BaRbIE project

An ion in a sea of ultracold atoms

Atom-ion hybrid setup

Within the BaRbIE project we have an hybrid atom-ion setup to investigate fundamental interactions between single trapped ions (either Ba+ or Rb+) and an ensemble of ultracold neutral atoms (Rb). The setup allows for investigating a multitude of fascinating phenomena on the quantum level.


Overview of the vacuum system. Initially, 87Rb atoms are laser-cooled and confined in a magneto-optical trap (MOT). From the MOT chamber these atoms are then magnetically transferred to the BEC chamber. Here, we produce an ultracold thermal cloud or a Bose-Einstein condensate (BEC) via forced evaporative cooling. Afterwards, the atom cloud is transported to the science chamber via an optical standing wave.  where the atoms are confined in a crossed optical dipole trap. In addition, an ion trap (linear paul trap) is located in the science chamber. It can be loaded either with single Ba+- or Rb+-ions. The trapping potentials for the atom cloud and the ion are finally overlaid such that collisions take place.

Combined atom-ion trap. Shown are the electrodes of the Paul trap and the laser beams of the optical dipole trap. If the trap centers are overlaid, the ion is immersed into the atom cloud as can be seen in the enlargement.

Current and planned research projects

  • State-to-state chemistry: Investigation of reactive (or inelastic) processes with full quantum state resolution of both the reactants and the products.
  • s-wave collisions: As the ion features driven micromotion in the Paul trap, its temperature is limited to about 1 mK in the current setup. Thus, we plan to implement an optical dipole trap in order to get to lower temperatures which will allow us to study atom-ion s-wave collisions.
  • Ion-Rydberg atom interactions: Rydberg atoms have characteristic properties like e.g. a large polarizability as compared to ground state atoms. Therefore, Rydberg atoms are interesting objects also for collision experiments with ions. We will study the control of atom-ion collisions by Rydberg dressing and search for novel kinds of atom-ion bound states.
  • Light-assisted collisions: We investigate light-assisted reactive and inelastic processes. One goal here is to use light in order to produce diatomic or triatomic molecules or molecular ions on demand via photoassociation.
  • Thermometry: A system for sideband cooling of ions can be used to determine the temperature of ions. This will serve as a tool to study atom-ion collisions.
  • Swap cooling: Cooling of an ion due to a charge exchange reaction with an ultracold atom.
  • Polaron physics: Study polaronic interactions of a charged impurity in a BEC.
  • Quantum information processing: Use hybrid atom-ion system for quantum information processing (e.g. quantum memories and entanglement gates).


Research highlights

State-to-state chemistry for three-body recombination

J. Wolf et al. Science 358, 921 (2017) and A. Härter et al. Nature Physics 9, 512 (2013)

We have developed a novel method to probe diatomic molecular product states after reaction. For this we make use of the hybrid atom ion setup. Once formed, the molecules are ionized via resonance-enhanced multi-photon ionization (REMPI) and subsequently captured in the ion trap where the molecular ions are detected with almost 100% efficiency. We have investigated three-body recombination of three neutral Rb atoms and measured the population distribution in absolute terms with a resolution down to the hyperine levels. From our experimental data it has been possible to extract propensity rules for the reaction process.

Figure: Measured rate constants due to three-body recombination into various molecular product channels as specified by the vibrational (v) and rotational (R) quantum numbers and the respective binding energy Eb. R is indicated next to the data points.

Reactive two-body and three-body collisons of Ba+ with Rb

A. Krükow et al. Phys. Rev. A 94, 030701(R) (2016) and A. Krükow et al. Phys. Rev. Lett. 116, 193201 (2016)

By mapping out the Ba+ loss rate dependence on the Rb atom density we have measured the rate coefficients for both two-body and three-body reactive collisons. Furthermore, we have determined the energy scaling of the three-body rate coefficient.

Figure: Density dependence of Ba+ loss in a Rb cloud: The plot shows the probability to detect the Ba+-ion after the interaction time t with a Rb cloud for ten different initial atom peak densities.

A single ion as a three-body reaction center

A. Härter et al. Phys. Rev. Lett. 100, 123201 (2012)

We have investigated the recombination process of a single Rb+-ion with two neutral Rb atoms and determined the three-body rate coefficient which is by a factor of 1000 larger than the rate coefficient for the recombination of three neutral Rb atoms.

Figure: Illustration of atom-atom-ion collision: (I) Two atoms simultaneously enter the interaction radius of the ion and a three-body process takes place. (II) The three-body reaction ejects the ion onto a trajectory much larger than the atom cloud.

Dynamics of a cold trapped ion in a BEC

S. Schmid et al. Phys. Rev. Lett. 105, 133202 (2010)

After setting up the hybrid atom-ion apparatus we performed first measurements with a Ba+-ion (respectively Rb+-ion) immersed in a Rb BEC. We have observed elastic as well as inelastic processes and we have demonstrated that a single ion can be used to probe the density profile of an ultracold atom cloud.

Figure: Ion as a probe for atom cloud density. Shown is the number of Rb atoms remaining in the trap depending on the position of the Rb+-ion relative to the center of the atom cloud. The measurements are performed with a thermal cloud (a), a partially condensed cloud (b) and an almost pure BEC (c).


We gratefully acknowledge funding through

  • Deutsche Forschungsgemeinschaft DFG (e.g. Priority Program GiRyd)
  • Center for Integrated Quantum Science and Technology IQST


The team

Front row from left to right: Joschka Wolf, Amir Mahdian, Amir Mohammadi

Back row from left to right: Markus Deiß, Artjom Krükow, Johannes Hecker Denschlag