Cooling molecules to quantum degeneracy

每个人都熟悉物质的三种正常状态--固体、液体和气体。还有另一种状态，称为玻色-爱因斯坦凝聚态(BEC)，这是一种只能用量子力学描述的物质状态。由于位置和速度之间的不确定关系，原子在放慢速度时会扩散开来。如果一种原子气体冷却到很低的温度，原子可能会扩散得太多，以至于它们都重叠在一起。在这一点上，它们协调一致，所有人都聚集在一起，进入具有最低能量的量子状态，并表现为一个单一的实体，而不是一个个体的集合。1924年，玻色和爱因斯坦预言了物质的这种状态，1995年，研究人员通过将原子气体冷却到低于微开尔文(比绝对零度高出百万分之一度)首次创造了这种状态。玻色-爱因斯坦凝聚是超流和超导体等特殊现象的基础。在过去的25年里，对原子的BEC的研究一直是一个非常有成果的研究课题，并且有强烈的动机将其扩展到分子。重要的是，分子可以是极性的，有正端和负端。由于这些电偶极子，分子之间的相互作用比原子强得多，距离也远得多。事实上，在极性分子的BEC中，每个分子都与其他分子相互作用，形成了一个相互作用很强的量子系统。从这些相互作用的系统中出现了新的、显着的现象，这些现象无法从成分的行为中预测出来，而且太复杂了，无法在正常的计算机上模拟。例子包括磁性和高温超导。分子BEC将是研究这些相互作用的量子系统的理想的、高度可控的系统。它还可能有助于量子计算机的发展，并提高我们对低温下的碰撞和化学的理解。最后，这样的低温将极大地提高正在进行的实验的精确度，这些实验使用分子来测试基础物理，例如寻找物质-反物质不对称的起源的测量。尽管有所有这些动机，分子还没有冷却到BEC所需的低温。我们的目标是在这个项目中做到这一点。我们将首先使用激光冷却，这是我们在过去几年中为分子开创的一种方法。然后我们将捕获分子，并使用碰撞来进一步冷却它们。在这里，有两种方法。在第一个阶段--蒸发冷却--最高能量的分子被从捕获器中移除，剩下的分子相互碰撞，重新分配减少的能量，从而冷却到较低的温度。在第二阶段--交感冷却--分子在较低温度下与原子碰撞时会冷却。除了这些关键的降温碰撞之外，还可能存在导致分子改变状态、反应或被从陷阱中弹出的严重碰撞。成功的关键是控制这些冲突，加强好的冲突，抑制坏的冲突。我们的理论和实验专业知识的结合将是我们的指南。我们产生的BEC将是一种全新类型的量子物质，其性质受强的、长程的偶极-偶极相互作用的支配。我们将研究它的行为，并学习如何利用电场和磁场来控制它，开辟一个丰富的强相互作用偶极物质的新领域。

Everyone is familiar with the three normal states of matter - solids, liquids and gases. There is also another state, known as a Bose-Einstein condensate (BEC), which is a state of matter that can be described only by quantum mechanics. Due to the uncertainty relation between position and speed, an atom spreads out when it slows down. If a gas of atoms is cooled to very low temperature, the atoms may spread out so much that they all overlap. At this point they coordinate, all gathering together into the quantum state that has the lowest energy, and behaving as a single entity instead of a collection of individuals. This state of matter was predicted in 1924 by Bose and Einstein, and in 1995 researchers created it for the first time by cooling a gas of atoms to less than a microkelvin (a millionth of a degree above absolute zero). Bose-Einstein condensation underlies some extraordinary phenomena such as superfluidity and superconductivity.The study of BECs of atoms has been an immensely fruitful research topic for the last 25 years, and there are strong motivations to extend this to molecules. Importantly, molecules can be polar, having a positive end and a negative end. Due to these electric dipoles, molecules can interact with one another far more strongly than atoms and over much larger distances. In fact, in a BEC of polar molecules, every molecule interacts with every other molecule, creating a strongly interacting quantum system. From these interacting systems emerge new and remarkable phenomena that could not be predicted from the behaviour of the constituents and are far too complex to simulate on a normal computer. Examples include magnetism and high-temperature superconductivity. A molecular BEC would be an ideal, highly controllable system for studying these interacting quantum systems. It may also contribute to the development of quantum computers and improve our understanding of collisions and chemistry at low temperatures. Finally, such low temperatures would hugely improve the precision of ongoing experiments that use molecules to test fundamental physics, such as measurements that search for the origins of matter-antimatter asymmetry.Despite all this motivation, molecules have not yet been cooled to the low temperatures needed for BEC. We aim to do that in this project. We will first use laser cooling, which is a method we have pioneered for molecules over the last few years. Then we will trap the molecules and use collisions to cool them further. Here, there are two approaches. In the first - evaporative cooling - the highest-energy molecules are removed from the trap and the remaining molecules collide and re-distribute the reduced energy, thereby cooling to lower temperatures. In the second - sympathetic cooling - the molecules cool as they collide with atoms at lower temperature. In addition to these crucial temperature-lowering collisions, there can also be bad collisions that cause molecules to change their state, react, or be ejected from the trap. The key to success is to control these collisions, enhancing the good ones and suppressing the bad ones. Our combination of theoretical and experimental expertise will be our guide. The BEC we produce will be a completely new type of quantum matter, whose nature is governed by the strong, long-range dipole-dipole interactions. We will study its behaviour and learn how to control it using electric and magnetic fields, opening up a rich new field of strongly-interacting dipolar matter.