Our manuscript “Tailored single-atom collisions at ultralow temperatures” has been published in Phys. Rev. Lett. 122, 013401 (2019). (See also the press release of TUK)
A single collision between two atomic particles in a vapor constitutes the foundation for the vapor’s macroscopic behavior, such as pressure or heat conductance, but also for its quantum properties. In our work, we study such individual collisions at ultracold temperatures, going beyond the changes of the particles’ motional state, i.e. their velocity or position. Together with our colleague Prof. E. Tiemann (University of Hanover), we rather investigate how the atoms’ internal structure, encoded in their angular momentum state, behaves upon the collision with another particle, and how this outcome can be controlled. Immersing individual Cs atoms at few hundred Nanokelvin temperature into an ultracold Rb gas, we could reveal a series of very narrow so-called Feshbach-Fano resonances, where the probability of a collision between two atoms is tunable via an external magnetic field in a range of few percent of the earth’s magnetic field. By studying single collision events between pairs of atoms, we have obtained information about the origin of these resonances, which is a giant molecular halo state, whose size in the micrometer range is large as our experimental system itself, bound at a very small energy. Furthermore, we have found that the colliding atoms can exchange angular momentum not only by a single quantum, but also by two. These observations yield novel insight into atomic interaction properties and the molecular potential directly at the dissociation threshold, so far not accessed in experiments. Both phenomena reveal only at very low energies in the system, thus they require us to keep relevant experiment parameters, such as the magnetic field and the collision energy under tight control, with fluctuations a level of only few kilo Hertz. With the experiment at hand, two intriguing research directions open. First, our system helps to precisely measure interaction phenomena at the level of a single collision, advancing ultracold chemistry and spectroscopy. Second, the exceptional level of control can help to build up and tailor the behavior of ultracold quantum matter, starting from its fundamental building block, the interaction of two particles.