Quantum solitons and cluster states with well-defined atom number

超低温下的粒子可以表现出令人惊讶的量子效应，如隧道效应和纠缠。通常情况下，单个粒子的量子效应最强，但大型多体系统的量子效应会减弱。特别是玻色子的多体系统，即允许同时占据同一状态的粒子，可以用集体波函数很好地描述，具有与经典流体相似的性质。目前，人们正在研究这一经典的类流体区域和具有单个粒子的系统之间的交叉，而观察量子效应所需的粒子数量也在激烈辩论中。该项目的目标是探测和研究这一交叉区域中的超冷原子气体的两个态--量子亮孤子和大团簇态。对于原子数在5到几百之间的相似实验参数，这两个态都有望存在，但它们从相反的方向接近交叉区域。亮孤子是无色散的波包，传播时不改变其形状，它们呈现出典型的非线性流体的性质。对于减少的粒子数，亮孤子有望获得单个量子对象的特征属性，如离散隧穿、不确定关系和纠缠。另一方面，团簇状态是由几个粒子组成的松散束缚状态，类似于分子。随着粒子数的增加，它们有望失去量子性质。该项目的目标是通过实验制备具有明确数量的超冷原子的亮孤子和团簇状态，并探索随着原子数量的变化系统的性质。这项研究将拓宽我们对少体和多体物理之间的边界的理解，并有可能促进技术应用，例如基于大和复杂量子态的新量子技术的发展。

Particles at ultra-low temperatures can show surprising quantum effects, such as tunnelling and entanglement. Typically, those quantum effects are strongest for individual particles but decrease for large many-body systems. Especially many-body systems of bosons, i.e. particles which allow for a simultaneous occupation of the same state, are well described by a collective wave function, with similar properties as a classical fluid. The crossover between this classical, fluid-like regime and systems with individual particles is currently studied, and the particle number, required to observe quantum effects, is actively debated. The goal of this project is to detect and study two states of ultracold atomic gases in this crossover regime - "quantum bright solitons" and large "cluster states". Both states are expected to exist for similar experimental parameters with between five and a few hundred atoms, but they approach the crossover regime from opposite sides.Bright solitons are dispersionless wave-packets that propagate without changing their shape, and they present a typical property of nonlinear fluids. For reduced particle number, bright solitons are expected to acquire properties which are characteristic for a single quantum object, such as discrete tunnelling, uncertainty relationships and entanglement. Cluster states on the other hand are loosely bound states of few particles, similar to molecules. They are expected to lose quantum properties with increasing particle number. The goal of the project is to experimentally prepare bright solitons and cluster states with a well-defined number of ultracold atoms, and to probe the properties of the system as the atom number is changed.The research will broaden our understanding of the boundary between few-body and many-body physics, and it has the potential to advance technical applications, e.g. with the development of new quantum technologies based on large and complex quantum states.