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Quantum liquid droplets in a mixture of Bose- Einstein condensates

Letitia Tarruell, ICFO, The Institute of Photonic Sciences, Barcelona, Spain

26.01.2018 at 09:00 

Self-bound states appear in contexts as diverse as solitary waves in channels, optical solitons in non-linear media and liquid droplets. Thei r 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 our recent experiments on 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
[D. S. Petrov, Phys. Rev. Lett. 115, 155302 (2015)].

We observe these self-bound droplets in a mixture of two Bose -Einstein condensates with attractive inter-state and repulsive intra -state interactions. Exploiting in situ imaging, we directly measure their ultra-low densities and micro-meter scaled sizes, and demonstrate the many-body origin of their stabilization mechanism. Furthermore, we observe that for small atom numbers quantum pressure is sufficient to dissociate the droplets and drive a liquid -to-gas  transition, which we map out as a function of atom number and interaction strength
[C. R. Cabrera et al., Science 14 December 2017 (10.1126/science.aao5686)].

In a second series of experiments, we study the difference existing between these liquid droplets and more conventional bright solitons. In analogy to non -linear optics, the former can be seen as one-dimensional matter-wave solitons stabilized by dispersion, whereas the latter correspond to high-dimensional solitons stabilized by a higher order non -linearity due to quantum fluctuations. We find that depending on the system parameters, solitons and droplets can be smoothly connected or remain distinct states coexisting only in a bi-stable region, and we determine experimentally its boundary
[P. Cheiney et al., arXiv:1710.11079].

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