*Carte non disponible*

## Solitons and droplets in two-component Bose-Einstein condensates

### Leticia Tarruell (ICFO-The Institute of Photonic Sciences, Castelldefels)

Self-bound states appear in contexts as diverse as solitary waves in channels, optical solitons in non-linear media and liquid droplets. Their binding results from a balance between attractive forces, which tend to make the system collapse, and repulsive ones, which stabilize it to a finite size. This talk will present experiments on three different types of self-bound states existing in two-component Bose-Einstein condensates.

In the first series of experiments, we study dilute quantum liquid droplets: macroscopic clusters of ultra-cold atoms that are eight orders of magnitude more dilute than liquid Helium, but have similar liquid-like properties. In particular, they remain self-trapped in the absence of external confinement due to the compensation of attractive mean-field forces and an effective repulsion stemming from quantum fluctuations [1]. We experimentally observe such droplets in a mixture of potassium Bose-Einstein condensates with repulsive intrastate and attractive interstate interactions [2], and study their relation with more conventional bright solitons [3].

As a second step, we consider instead a system where each atom is placed in a coherent superposition of these two internal states. We measure the elastic and inelastic properties of the gas, and show that they can be flexibly controlled via the parameters of the coupling field [4,5]. In the attractive regime, we exploit this method to form bright solitons formed by dressed-state atoms, and to create bright soliton trains.

Finally, we explore the situation where the coupling field imparts a momentum kick to the atoms, breaking Galilean invariance and creating an artificial vector potential. For potassium condensates, the intrastate interactions have very different values and this synthetic gauge field becomes density-dependent [6]. We show that our system implements a one-dimensional Chern-Simons gauge theory that contains a chiral current term, and experimentally observe the stabilization of chiral bright solitons.

[1] D. S. Petrov, Phys. Rev. Lett. 115, 155302 (2015)

[2] C. R. Cabrera et al., Science 359, 301 (2018)

[3] P. Cheiney et al., Phys. Rev. Lett. 120, 135301 (2018)

[4] C. P. Search and P. R. Berman, Phys. Rev. A 63, 043612 (2001)

[5] D. S. Petrov, Phys. Rev. Lett. 112, 103201 (2014)

[6] M. J. Edmonds et al., Phys. Rev. Lett. 110, 085301 (2013)