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Prof. Dr. Michael Köhl

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Ultracold atoms & Bose-Einstein condensation


Bose-Einstein condensation is a fascinating demonstration of the quantum character of matter. Weakly interacting particles populate the motional ground state and establish a macroscopic wave function. Its experimental realization in 1995 (Nobel prize 2001) sparked an ongoing vivid experimental and theoretical research on this novel quantum state. Bose-Einstein condensates give experimental access to weakly interacting quantum gases. Superfluid properties - otherwise known only from (strongly interacting) superfluid helium - can be studied. Last but least, Bose-Einstein condensates give access to precise studies of phase transitions. Due to their exceptional experimental controllabilty the order parameter and its correlation functions can be measured directly across the phase transition both in and out of equilibrium.


Decoherence of a Single-Ion Qubit Immersed in a Spin-Polarized Atomic Bath

Phys. Rev. Lett. 110, 160402 (2013).

spinsWe report on the immersion of a spin qubit encoded in a single trapped ion into a spin-polarized neutral atom environment, which possesses both continuous (motional) and discrete (spin) degrees of freedom. The environment offers the possibility of a precise microscopic description, which allows us to understand dynamics and decoherence from first principles. We observe the spin dynamics of the qubit and measure the decoherence times (T1 and T2), which are determined by the spin-exchange interaction as well as by an unexpectedly strong spin-nonconserving coupling mechanism.


Controlling chemical reactions of a single particle

Nature Physics 8, 649 (2012).

collisionsTraditionally, chemical reactions have been investigated by tuning thermodynamic parameters, such as temperature or pressure. More recently, laser or magnetic field control methods have emerged to provide new experimental possibilities, in particular in the realm of cold collisions. The control of reaction pathways is also a critical component to implement molecular quantum information processing. For these studies, single particles provide a clean and well-controlled experimental system. Here, we report on the experimental tuning of the exchange reaction rates of a single trapped ion with ultracold neutral atoms by exerting control over both their quantum states.We observe the influence of the hyperfine interaction on chemical reaction rates and branching ratios, and monitor the kinematics of the reaction products. These investigations advance chemistry with single trapped particles towards achieving quantum-limited control of chemical reactions and indicate limits for buffer-gas cooling of single-ion clocks.


A trapped single ion inside a Bose–Einstein condensate

Nature 464, 388 (2010).

Hybrid trapImproved control of the motional and internal quantum states of ultracold neutral atoms and ions has opened intriguing possibilities for quantum simulation and quantum computation. Many-body effects have been explored with hundreds of thousands of quantum degenerate neutral atoms, and coherent light–matter interfaces have been built. Systems of single or a few trapped ions have been used to demonstrate universal quantum computing algorithms and to search for variations of fundamental constants in precision atomic clocks. Until now, atomic quantum gases and single trapped ions have been treated separately in experiments. Here we investigate whether they canbe advantageously combined into one hybrid system,by exploring the immersion of a single trapped ion into a Bose–Einstein condensate of neutral atoms. We demonstrate independent control over the two components of the hybrid system, study the fundamental interaction processes and observe sympathetic cooling of the single ion by the condensate. Our experiment calls for further research into the possibility of using this technique for the continuous cooling of quantum computers. We also anticipate that it will lead to explorations of entanglement in hybrid quantum systems and to fundamental studies of the decoherence of a single, locally controlled impurity particle coupled to a quantum environment.

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