A single impurity in a Bose-Einstein condensate

点击翻译按钮获取中文摘要

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.