Mini-Tornadoes in Superfluid Demonstrate Quantum Mechanics on a Large Scale
Quantum mechanics is, by nature, abstract and often unobservable, as it describes the behavior of particles on a nanoscopic scale. But a substance called superfluid helium has singular properties that allow quantum properties to be observed on a larger scale. Researchers at the SLAC National Accelerator Laboratory used an X-ray laser to observe tiny "tornadoes" in helium superfluid droplets that could shed light on quantum states.
"We were able to see a manifestation of the quantum world on a macroscopic scale," said Ken Ferguson, a PhD student from Stanford University working at SLAC's Linac Coherent Light Source (LCLS).
These vortices had been observed in larger particles of helium, but this was the first time that they were observed on a microscopic scale (although still large compared to most quantum observations). These formations provided the first definitive evidence of these droplets' quantum states.
"Helium nanodroplets are considered ideal model systems to explore quantum hydrodynamics in self-contained, isolated superfluids," according to the research paper.
For the experiment, they shot a stream of nanoscale helium droplets into a vacuum, and then rotated the droplets in such a way that they were cooled to a temperature colder than outer space. These extreme temperatures chilled the droplets to a superfluid state, or turned them into a cold, frictionless, entropy-less liquid whose weakly attracted atoms are constantly in motion, preventing freezing. As a result of this weak attraction, the quantum properties of the liquid prevail, and superfluids are able to roll uphill and diffuse through molecule-wide holes. When the droplets reached superfluid state, the X-ray laser then took pictures of the individual droplets, revealing the properties of the vortices.
In addition to observing the droplets' quantum states, the researchers observed distinctive properties of their rotation. In normal liquids, rapidly rotated droplets will warp into peanut-like shapes, but approximately 1% of the helium droplets acquired a wheel-like shape, and were able to reach rotation speeds far beyond those of droplets that obey the laws of classical physics without disintegrating.
Oliver Gessner, a co-leader in the experiment, said, "Now that we have shown that we can detect and characterize quantum rotation in helium nanodroplets, it will be important to understand its origin and, ultimately, to try to control it."