# Synopsis: Heteronuclear tango

Weakly bound three-body states have now been observed in mixtures of two different ultracold atoms.

In the 1970s, Vitaly Efimov theoretically showed that a wide range of systems from nucleons to atoms could exhibit weakly bound three-body states. Thirty-five years later, these Efimov states were experimentally observed in ultracold cesium atoms where the three bound atoms were all the same. Now, writing in Physical Review Letters, Giovanni Barontini and colleagues at the University of Florence, Italy, and the University of Bonn, Germany, report their observations of Efimov states in ultracold gases containing a mixture of potassium and rubidium, thus extending the experimental envelope to heteronuclear systems.

The first experimental observation of Efimov effects relied on tuning the interatomic interaction with a Feshbach resonace, where the influence of an adjustable external magnetic field alters the energy levels to create a three-body bound state during a collision. To detect the heteronuclear Efimov states, Barontini et al. cooled a mixture of ${}^{41}\text{K}$ and ${}^{87}\text{Rb}$ to a few hundred nanokelvin in an optical dipole trap. After releasing the atoms from the trap, they followed the total number of atoms to track losses by three-body collisions. At specific magnetic field strengths, they observed dips in the atom count showing the resonant formation of bound states. This occured at two field values, one for $\text{K-K-Rb}$ states and another for $\text{K-Rb-Rb}$ states.

Extending Efimov physics to mixtures of dissimilar atoms may help with the observation of three-body states in a much more difficult setting: nuclear physics. This is the context in which Efimov carried out his original theoretical studies, and the results in atomic systems may be the first step toward observing Efimov physics in nuclei. – David Voss

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