Abstract
Self-bound quantum droplets are a newly discovered phase in the context of ultracold atoms. In this Letter, we report their experimental realization following the original proposal by Petrov [Phys. Rev. Lett. 115, 155302 (2015)PRLTAO0031-900710.1103/PhysRevLett.115.155302], using an attractive bosonic mixture. In this system, spherical droplets form due to the balance of competing attractive and repulsive forces, provided by the mean-field energy close to the collapse threshold and the first-order correction due to quantum fluctuations. Thanks to an optical levitating potential with negligible residual confinement, we observe self-bound droplets in free space, and we characterize the conditions for their formation as well as their size and composition. This work sets the stage for future studies on quantum droplets, from the measurement of their peculiar excitation spectrum to the exploration of their superfluid nature.
Highlights
Ultracold atoms are commonly known and studied in their gas-phase
Thanks to an optical levitating potential with negligible residual confinement we observe self-bound droplets in free space and we characterize the conditions for their formation as well as their equilibrium properties
This work sets the stage for future studies on quantum droplets, from the measurement of their peculiar excitation spectrum, to the exploration of their superfluid nature
Summary
The optical potential used to levitate the atoms against gravity is created by a single elliptical beam, whose vertical position is modulated in time. Along the other two directions, the curvature is positive but very weak: ωx = 2π × 2.2 Hz and ωy = 2π × 7 Hz. While the confinement along these two axes is well controlled by measuring the power and waists of the laser beam, we would like to perform an in-situ calibration of the curvature along z, which is more sensitive to misalignments. We perform two different measurements: in the first one we provide an upper bound to the local curvature in z = 0, while in the second one we estimate the global effect produced by the levitating potential on a larger scale. The fact that we do not observe any deformation of the droplet from its predicted spherical geometry let us conclude that the effect of the levitating potential is negligible on the vertical direction
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