A Quantum Gas of Ultracold Polar Molecules

激光冷却的出现彻底改变了原子物理学，并促成了量子简并气体的实现，在量子简并气体中，粒子的量子力学性质支配着它们的经典行为。这些稀释的原子气体，以玻色-爱因斯坦凝聚物（BEC）和费米简并气体的形式存在，被证明是惊人的丰富，现在在世界范围内进行常规研究。最近，追求创造超冷和量子简并分子样品已经成为原子和分子物理学以及物理化学社区的首要兴趣。分子丰富的内部结构加上超冷系统提供的卓越控制，为从精密测量和高分辨率光谱到超冷化学和量子信息处理等领域提供了巨大的应用范围。也许最吸引人的是产生异核分子的超冷量子气体的可能性，在这种气体中，远距离的、各向异性的偶极子-偶极子相互作用被预测会产生丰富的新量子相谱。然而，由于分子内部复杂的旋转和振动结构，作为原子物理学惊人实验进展核心的激光冷却技术并不适用于分子。这促使人们开发了许多制造超冷分子气体的替代方法，这些方法都依赖于在室温下冷却已有的分子。然而，这一提议遵循了另一种方案，该方案利用激光冷却和原子气体捕获的巨大进步，通过从超冷原子中仔细组装超冷分子。从Rb和Cs的超冷混合量子气体开始，目标是通过两步转换过程在旋转振动基态中创建超冷rbc分子。第一步依赖于超冷原子碰撞中散射共振的存在，这种共振是由自由原子和准束缚分子状态之间的耦合产生的，称为费什巴赫共振。在费什巴赫共振附近简单地应用适当的磁场坡道，可以高效地将原子转化为分子，同时保持原始原子样品的相空间密度。然而，这样的fishbach分子是非常脆弱的；它们以非常弱的束缚态存在，接近解离阈值，它们在相互碰撞时通常不稳定。第二步的挑战是在不加热样品的情况下将这些分子转移到碰撞稳定的基态。这可以通过一个被称为受激拉曼绝热通道（STIRAP）的过程来实现，在这个过程中，两个激光场被应用到分子上，通过第三个激发态将初始弱束缚态连接到基态。值得注意的是，在适当的时变激光脉冲下，STIRAP过程允许分子相干转移到基态，而不填充激发态，从而消除了自发衰变造成损失的可能性。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。在红细胞的情况下，预测使用单个STIRAP阶段可以产生旋转振动基态分子，创造稳定的玻色子分子偶极量子气体，可以被捕获并进一步冷却到量子简并。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 advent of laser cooling revolutionized atomic physics and precipitated the realization of quantum degenerate gases in which the quantum mechanical nature of the particles dominates over their classical behaviour. These dilute atomic gases, in the form of Bose-Einstein condensates (BEC) and Fermi-degenerate gases, have proved surprisingly rich and are now routinely studied throughout the world. More recently, the quest for the creation of ultracold and quantum degenerate molecular samples has become of paramount interest to both the atomic and molecular physics and physical chemistry communities. The rich internal structure of molecules coupled with the remarkable control afforded by ultracold systems offers enormous scope for applications in fields ranging from precision measurement and high-resolution spectroscopy to ultracold chemistry and quantum information processing. Perhaps most intriguing of all is the possibility to produce ultracold quantum gases of heteronuclear molecules where the long-range, anisotropic dipole-dipole interaction is predicted to give rise to a rich spectrum of novel quantum phases.The laser cooling techniques at the heart of the spectacular experimental advances in atomic physics do not, however, work for molecules due to their complex internal rotational and vibrational structure. This has prompted a host of alternative approaches to create ultracold molecular gases to be developed which all rely on cooling pre-existing molecules from room temperature. This proposal, however, follows an alternative scheme which exploits the huge advances in laser cooling and trapping of atomic gases by carefully assembling ultracold molecules from ultracold atoms. Starting from an ultracold mixed species quantum gas of Rb and Cs, the objective is to create ultracold RbCs molecules in the rovibrational ground state following a two-step conversion process.The first step relies upon the existence of scattering resonances in the collisions between ultracold atoms that result from a coupling between the free atoms and a quasibound molecular state known as a Feshbach resonance. The simple application of an appropriate magnetic field ramp in the vicinity of a Feshbach resonance results in the highly efficient conversion of atoms to molecules whilst preserving the phase-space density of the original atomic sample. However, such Feshbach molecules are extremely fragile; existing in very weakly bound states close to the dissociation threshold they are generally unstable when colliding with each other. The challenge of the second step is to transfer these molecules to the collisionally stable ground state without heating the sample. This can be achieved using a process known as stimulated Raman adiabatic passage (STIRAP) in which two laser fields are applied to the molecule connecting the initial weakly bound state to the ground state via a third excited state. Remarkably, with the appropriate time-dependent laser pulses, the STIRAP process permits the coherent transfer of the molecules to the ground state without populating the excited state thereby removing the possibility of loss due to spontaneous decay. 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.In the case of RbCs, it is predicted that rovibrational ground state molecules can be produced using a single STIRAP stage, creating a stable bosonic molecular dipolar quantum gas which could be trapped and further cooled to quantum degeneracy. 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.