The sloshing of a quantum fluid reveals superfluid properties

An Australian-led team of physicists have gained new insights into superfluidity by creating sloshing quantum liquids comprised of light and matter, in a bucket formed by lasers.

“These quantum fluids are expected to be as wavy as the oceans, but catching clear pictures of the waves is an experimental challenge,” said lead author of the new paper in Physical Review Letters, Dr Eliezer Estrecho.

The team serendipitously observed the wavy motion of the quantum fluid and were able to calculate the speed of sound in the fluid, using an optically-controlled bucket filled with hybrid light-matter particles known as exciton-polaritons.

Dr Estrecho is a researcher in the Nonlinear Physics Centre in the Research School of Physics and is also a member of the ARC Centre of Excellence in Future Low-Energy Electronics Technologies (FLEET). 

Superfluidity is one of FLEET’s research focuses - particles that flow without suffering resistance could have important future applications in low-energy electronics.

To create and confine the exciton-polariton quantum fluid, the team used laser light shaped into a ring, which they shone onto a semiconductor. The laser light combined with electron-hole pairs within the semiconductor to form the exciton-polaritons and trapped them inside the ring.

As these particles cool down they form a giant quantum object called a Bose-Einstein condensate (sometimes referred to as the fifth state of matter), in which quantum phenomena can be seen on a macroscopic scale.

“The excess energy lost by the cooling particles does not disappear easily, so the condensate will display some sort of wavy, oscillating behaviour, which is random for every realisation of the condensation,” said corresponding author Professor Elena Ostrovskaya, also from the Nonlinear Physics Centre and FLEET.

That randomness makes it hard to detect the transient oscillations with imaging cameras, since it will average out in the experiment.

However, fortuitously, the ‘bucket’ is tilted.

“In most experiments, we try to avoid the tilt since it complicates the analysis,” said Dr Estrecho.

“But in this case the ‘annoying’ tilt enabled the observation of the oscillation because it is favourable for the condensate to slosh along the tilt direction.”

The sloshing oscillation was observed in both the position and momentum of the condensate, elegantly displaying the laws of quantum mechanics at a macroscopic scale visible with an ordinary microscope. However, the oscillations are extremely fast, so that it was only possible to observe them using a camera with picosecond-scale temporal resolution.

The analysis of the oscillations gave insights into the superfluid properties of the quantum fluid, said Dr Estrecho.

“The true beauty of the experiment lies in the analysis of the oscillation frequencies, since it is directly related to the speed of sound. 

“Probing the superfluidic properties is relevant because this quantum fluid can exist at room temperature, and so is promising for device applications,” he said.

The team found that the speed of sound extracted from the experimental data was smaller than expected from prevailing theories. 

“We think the discrepancy arises from the existence of an invisible reservoir of hot, uncondensed particles that interact with the condensate,” Dr Estrecho said.

The experiment also provides clues to the possible effects that can slow down the superfluid. At absolute zero temperature, the oscillations are expected to never end since there is no resistance to motion. However, at finite temperature, this is not the case, so studying the damping rates of the oscillations is essential in understanding the superfluid.

Initial results, however, are puzzling.

“Neither the reservoir particles, the finite temperature, nor the inherent short lifetime of exciton-polaritons can solely explain the observed damping rates,” Professor Ostrovskaya said.

“Further theoretical studies that combine these effects and carefully controlled experiments are needed to better understand the non-equilibrium quantum fluid.”

Low-energy collective oscillations and Bogoliubov sound in an exciton-polariton condensate was published as an Editor’s Suggestion in Physical Review Letters in February 2021. (DOI 10.1103/physrevlett.126.075301)

ANU researchers from FLEET, Nonlinear Physics Centre and the Laser Physics Centre worked with collaborators at the Department of Physics and Astronomy, University of Pittsburgh (USA) and Department of Electrical Engineering, Princeton University (USA).

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Dr Eliezer Estrecho
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