Researchers from Skoltech, the University of Southampton, and the University of Iceland have created quantum vortices in a rotating bucket of liquid light after overcoming a long-standing experimental hurdle. The study is reported in a recent article in Science Advances and goes back in part to Newton’s rotating bucket experiment: When a bucket of water is set to spin, the fluid forms a spiraling vortex at its center, much like bath water running down the drain. However, everything changes when superfluids — such as liquid helium and atomic Bose-Einstein condensates — are involved, since they are frictionless. When a superfluid is stirred, instead of forming one spiraling vortex like a classical liquid would, above some critical rotation frequency, multiple quantized vortices are created. Interestingly, faster rotation produces more vortices.
In the case of liquid light (otherwise known as a polariton condensate), which is the result of strong coupling between light and matter (photons and excitons) at cryogenic temperatures, the fluid is instead contained within a “bucket” made from a laser emission. Suddenly, when that laser beam imitating a bucket rotates, the polariton condensate in it also rotates, and above some critical rotation frequency a quantum vortex appears at the center of the fluid.
Until now, creating such a vortex has been prohibited by limitations on the rotation speed of the bucket. Due to the fundamental dynamics of liquid light, a bucket rotating at a gigahertz frequency is required to induce a quantum vortex in the polariton condensate. This, however, is many orders of magnitude faster than the lab equipment typically used to shape a laser beam, which has a maximum update rate well below 100 Hz.
Skoltech PhD student Ivan Gnusov designed an optical setup to overcome this limitation. The apparatus was built in Skoltech’s Hybrid Photonics Laboratory, led by Professor Pavlos Lagoudakis. The approach used relies on two laser beams with a slight difference in energy. Combining them results in a beating note profile rotating in accordance with their energy difference.