A single impurity in a Bose-Einstein condensate

接近绝对零度时，粒子几乎静止不动。这一机制为多体系统的量子本质打开了一扇窗。在原子气体中达到零温度的路线利用激光的机械效应来减慢原子，然后在磁阱中蒸发冷却原子。在达到100纳开尔文及以下的温度时，实验努力得到了最终可控性和微观理解的量子多体系统的回报。仅在10年前才首次在实验上达到这种制度。可以看到中性原子的稀释气体如何凝聚成单个量子态，表现出一种宏观量子力学波函数的行为。玻色-爱因斯坦凝聚的这一成就引发了量子简并气体的研究热潮。最初的实验强调了它的无摩擦流动，并证明了原子激光束的产生，强烈和高度定向的物质束。目前的研究探索量子相变，可调原子相互作用，粒子之间的相关效应。在不久的将来，可能会看到开放问题的答案和新的效果。例如，有人提出，高温超导的物理基础可以用冷原子气体来模拟。为了达到这些目标，需要新的探针，允许在几纳米的长度尺度上对量子多体系统进行局部调查和操纵。与玻色-爱因斯坦凝聚研究的进展相平行，对单个囚禁离子的研究大大有助于我们理解少数粒子的量子力学行为。特别是，通用的量子计算算法已被证明与捕获的离子，从而将量子力学的特殊功能，如量子纠缠，转化为实际应用。即将到来的向量子计算机的根本性技术转变有望有效处理某些经典计算机技术难以处理的计算任务。到目前为止，阻碍这些量子计算机广泛使用的一个障碍在于苛刻的技术要求。特别是在计算操作期间发生的离子的不必要的加热削弱了量子控制的水平，并且经常地计算必须暂停以进行冷却。冷却本身是一个非常苛刻的操作，需要国家的最先进的激光系统和精确的控制的电磁场捕获的ions.In可能同时解决这两个讨论的问题，我们建议沉浸在超冷玻色爱因斯坦凝聚的捕获离子。冷中性原子与被囚禁离子的相互作用提供了一系列有趣的物理问题。例如，带电离子极化中性量子气体，导致中性原子被捕获到束缚态，并产生数百个粒子的介观大离子复合物。这提供了一个独特的系统，其粒子数介于被困离子的少粒子世界和具有数百万原子的多粒子玻色-爱因斯坦凝聚体之间。此外，离子构成超流体内部的局部探针。询问它的量子态将以最小侵入的方式揭示有关超流体环境局部性质的信息。在超流中移动的离子的耗散和拖曳效应对超流的基本概念有影响，其基础物理与电路中的超导性密切相关。此外，超冷中性原子环境构成了离子的冰箱，并且将研究其连续冷却的可能性，为离子阱量子计算机中更广泛的计算铺平道路。

Close to absolute zero temperature particles almost come to a standstill. This regime opens a window on the quantum nature of many-body systems. The route towards zero temperature in a gas of atoms exploits the mechanical effect of laser light to slow down atoms, followed by evaporative cooling of the atoms in a magnetic trap. Upon reaching temperatures of 100 Nanokelvin and below, the experimental efforts are rewarded with a quantum many-body system of ultimate controllability and access to microscopic understanding. Experimentally this regime has been reached for the first time only ten years ago. It could be witnessed how a dilute gas of neutral atoms condensed into a single quantum state displaying the behaviour of one macroscopic quantum mechanical wave function. This achievement of Bose-Einstein condensation has initiated a wave of research on quantum degenerate gases. Initial experiments highlighted its frictionless flow and demonstrated the production of atom laser beams, intense and highly directional beams of matter. Current investigations explore quantum phase transitions, tunable atomic interactions, and correlation effects between the particles. In the near future, answers to open questions and new effects will probably be seen. For example, it has been proposed that the physics underlying high-temperature superconductivity may be mimicked using cold atomic gases. To reach these goals new probes are needed which allow for a local investigation and manipulation of the quantum many-body systems on a length scale of a few nanometers. Parallel to the progress in research on Bose-Einstein condensates, the investigation of single trapped ions has contributed significantly to our understanding of the quantum mechanical behaviour of few particles. In particular, universal quantum computing algorithms have been demonstrated with trapped ions thereby transforming the peculiar features of quantum mechanics, such as quantum entanglement, into a practical application. The upcoming radical technological shift towards quantum computers promises efficient processing of certain computational tasks which are intractable with classical computer technology. One obstacle hindering so far the widespread use of these quantum computers lies in the demanding technical requirements. Especially the unwanted heating of the ion occurring during the computing operations weakens the level of quantum control and regularly the computation has to be paused for cooling. The cooling itself is a very demanding operation requiring state-of-the-art laser systems and a precise control of the electromagnetic fields for trapping the ions.In order to potentially solve both of the discussed problems simultaneously we propose to immerse a trapped ion into an ultracold Bose-Einstein condensate. The mutual interaction of the cold neutral atoms and the trapped ion gives access to a variety of interesting physical problems. For example, the charged ion polarizes the neutral quantum gas leading to a capture of neutral atoms into bound states and creating mesoscopically large ion complexes of several hundred particles. This provides a unique system with a particle number in between the few-particle world of trapped ions and many-particle Bose-Einstein condensates with millions of atoms. Moreover, the ion constitutes a local probe inside the superfluid. Interrogating its quantum state will reveal information about the local properties of the superfluid environment in a minimally invasive way. The effects of dissipation and drag of the ion moving through the superfluid have implications for the fundamental concept of superfluidity whose underlying physics is closely connected to superconductivity in electrical circuits. In addition, the ultracold neutral atom environment constitutes a refrigerator for ions and the possibility for its continuous cooling will be investigated paving the way for more extensive computations in ion trap quantum computers.