Charged Polarons and Molecules in a BEC
Recent theoretical paper in Physical Review Letters by CCQ’s Esben Rohan Christensen and Georg M. Bruun, and former CCQ assistant professor Arturo Camacho-Guardian, who answer the question of what happens when you put an ion into a BEC.
A real CCQ project, this theoretical study of an ion immersed in a Bose-Einstein condensate (BEC) is inspired by recent experimental advances in producing atomic BECs, beautifully exemplified in the CCQ group of Jan Arlt, and trapping of single ions, which the CCQ group of Michael Drewsen has great expertise in. The natural question is: What happens if two such experimental groups join forces and put a single ion into a BEC? Already many experimental groups around the world are working on this, and the authors of the present Letter have provided an answer from a theoretical point of view to this question.
Atom-ion hybrid systems are very important to get a fundamental understanding of because these are an exciting frontier for the realization of quantum simulators. Such simulators can extract the important physics of quantum systems that no super computer or ordinary experiment would be able to model. Specifically, understanding the system of an ion in a BEC is an interesting first step in this direction and has implications for e.g. controlling, cooling and transporting qubits in a future quantum simulator.
The charge of the ionic impurity gives rise to a long-range interaction between the ion and the atoms of the BEC which is more complicated to handle than in the neutral impurity case. The authors have had to make non-trivial extensions of the tried-and-tested methods and combined several of these to paint a picture of an interesting interplay between few- and many-body physics. They find the existence of several polaronic and molecular states due to the binding of an increasing number of bosons to the ion, which is well beyond what can be described by a short-range pseudopotential used in the neutral case.
Furthermore, they characterize the dynamics of these states and estimate that the many-body effects should be observable in state of the art experiments. This should be interesting to follow, especially if such an experiment is undertaken in CCQ. In the meantime, the theoretical group of Georg Bruun is attempting to describe what happens if there are multiple ions in the BEC. In this case, sound modes of the BEC can mediate interactions between ions that give rise to some interesting physics. Overall, the present Letter is an important benchmark and valuable guide for future theoretical- and experimental studies on hybrid ion-atom systems.