01.01.2021

Dancing helium

University of Queensland researchers are using a fluid that never settles to create quantum tornadoes on a chip.

The team, led by Professor Warwick Bowen has been working to understand and harness interactions between waves in superfluid helium and light. Their research leads on to new light-based technologies that require very little energy and are hypersensitive, while at the same time providing an insight into turbulent systems such as cyclones and interstellar objects.

Professor Bowen’s research is centred on a raised silicon disk that was nanofabricated with help from ANFF-Q equipment and expertise. The structure’s shape traps light at its edges, creating a cavity that can keep photons circling for a limited time.

This disk is then coated with superfluid helium – this is an approach unique to Professor Bowen’s laboratory and the work builds on the team’s previous endeavours that showed laser cooling can be used to sustain the temperatures required to keep the helium film in this superfluid state.

Superfluids are quantum liquids that are characterised by the fact that they have no viscosity, and flow without losing any energy. This means if a wave or a vortex starts in the superfluid, it won’t ever stop of its own accord. In Professor Bowen’s device, a superfluid acoustic wave is triggered to flow around the disks edge, travelling in the exact same space as the trapped light.

This colocation leads to the light interacting with the acoustic wave, which triggers a phenomenon called Brillouin scattering. The effect causes the light particles to be absorbed and re- emitted with lower energy. The energy difference is taken up by the acoustic wave, amplifying it. The result is in effect an incredibly low power, and low threshold type of laser for sound, rather than light.

This result can be harnessed to create ultraprecise sensors, ultranarrow light filters, low power devices, precision silicon chips, and more. Due to the forces at play, they are less susceptible to shock and vibration than their MEMS-based equivalents, and they could also have performance benefits.

Another bizarre behaviour of quantum liquids is that they can only carry integer circulation, which means they can only flow at fixed velocities. They can therefore be used to provide an absolute reference frame for acceleration measurement. This would allow a system to track its position without the need for satellites, making location pinpointing in submarines, in-door shopping centres, and underground far more accurate.

There’s yet more novel effects to unlock however – Professor Bowen’s devices can stir the superfluid helium to trigger vortices that will go on forever, and this provides a significant opportunities as a method to study turbulence. Professor Bowen said it was postulated more than 50 years ago that the turbulence problem could be simplified using quantum liquids.

“Our new technique is exciting because it allows quantum turbulence, which mirrors the sort of behaviour you see in cyclones, to be studied on a silicon chip for the first time,” Professor Bowen said. The research also had implications in space, where quantum liquids are predicted to exist within dense astrophysical objects.

“Our finding allows us to observe this nanoscale quantum turbulence in the lab, and this research could help to explain how these objects behave,” he said.

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