Quantum wind: From oceanography to quantum physics

Richard Juggins and Marzena Szymanska from the Department of Physics and Astronomy at UCL, together with their experimental collaborators at INFN in Lecce, have used a microscopic analogue of oceanographic techniques to measure the excitation spectrum of a thermalized polariton condensate.

In a paper published today in Nature Communications, our researchers detail how quantum fluids of light can be studied using a technique inspired by oceanography. Discussing how they made the unlikely link between oceanography and quantum physics, Juggins said:

“Our experimental collaborators at INFN in Italy, had done an experiment and had obtained a weird result that they couldn’t explain. After some simulations, we noticed that there are parallels in the way quantum fluids of light in the experiment behave and the way in which macroscopic fluids such as oceans behave. It essentially comes down to an idea of wind.”

Quantum wind

Out to sea, as the wind whips across the water it generates waves which, quite intuitively, move in the same direction as the air propelling them. By taking a series of pictures from above, it is possible to infer the wavelength and frequency of the waves, and from these the ocean depth.

However, in the experiment, instead of water the ‘sea’ is made of light trapped between tiny semiconductor mirrors. Inside these, particles of light couple to electrons to form a fluid of composite particles known as polaritons. As light tends to leak out from the mirrors, a laser must be used to maintain the fluid. The positioning of this laser produces an outward flow of light that blows over the polaritons like a breeze on the ocean, exciting waves in only one direction. This quantum wind shows how nature can exhibit similar behaviour on radically different scales.

Just like the oceanographic case, the researchers produced images of the polariton waves and used them to calculate the waves’ wavelength and frequency. From which they can find the density of the polaritons and whether they display any of the attributes associated with the strange phenomenon of superfluidity.

Polaritons and superfluids explained

In this video, Richard Juggins and Marzena Szymanska describe polaritons and their recent research into superfluidity.

Using modern technology, it is possible to trap particles of light and turn them into a fluid. Unlike ordinary light, the particles in the fluid have mass and can interact with each other. We can observe this liquid light as it flows and watch it deform as it passes obstacles. As well creating a fluid, it is also possible to build synthetic quantum matter out of light.

Quantum mechanics is usually only visible on the tiniest scales – those of individual particles. However, if we engineer the conditions correctly, trillions of trillions of particles of light will combine and show quantum effects on larger scales too. That is, we can synthesise matter that has quantum features on all scales.

An example of quantum matter is a superfluid. First observed in liquid helium, superfluids flow with no viscosity and cannot rotate, except in the form of tiny tornado-like vortices, and can even spontaneously empty themselves from their container by climbing up the walls. Experiments have shown that it is possible to synthesise superfluids of light that share these bizarre properties.

In the last decade or so, experiments have shown that liquid light can become a superfluid — a fluid which flows with no viscosity or friction. Because of the lack of friction, superfluids cannot be stirred or rotated. If you put a superfluid in a bucket and rotate the bucket, the fluid itself will remain stationary.

Marzena Szymanska said, “The field of quantum technologies has been rapidly developing in recent years. Research groups are beginning to build quantum devices out of light and even hope in the future to build quantum computers. In pursuit of this goal, deepening our understanding of various forms of light – and how to synthesise and manipulate it – will be invaluable."

You can learn more about polaritons and this research by visiting Prof. Szymanska group's website: qlm-ucl.org

PAPER: https://www.nature.com/articles/s41467-019-13733-x

COMMUNICATIONS CONTACT: Henry Bennie, UCL Quantum Science and Technology Institute

IMAGE CAPTION: Light from the pump blows over the polaritons like wind, forming waves which propagate in the same direction, Dario Ballarini, INFN.