PACS numbers:Improved 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 1 and coherent light-matter interfaces have been built 2,3 . Systems of single or a few trapped ions have been used to demonstrate universal quantum computing algorithms 4 and to detect variations of fundamental constants in precision atomic clocks 5 .Until now, atomic quantum gases and single trapped ions have been treated separately in experiments. Here we investigate whether they can be 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 within 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 6 . 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 7,8 .Hybrid systems allow the exploration of physics much more advanced than that which can be studied using the component systems alone. In particular, the immersion of distinguishable particles into a quantum liquid has contributed significantly to our understanding of many-body systems. In liquid helium, for example, vortex lattices have been observed using charged particles as markers for the vortex lines 9 . Moreover, in conventional 10,11 and high-Tc 12 superconductors single impurity atoms lead to quasiparticle excitations which profoundly affect the superconducting properties. In future investigations of distinct single particles in combination with quantum matter, it will be important to have a high degree of control over these particles. In this regard, the excellent control possible over the external and internal degrees of freedom of a single ion trapped in a Paul trap is highly promising. Immersed into a quantum gas, a single trapped ion not only acts as a probe but could also be used for local manipulation. Numerous applications of this hybrid system have been foreseen, including sympathetic cooling 13 , the nucleation of localized density fluctuations in a Bose gas [14][15][16] , scanning probe microscopy with previously unattainable spatial resolution 17,18 , and hybrid atom-ion quantum processors 19 . The majority of these proposals are based on there being a strong collisional interaction between the ion and the neutral atoms.The interaction between a single ion and a neutral atom is dominated by the polarization potential. Asymptotically this behaves as 20 V (r) = − 1 (4πǫ0) 2 αq 2 2r 4 and therefore decays more slowly than...

We investigate the propagation of spin impurity atoms through a strongly interacting one-dimensional Bose gas. The initially well localized impurities are accelerated by a constant force, very much analogous to electrons subject to a bias voltage, and propagate as a one-dimensional impurity spin wave packet. We follow the motion of the impurities in situ and characterize the interaction induced dynamics. We observe a very complex nonequilibrium dynamics, including the emergence of large density fluctuations in the remaining Bose gas, and multiple scattering events leading to dissipation of the impurity's motion. PACS numbers: 03.75.Pp,05.60.Gg, 37.10.Jk, The impetus for miniaturization has resulted in the creation of nanostructures in which the motion of particles is purely one-dimensional. In these systems motional degrees of freedom can be excited only along one direction whereas in the two orthogonal directions the system occupies the quantum mechanical ground state. To reach the one-dimensional regime the chemical potential and the temperature need to be much smaller than the transverse level spacing. Interacting particles confined to a one-dimensional wave guide are fundamentally governed by many-body quantum mechanics [1]. In non-equilibrium situations this gives rise to genuine quantum dynamics, examples of which have been seen in single mode nanowires [2] and in atom traps [3,4,5,6]. In this paper, we study the non-equilibrium transport of single or few impurity particles through a one-dimensional, strongly interacting Bose gas. The impurities are accelerated by a constant force, very much analogous to electrons subject to a bias voltage, and undergo scattering with the atoms in the Tonks-Girardeau gas.An interacting one-dimensional Bose gas realizes a bosonic Luttinger liquid. Its many-body quantum state in the homogeneous case is characterized by a single parameter γ = mg1D 2 n1D [7,8]. Here m is the atomic mass, g 1D is the 1D coupling constant, and n 1D is the 1D density. For weak interactions (γ ≪ 1) Bose-Einstein condensation and superfluidity are possible in harmonically confined 1D systems. For strong interactions (γ ≫ 1) the longitudinal motion of the particles is highly correlated. In this so-called Tonks-Girardeau regime the Bose gas "fermionizes", i.e. its N -particle wave function can be related to that of a N -particle spin-polarized Fermi gas [9,10,11]. The density of the Bose gas as well as the density dependent correlation functions become Fermion-like and superfluidity vanishes [12,13].Both the weakly [3] and the strongly interacting [14, 15] regimes of one-dimensional Bose gases have been accessed a few years ago. This experimental realization of the onedimensional Bose gas with δ-functional interactions has triggered significant research efforts both experimentally and theoretically [16]. Of particular interest have been dynamical experiments [3,4,5,6]. Elementary transport experiments have seen the suppression of dipole oscillations in a corrugated potential [5] and the absence of th...

. 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.The full control over all quantum mechanical degrees of freedom of a chemical reaction allows the identification of fundamental interaction processes and the steering of chemical reactions. This task is often complicated in heteronuclear systems by a multitude of possible reaction channels, which make theoretical treatments very challenging. Therefore, focussing on the best-controlled experimental conditions, such as using state-selected single particles and low temperatures, is crucial for the investigation of chemical processes at the most elementary level. The hybrid system of trapped atoms and ions offers key advantages in this undertaking. On the one hand, ion traps offer a large potential well depth to trap the reaction products for precision manipulation and investigation. On the other hand, contrary to pure ionic systems, there is no Coulomb barrier between the particles to fundamentally prevent chemical reactions at low temperatures. Therefore, the efforts to control the motional degrees of freedom of one 4-6 and both 7-13 reactants in hybrid atom-ion systems have paved new ways towards cold chemistry. The yet missing component is the simultaneous control of the internal degrees of freedom.The interaction between an ion and a neutral atom at long distances is dominated by the attractive polarization interaction potential V (r), which is of the formHere, C 4 = α 0 q 2 /(4π 0 ) 2 is proportional to the neutral particle polarizability α 0 , q is the charge of the ion, 0 is the vacuum permittivity, and r is the internuclear separation. Inelastic collisions take place at short internuclear distances. In the cold, semiclassical regime this requires collision energies above the centrifugal barrier [14][15][16] . Such processes are referred to as Langevin-type collisions and happen, even for cold collisions 7,9 , at an energyindependent rate γ Langevin = 2π √ C 4 /µ n a . Here µ is the reduced mass of the collision partners and n a is the neutral atom density.Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, UK. *e-mail: cs540@cam.ac.uk. More subtle effects, such as the hyperfine interaction, which may lead to atom-ion Feshbach resonances, are not included in the polarization potential and have been investigated only theoretically so far 17 . Experimentally, reactive Langevin collisions in the polarization potential have been investigated in ground state collisi...

We study cold heteronuclear atom-ion collisions by immersing a trapped single ion into an ultracold atomic cloud. Using ultracold atoms as reaction targets, our measurement is sensitive to elastic collisions with extremely small energy transfer. The observed energy-dependent elastic atom-ion scattering rate deviates significantly from the prediction of Langevin but is in full agreement with the quantum mechanical cross section. Additionally, we characterize inelastic collisions leading to chemical reactions at the single particle level and measure the energy-dependent reaction rate constants. The reaction products are identified by in-trap mass spectrometry, revealing the branching ratio between radiative and nonradiative charge exchange processes.

The immersion of a single ion confined by a radiofrequency trap in an ultracold atomic gas extends the concept of buffer gas cooling to a new temperature regime. The steady state energy distribution of the ion is determined by its kinetics in the radiofrequency field rather than the temperature of the buffer gas. Moreover, the finite size of the ultracold gas facilitates the observation of backaction of the ion onto the buffer gas. We numerically investigate the system's properties depending on atom-ion mass ratio, trap geometry, differential cross-section, and non-uniform neutral atom density distribution. Experimental results are well reproduced by our model considering only elastic collisions. We identify excess micromotion to set the typical scale for the ion energy statistics and explore the applicability of the mobility collision cross-section to the ultracold regime.