Tailored Single-Atom Collisions at Ultralow Energies.

We employ collisions of individual atomic cesium (Cs) impurities with an ultracold rubidium (Rb) gas to probe atomic interaction with hyperfine- and Zeeman-state sensitivity. Controlling the Rb bath's internal state yields access to novel phenomena observed in interatomic spin exchange. These can be tailored at ultralow energies, owing to the excellent experimental control over all relevant energy scales. First, detecting spin-exchange dynamics in the Cs hyperfine-state manifold, we resolve a series of previously unreported Feshbach resonances at magnetic fields below 300 mG, separated by energies as low as h×15 kHz. The series originates from a coupling to molecular states with binding energies below h×1 kHz and wave function extensions in the micrometer range. Second, at magnetic fields below ≈100 mG, we observe the emergence of a new reaction path for alkali atoms, where in a single, direct collision between two atoms two quanta of angular momentum can be transferred. This path originates from the hyperfine analog of dipolar spin-spin relaxation. Our work yields control of subtle ultralow-energy features of atomic collision dynamics, opening new routes for advanced state-to-state chemistry, for controlling spin exchange in quantum many-body systems for solid-state simulations, or for determination of high-precision molecular potentials.

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

We report on spin dynamics of individual, localized neutral impurities immersed in a Bose-Einstein condensate. Single cesium atoms are transported into a cloud of rubidium atoms and thermalize with the bath, and the ensuing spin exchange between localized impurities with quasispin F_{i}=3 and bath atoms with F_{b}=1 is resolved. Comparing our data to numerical simulations of spin dynamics, we find that, for gas densities in the Bose-Einstein condensate regime, the dynamics is dominated by the condensed fraction of the cloud. We spatially resolve the density overlap of impurities and gas by the spin population of impurities. Finally, we trace the coherence of impurities prepared in a coherent superposition of internal states when coupled to a gas of different densities. For our choice of states, we show that, despite high bath densities and, thus, fast thermalization rates, the impurity coherence is not affected by the bath, realizing a regime of sympathetic cooling while maintaining internal state coherence. Our work paves the way toward the nondestructive probing of quantum many-body systems via localized impurities.

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.

Extreme event statistics in a drifting Markov chain.

We analyze extreme event statistics of experimentally realized Markov chains with various drifts. Our Markov chains are individual trajectories of a single atom diffusing in a one-dimensional periodic potential. Based on more than 500 individual atomic traces we verify the applicability of the Sparre Andersen theorem to our system despite the presence of a drift. We present detailed analysis of four different rare-event statistics for our system: the distributions of extreme values, of record values, of extreme value occurrence in the chain, and of the number of records in the chain. We observe that, for our data, the shape of the extreme event distributions is dominated by the underlying exponential distance distribution extracted from the atomic traces. Furthermore, we find that even small drifts influence the statistics of extreme events and record values, which is supported by numerical simulations, and we identify cases in which the drift can be determined without information about the underlying random variable distributions. Our results facilitate the use of extreme event statistics as a signal for small drifts in correlated trajectories.