A Stable Quantum Gas of Fermionic Polar Molecules

量子力学理论对孤立的原子提供了极好的描述，并使我们对这种系统的物理学的理解达到了前所未有的水平。同样的量子物理最终支配着所有物质，甚至是块状材料，并导致许多重要而有趣的现象，如高温超导和奇异形式的磁性。然而，在固体材料中，单个原子不再彼此孤立，并且通常与材料中的许多其他粒子发生强烈的远程相互作用。在这种情况下，量子力学的本质意味着多体系统的精确解通常是不可能的。相反，我们必须建立一个简单的系统模型，它能捕捉到基本的物理特性，然后尝试为有限数量的粒子求解这个模型。然而，即使这种方法在超过10到100个粒子的经典计算机上也变得难以处理。另一种策略最初是由理查德·费曼提出的，即使用另一个量子系统来“模拟”描述感兴趣系统的模型或哈密顿量。开发这样的“量子模拟器”已经成为研究的一个主要主题，因为它们有可能改变我们理解新材料的方式，并最终影响到造福全社会的未来设备和技术。激光冷却技术的发展使我们能够将原子气体冷却到绝对零度以上不到百万分之一度的温度，在这种温度下，粒子的量子力学性质支配着它们的热运动。在这种状态下，新的物质状态以玻色-爱因斯坦凝聚体和费米简并气体的形式出现。这种气体具有高度可控性，为实现量子模拟协议提供了一个有前途的平台。特别是，超冷异核分子具有可控的远程相互作用，这是设计与凝聚态物理相关的一类重要问题所必需的。此外，利用激光驻波将分子限制在光学晶格中，可以很容易地复制真实材料的晶体结构。然而，由于分子内部复杂的旋转和振动结构，为原子开发的激光冷却和俘获技术通常不适用于分子。尽管如此，它们仍然可以通过小心地将超冷原子组装成超冷分子来加以利用。这种方法被证明是非常成功的，已经产生了许多不同的分子。该技术使用两个不同的步骤将分子联系起来。首先，弱结合分子是通过碰撞共振形成的，这种共振被称为费什巴赫共振，它将自由原子偶联到接近阈值的分子状态。其次，利用被称为受激拉曼绝热通道（STIRAP）的双光子光学传递过程将分子转移到绝对基态。值得注意的是，整个转换过程可以非常高效，加热可以忽略不计，因此产生的分子量子气体的温度和密度反映了原子混合物的初始参数。本提案的目标是通过将K和Cs的预冷原子相结合来实现超冷费米子KCs分子气体。与其他双碱分子相比，这种分子具有抗反应性碰撞稳定的优势，并提供费米子和玻色子同位素。通过将分子限制在二维煎饼陷阱阵列中，我们将为量子模拟应用提供一个测试平台。为了实现这一雄心勃勃的目标，我们建议将最先进的实验与世界领先的理论支持结合起来，形成一个变革性的研究项目，以巩固英国在这一激动人心的国际领域的前沿地位。

The theory of quantum mechanics provides an excellent description of isolated atoms and has allowed us to develop our understanding of the physics of such systems to unprecedented levels. The same quantum physics ultimately governs all matter, even in bulk materials, and leads to many important and interesting phenomena, such as high temperature superconductivity and exotic forms of magnetism. However, in solid materials individual atoms are no longer isolated from one another and commonly experience strong long-range interactions with many other particles in the material. In this case, the nature of quantum mechanics means that an exact solution of the many-body system is usually impossible. Instead we must develop a simple model of the system which captures the essential physics and then try to solve this model for a finite number of particles. However, even this approach becomes intractable on a classical computer for more than 10 to 100 particles. An alternative strategy, originally proposed by Richard Feynman, is to use another quantum system to 'simulate' the model or Hamiltonian describing the system of interest. Developing such 'quantum simulators' has become a major theme of research, as they have the potential to change the way we understand new materials and could ultimately impact on future devices and technologies of benefit to all of society.The development of laser cooling has allowed us to cool atomic gases to temperatures less than a millionth of a degree above absolute zero where the quantum mechanical nature of particles dominates over their thermal motion. In this regime new states of matter emerge in the form of Bose-Einstein condensates and Fermi-degenerate gases. Such gases are highly controllable and offer a promising platform to implement quantum simulation protocols. In particular, ultracold heteronuclear molecules possess the controllable long-range interactions needed to engineer an important class of problems relevant to condensed-matter physics. Moreover, the crystalline structures of real materials can easily be replicated using standing waves of laser light to confine the molecules in optical lattices.The laser cooling and trapping techniques developed for atoms do not generally work, however, for molecules due to their complex internal rotational and vibrational structure. Nevertheless, they can still be exploited by carefully assembling ultracold molecules from ultracold atoms. This approach has proved remarkably successful, with a number of different molecules having been created. The technique uses two distinct steps to associate the molecules. First, weakly bound molecules are formed using a collision resonance, known as a Feshbach resonance, which couples the free atoms into a near threshold molecular state. Secondly, the molecules are transferred to the absolute ground state using a two-photon optical transfer process, known as stimulated Raman adiabatic passage (STIRAP). Remarkably, the overall conversion process can be highly efficient with negligible heating so that the temperature and density of the resulting molecular quantum gas mirror the initial parameters of the atomic mixture. The goal of this proposal is to realise a gas of ultracold fermionic KCs molecules by associating pre-cooled atoms of K and Cs. This molecule has the advantage over other bi-alkali molecules of being stable against reactive collisions and offers both fermionic and bosonic isotopes. By confining the molecules in an array of two-dimensional pancake traps we will deliver a test platform for quantum simulation applications. To achieve this ambitious objective we propose to combine state-of-the-art experiments in synergy with world leading theoretical support into a transformative program of research that stands to cement the UK's position at the forefront of an exciting international field.