Laser Cooling Molecules

激光冷却原子是一项非常成功的技术。不仅有可能将原子气体的温度降低到纳米开尔文的范围，技术的改进还可以产生原子玻色-爱因斯坦凝聚物，这是物理定律所允许的最冷的物质。现在人们对探索超冷分子的物理学有着极大的兴趣，例如研究超低温下的化学反应，光谱学和高精度测量，量子信息处理，以及研究具有远距离各向异性相互作用的量子简并物质的基本物理学。然而，到目前为止，用于产生超冷原子的关键技术——激光冷却，还没有应用于分子——甚至是简单的双原子分子——因为电子基态有许多振动和旋转能级。问题是激光冷却需要吸收和发射成千上万的光子。大多数分子很快被泵送到不再与激光相互作用的水平。激光冷却分子的关键是使用额外的激光波长来堵塞所有这些泄漏，这对大多数分子物种来说是一项艰巨的任务。在本研究中，我们将探索SrF和BaF的激光冷却。这些相当简单的分子具有这样的特性：电子跃迁（用来从分子中提取能量，从而使它们冷却）往往不会改变分子的振动量子数。这意味着只需要几个激光波长就可以将泄漏堵塞到其他旋转和振动状态。我们计划使用四个二极管激光器来产生这种光，这是非常可靠和相对便宜的。我们将使用超音速膨胀来制造分子，我们的研究小组已经非常熟悉这项技术。分子在几个开尔文的温度下从膨胀中产生，与室温相比非常冷，但仍然太热，无法研究有趣的新物理和应用。激光冷却会使分子的温度降低数千倍。我们的计划是首先展示一维的冷却，然后将技术扩展到二维和三维。该计划的最终目标是利用磁场和电场的结合来捕获分子，并使用激光将它们冷却到微开尔文的温度。我们相信，这样一个冷捕获分子的样本将成为探索新的科学领域的一个迷人的工具。

Laser cooling of atoms has been a fantastically successful technique. Not only is it possible to reduce the temperature of a gas of atoms to the nanokelvin range, refinements of the technique allow the production of atomic Bose-Einstein condensates, the coldest matter allowed by the laws of physics. There is now enormous interest in exploring the physics of ultracold molecules, for example to study chemical reactions at ultralow temperatures, for spectroscopy and high-precision measurement, for quantum information processing, and for the study of the basic physics of quantum degenerate matter with long range anisotropic interactions. Until now however, the key technique used to produce ultracold atoms, laser cooling, has not been applied to molecules - even simple diatomic molecules - because the electronic ground state has many vibrational and rotational levels. The problem is that laser cooling requires the absorption and emission of many thousands of photons. Most molecules are quickly pumped into levels where they no longer interact with the laser light. The key to laser cooling molecules is to plug all of these leaks using additional laser wavelengths, a formidable task for most molecular species.In this research, we will explore the laser cooling of SrF and BaF. These rather simple molecules have the property that electronic transitions (which are used to extract energy from the molecules and thereby cool them) tend not to change the molecule's vibrational quantum number. This means that only a few laser wavelengths are required to plug the leaks into the other rotational and vibrational states. We plan to generate this light using four diode lasers, which are very reliable and relatively cheap. We will produce the molecules using a supersonic expansion, a technique that our research group is already very familiar with. Molecules emerge from the expansion with a temperature of a few Kelvin, very cold compared to room temperature but still far too warm to investigate interesting new physics and applications. Laser cooling will make the molecules thousands of times colder. Our plan is first to demonstrate cooling in one dimension, then to extend the techniques to 2d and 3d. The ultimate goal of the programme is to trap the molecules using a combination of magnetic and electric fields and to use laser light to cool them to microkelvin temperatures. We believe such a sample of cold trapped molecules will be a fascinating tool with which to explore new areas of science.