Engineering tunable atom-atom interactions in a quantum gas

HERBERT WALTHER

Alexander Guthmann, Felix Lang, Louisa Marie Kienesberger +2 more

Scattering resonances are fundamental to many areas of physics, occurring across a wide range of energy scales. In ultracold quantum gases, magnetic Feshbach resonances have transformed quantum many-body research by enabling precise interaction control between atoms. Here, we demonstrate unprecedented control to engineer Feshbach resonances at tunable positions via Floquet driving in a lithium-6 (^{6}Li) atom gas, achieved through strong magnetic field modulation at megacycles per second frequencies. This periodic modulation creates scattering resonances whenever dressed molecular levels cross the atomic threshold. By adding a second modulation at twice the base frequency, we tune the asymmetry of resonance loss profiles and suppress two-body losses from Floquet heating. This technique enhances control over atomic interactions, expanding possibilities for quantum simulations of complex systems and studies of exotic quantum phases.

D. Giambastiani, M. Barsanti, M. L. Chiofalo

We explore the evolution of an ultracold quantum gas of interacting fermions crossing from a Bardeen-Cooper-Schrieffer (BCS) superfluidity to a Bose-Einstein condensation (BEC) of molecular bosons in the presence of a tunable-range interaction among the fermions and of an artificial magnetic field, which can be used to simulate a pseudo-spin-orbit coupling (SOC) and to produce topological states. We find that the crossover is affected by a competition between the finite range of the interaction and the SOC and that the threshold for the topological transition is affected by the interactions only in the small pair size, BEC-like, regime. Below , we find persistence of universal behavior in the critical temperature, chemical potential, and condensate fraction, provided that the pair correlation length is used as a driving parameter. Above threshold, universality is lost in the regime of large pair sizes. Here, the limiting ground state departs from a weakly interacting BCS-like one so that a different description is required. Our results can be relevant in view of current experiments with cold atoms in optical cavities, where tunable-range effective atomic interactions can be engineered.

Carl J. Williams, Eite Tiesinga, Éric Charron +2 more

Kevin E. Strecker, Guthrie B. Partridge, Andrew G. Truscott +1 more

Leon Mixa, Milan Radonjić, Axel Pelster +1 more

We investigate a dilute Bose gas with both a short-range contact and an effective long-range interaction between the atoms. The latter is induced by the strong coupling to a cavity light mode and is spatially characterized by a periodic signature and a tunable envelope rooted in the pumping of the cavity. We formulate a Bogoliubov theory based on a homogeneous mean-field description and quantum fluctuations around it. The competition between the repulsive contact interaction and the long-range interaction allows the formation of self-bound quantum droplets. This generic approach is applied to two cavity setups, one without and one with a momentum-conserving effective long-range interaction between the atoms in the form of a driven dispersive cavity mode and a multimode cavity, respectively. For both cases we show analytically how the size and the central density of the cavity-induced quantum droplets depend on the contact interaction strength and on the shape of the spatial envelope of the long-range interaction.