The University of Southampton

'Giant quantum tornados' offer new optical insight into fluid dynamics

Published: 29 June 2021
Varying giant vorticies in heptagons of polariton condensates.

Researchers from the University of Southampton and Skolkovo Institute of Science and Technology have created a stable giant vortex in a hybrid light-matter system, addressing a longstanding challenge in quantised fluid dynamics.

The findings open possibilities to create unique coherent light sources and explore many-body physics under extreme conditions. The paper was published in the journal Nature Communications.

In fluid dynamics, a vortex is a region where a fluid revolves around a point (2D) or a line (3D); you have seen them when draining a sink or may have felt one in the form of turbulence while flying. The quantum world also has vortices: the flow of a quantum fluid can create a zone where the particles revolve persistently around some point. The prototypical signature of such quantum vortices is their singular phase at the core of the vortex.

Professor Pavlos Lagoudakis, Head of Hybrid Photonics at the University of Southampton, partnered with Skoltech Professor Natalia Berloff and colleagues to study vortices created by polaritons - odd hybrid quantum particles that are half-light (photon) and half-matter (electrons) - forming a quantum fluid under the right conditions.

The scientists were looking for a way to create vortices in these polariton fluids with intense rotation. These vortices, also known as giant vortices, are generally very hard to obtain as they tend to break apart into many smaller vortices with low rotation in other systems.

Creating stable giant vortices shows that non-equilibrium (open) quantum systems, like polariton condensates, can overcome some severe limits of alternatives such as Bose-Einstein condensates of cold atoms. Control over the vorticity of a polariton fluid could open new perspectives on simulation of gravity or black hole dynamics in the microscopic world, and become an important tool for optical data storage, distribution and processing applications.

Professor Lagoudakis, says: "This is a very nice demonstration of how polaritons can provide a very flexible sandbox to probe some of the more complex phenomena of nature. What we have shown here is a system that shares a lot of characteristics with a black hole while still emitting light, much like a 'white hole'."

The researchers had been working on using interacting polariton condensates as candidates to simulate a planar vector model known as the XY model. They realised that when multiple condensates were arranged into a regular polygon with an odd number of vertices, the ground state of the whole system could correspond to a particle current along the polygon edge. By going from a triangle to pentagon, heptagon, and so on, the authors showed that the current rotated faster and faster, forming a giant vortex of varying angular momentum.

Dr Tamsin Cookson, first author and joint Southampton-Skoltech researcher, says: "The formation of stable clockwise, or anticlockwise, polariton currents along the perimeter of our polygons can be thought of as a result of geometric frustration between the condensates. The condensates interact like oscillators that want to be in antiphase with each other. But an odd numbered polygon cannot satisfy this phase relationship because of its rotational symmetry, and therefore the polaritons settle for the next-best-thing which is a rotating current."

Professor Lagoudakis is based at both the University of Southampton’s School of Physics and Astronomy and Skoltech in Moscow, Russia. Other organisations involved in this latest research include the University of Cambridge, and Cardiff University.

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