University of Southern California

Helium nanodroplets have long been considered ideal model systems to explore quantum hydrodynamics in self-contained, isolated superfluids. However, the dynamic properties of individual droplets remained beyond the reach of experimentalists. Here, we investigated the rotation of single, isolated, superfluid 4He droplets by using single shot femtosecond x-ray coherent diffractive imaging at the LCLS free electron laser. The droplets were doped with Xe atoms, which are attracted to the cores of quantum vortices inside the droplets. Using the trapped Xe atoms as contrast, the coordinates of the vortices were determined from the diffraction images using iterative Error-Reduction phase retrieval algorithm (ERA). The obtained images reveal a plethora of possible vortex configurations within He droplets.