到深度结合，极性状态使用双光子光学转移过程，称为受激拉曼绝热通道（STIRAP）。整个过程不需要加热，因此产生的分子气体的温度与原子混合物的温度成镜像。在我们的分子可以用于新的量子设备之前，我们需要了解更多关于它们的相互作用和碰撞的信息。一个关键的问题是，当两个分子碰撞时，它们是否会长时间地“粘在一起”，而这一问题在原子中并不存在。如果它们这样做了，那么第三个分子可能会沿着出现并破坏前两个分子，从而缩短分子样品的寿命。该项目的目标是研究超冷分子的碰撞，包括实验和理论，以了解所涉及的过程。我们将研究两体和三体碰撞，并通过将我们的分子加载到由激光驻波形成的光学晶格中来区分它们。这些方法可以将分子限制在一堆扁平的煎饼、一束束管甚至单个盒子中。即使分子碰撞是“粘性的”，我们也希望能够找到控制它们的方法，从而将分子样品保存足够的时间来进行有趣的实验。我们将研究如何利用外加电场定向受限分子，并利用微波光子对其进行修饰，从而创造出将分子分开并防止碰撞的力。这个雄心勃勃的项目将最先进的实验与世界领先的理论相结合，将巩固英国在这一激动人心的国际领域的前沿地位。

At temperatures about a millionth of a degree above absolute zero, atoms and molecules enter a new regime where all their motions are governed by the laws of quantum mechanics. Over the last 20 years, there has been a renaissance in atomic physics based on the new phenomena that emerge in this regime. Experiments with atomic Bose-Einstein condensates (recognised by the 2001 Nobel Prize in Physics) have explored a wide range of topics including nonlinear atom optics, quantum vortices, and phase-transitions in optical lattices. At the same time, degenerate Fermi gases have been used, for example, to improve our understanding of Fermionic superfluidity and the physics of polarons. At the heart of all these advances is a deep and detailed understanding of ultracold atomic collisions that has developed over the last two decades through the close interplay of theory and experiment.Attention in this field is now turning to the new possibilities offered by ultracold molecules. Unlike atoms, molecules can possess an electric dipole moment, with one end positively charged and the other negatively charged. These dipoles mean that molecules can interact with one another more strongly than atoms, and crucially at longer range. Molecules also have more complicated internal structure than atoms: they have multiple spinning nuclei, and they can rotate and be oriented by external fields. Because of this, ultracold molecules offer many new possibilities for the study of novel physical phenomena and the development of new quantum technologies. Examples include the study of exotic forms of quantum magnetism and the potential to design new material properties using so-called "quantum simulators" based on arrays of molecules confined in optical lattices. We have recently become only the third group in the world to succeed in forming a sample of ultracold polar molecules. In our case, this was done by cooling gases of rubidium and cesium atoms to ultracold temperatures, and then pairing up the atoms to form molecules. The molecules we formed were initially nonpolar and only very weakly bound, but we succeeded in transferring them to deeply bound, polar states using a two-photon optical transfer process, known as stimulated Raman adiabatic passage (STIRAP). The entire process occurs without heating, so that the temperature of the resulting molecular gas mirrors that of the atomic mixture.Before our molecules can be used in new quantum devices, we need to understand a lot more about their interactions and collisions. A key question, which does not arise for atoms, is whether pairs of molecules "stick together" for a long time when they collide. If they do, then a third molecule may come along and destroy the first two, shortening the lifetime of the molecular sample. The objective of this project is to investigate collisions of ultracold molecules, both experimentally and theoretically, in order to understand the processes involved. We will investigate both 2-body and 3-body collisions, and distinguish between them by loading our molecules into optical lattices formed by standing waves of laser light. These can confine the molecules in stacks of flat pancakes, bundles of tubes, or even individual boxes.Even if molecular collisions are "sticky", we expect to be able to find ways to control them and thereby preserve the molecular sample for sufficient time to perform interesting experiments. We will investigate orienting the confined molecules with applied electric fields and dressing them with microwave photons, allowing us to create forces that will hold the molecules apart and prevent collisions.This ambitious project, combining state-of-the-art experiments with world-leading theory, will cement the UK's position at the forefront of an exciting international field.