Scientists catch spirals of Kelvin waves in superfluid helium for the first time

A team of Japanese researchers has discovered a method to control Kelvin wave excitation in superfluid helium-4. This breakthrough can refine our understanding of how energy moves in quantum fluids, improving quantum sensors and making quantum devices more efficient.

Kelvin waves are tiny, spiral-like disturbances that move along the length of a quantum vortex in superfluids (like liquid helium at extremely low temperatures) and ultracold atomic gases. These waves are different from the large-scale oceanic Kelvin waves and play an important role in deciding energy loss and turbulence in quantum systems.

However, until now scientists have been struggling to find ways to observe and control these spiral waves. This is why not much information is available on them. The current study is the first to reveal a practical way to control Kelvin waves and confirm their helical nature.

“This work elucidates the dynamics of Kelvin waves and initiates an approach for manipulating and observing quantized vortices in three dimensions, thereby opening avenues for exploring quantum fluidic systems,” the study authors note.

An accidental but exciting discovery

The researchers were not looking for Kelvin waves but were instead performing an experiment that involved applying an electric field to a tiny nanoparticle attached to a quantum vortex. Their goal was to move the whole vortex structure by making the nanoparticle oscillate.

For the unversed, a quantum vortex is like a tiny whirlpool that forms in a superfluid. It’s special because it has a fixed amount of spin that can’t be changed, unlike normal whirlpools.

The scientists couldn’t move the vortex but they observed a wavy motion of the vortex core (Kelvin waves). "This unexpected result prompted us to shift our focus toward studying the excitation of Kelvin waves in-depth," Yosuke Minowa, lead researcher and an associate professor at Kyoto University, said.

So they created tiny silicon particles in superfluid helium-4 by hitting a silicon wafer with a laser. This action disturbed the fluid, trapping some particles in vortex cores, and making the vortices visible.

Next, they applied an electric field, causing the particles to oscillate, forming helical Kelvin waves along the vortices. "In previous studies, Kelvin wave-like oscillations were observed only accidentally. We developed a novel method to manipulate an ideal vortex in superfluid helium, providing a new way to study the behavior of these quantized vortices," Minowa added.

Confirming the spiral curves of Kelvin waves

The study authors didn’t stop with observing Kelvin waves. They used a dual camera setup and applied different excitation frequencies (ranging between 0.8 Hz to 3 Hz) to see how the waves behave and how they appear in 3D.

"The three-dimensional image reconstruction played a critical role in confirming the helical nature of the Kelvin waves. By visualizing the vortex's three-dimensional dynamics, we obtained direct and concrete evidence that the observed oscillations were indeed Kelvin waves," Minowa said.

Moreover, the researchers also collected information related to how the wavelength and speed of Kelvin waves changed with their frequency. These findings show that the proposed approach could prove to be an effective tool for studying quantum vortices, Kelvin waves, and their properties.

“We have introduced a new tool to study Kelvin waves in superfluid helium, paving the way for numerous experimental investigations,” Minowa concluded.

The study is published in the journal Nature Physics.