Tobias Lausch

Neutral impurities in a Bose-Einstein condensate for simulation of the Fröhlich-polaron

We present an experimental system to study the Bose polaron by immersion of single, well-controllable neutral Cs impurities into a Rb Bose-Einstein condensate (BEC). We show that, by proper optical traps, independent control over impurity and BEC allows for precision relative positioning of the two sub-systems as well as for dynamical studies and independent read-out. We furthermore estimate that measuring the polaron binding energy of Fröhlich-type Bose polarons in the low and intermediate coupling regime is feasible with our experimental constraints and limitations discussed, and we outline how a parameter regime can be reached to characterize differences between Fröhlich and Bose-polaron in the strong coupling regime.

Individual Tracer Atoms in an Ultracold Dilute Gas

We report on the experimental investigation of individual Cs atoms impinging on a dilute cloud of ultracold Rb atoms with variable density. We study the relaxation of the initial nonthermal state and detect the effect of single collisions which has so far eluded observation. We show that, after few collisions, the measured spatial distribution of the tracer atoms is correctly described by a Langevin equation with a velocity-dependent friction coefficient, over a large range of Knudsen numbers. Our results extend the simple and effective Langevin treatment to the realm of light particles in dilute gases. The experimental technique developed opens up the microscopic exploration of a novel regime of diffusion at the level of individual collisions.

Nonergodic diffusion of single atoms in a periodic potential

Drawing microscopic information out of the diffusive dynamics of complex processes often requires an assumption of ergodicity. Precision experiments on a single atom in a periodic potential suggest that this may be too simplistic in many cases. Diffusion can be used to infer the microscopic features of a system from the observation of its macroscopic dynamics. Brownian motion accurately describes many diffusive systems, but non-Brownian and nonergodic features are often observed on short timescales. Here, we trap a single ultracold caesium atom in a periodic potential and measure its diffusion1,2,3. We engineer the particle–environment interaction to fully control motion over a broad range of diffusion constants and timescales. We use a powerful stroboscopic imaging method to detect single-particle trajectories and analyse both non-equilibrium diffusion properties and the approach to ergodicity4. Whereas the variance and two-time correlation function exhibit apparent Brownian motion at all times, higher-order correlations reveal strong non-Brownian behaviour. We additionally observe the slow convergence of the exponential displacement distribution to a Gaussian and—unexpectedly—a much slower approach to ergodicity5, in perfect agreement with an analytical continuous-time random-walk model6,7,8. Our experimental system offers an ideal testbed for the detailed investigation of complex diffusion processes.

Precision measurement of theRb87tune-out wavelength in the hyperfine ground stateF=1at 790 nm

We report on a precision measurement of the

Extreme event statistics in a drifting Markov chain

D

Optimizing quantum gas production by an evolutionary algorithm

line tune-out wavelength of

A Single Atom Thermometer for Ultracold Gases

^{87}

Quantum Spin Dynamics of Individual Neutral Impurities Coupled to a Bose-Einstein Condensate

Rubidium in the hyperfine ground state

Tailored single-atom collisions at ultra-low energies

|F=1, m_F=0,\pm1 \rangle

Controlled doping of a bosonic quantum gas with single neutral atoms

manifold at 790 nm, where the scalar ac Stark shifts of the