Research
My research focuses on the quantum hydrodynamics of superfluid helium.
At just 2.17 degrees above absolute zero, liquid helium-4 undergoes a remarkable transformation into the superfluid phase. In this exotic state, quantum mechanics partially takes over its dynamics. Superfluid helium can flow without friction, carry heat more efficiently than any other known material and behave not like a collection of individual atoms, but rather as a single, coherent quantum wave of matter.
What facsinates me about superfluid helium is the presence of quantum vortices, tiny tornado-like structures that can form within the fluid. These vortices are topologically protected, meaning they can’t simply appear or vanish. They offer a rare glimpse into the hidden order of quantum systems. Remarkably, when they form a chaotic tangle, they can recreate features of classical turbulence, providing new insights not only into unresolved problems in fluid dynamics, but also into phenomena as far-reaching as black holes and the early universe.
Quantum turbulence
Turbulence, the seemingly chaotic motion of fluids, remains one of the great unsolved problems in physics. In classical fluids, vortices vary in size and strength, whereas quantum fluids like superfluid helium support only discrete, singly-quantised vortices or their bundles. The complex dynamics of these tangled vortex structures are central to quantum turbulence. During my PhD at Charles University, I investigated this phenomenon using cryogenic flow visualisation and second sound attenuation techniques.
Cryogenic flow visualisation
Flow visualisation is a powerful approach for experimental studies of turbulence. I visualised mechanically and thermally driven flows of superfluid helium by following small flakes of frozen hydrogen dispersed in the superfluid.
These particles do more than trace the flow—they interact with quantum vortices. Circulating superflow creates pressure forces that pull particles toward vortex cores, sometimes forming particle-decorated lines. In turbulent helium, these interactions are brief, but they leave statistical fingerprints. I analysed millions of particle trajectories to understand these interactions and shed light on the underlying quantum vortex tangle. I found that these quantum signatures depend on both the probing scale and the vortex line density, providing an new insight into quantum turbulence.
Second sound attenuation
Sound is a wave of pressure and density moving through a medium like air. In superfluid helium, these waves are called first sound. But quantum fluids also support second sound, a unique kind of wave, where temperature, not pressure, oscillates and propagates through the fluid. I studied second sound using tiny heater-thermometer pairs and custom-made ‘speakers’.
When a second sound wave travels through a dense tangle of quantum vortices, it gets attenuated. This process allowed me to measure the density of quantum vortex lines in superfluid helium. Although second sound attenuation is typically used in homogeneous turbulence, I implemented this technique for the first time in spatially inhomogeneous flows: vortex rings and superfluid jets.
Vortex rings
Quantum vortices sometimes tend to self-organise. Their collective behaviour and the formation of large-scale structures can be revealed, for example, by measuring scale-resolved turbulent energy spectra, suggesting that quantum turbulence supports eddies larger than a single quantum vortex. As part of my PhD at Charles University, I studied a special type of macroscopic eddy known as a vortex ring.
These doughnut-shaped vortices are notable for their long lifetimes and self-propulsion. I tracked the motion of thermally generated superfluid vortex rings and measured their strength using a custom-built experimental channel equipped with two pairs of second sound sensors. My measurements showed, for example, that the rings’ dynamics depend solely on the strength of the initial heat pulse, as determined by the brief temperature rise during the ring generation process.
Black hole simulations
Superfluid helium enables tabletop simulations of curved spacetime thanks to a mathematical analogy: the wave equation for surface waves on a flowing fluid has the same form as that of a quantum field in the curved spacetime, e.g. near a black hole. The effective geometry is given by the flow field, and is known as the acoustic metric.
In my postdoctoral work at the University of Nottingham, I investigated draining vortex flows (just a fancy name for the same flow that forms in a bathtub), which recreate the spacetime similar to a rotating black hole. To realise this in superfluid helium, I designed and built a custom experimental setup in which a driven recirculation loop generated a stable giant quantum vortex, thereby sustaining a steady analogue spacetime.
Superfluid interface visualisation
To test whether the experiment truly simulated black hole phenomena, I needed to detect small surface waves on the superfluid with both spatial and temporal resolution. I achieved this by imaging the interface against a checkerboard pattern, which appears regular only when the surface is flat. Waves distort the pattern in predictable ways, and by capturing these distortions with a high-speed camera, I reconstructed surface deformations down to one micrometre. This allowed me and my collaborators to analyse their spatial and temporal structure and compare the results with theoretical predictions from black hole physics.
To find whether this experimental setup simulates black hole phenomena, I needed to find a way to detect small waves that propagate on the superfluid interface with simultaneous resolution in space and time. I achieved this by observing the interface against checkerboard pattern. This pattern appears nice and regular only when the superfluid interface is calm and flat. Once a wave is excited in the fluid, the pattern gets distorted in a specific way. By capturing these deformations by a high speed camera, I was able to reconstruct surface deformations down to one micrometre, study their periodicity in both space and time, and compare these measurements with theoretical predicitions using methods developed for black hole physics.