Turbulence in solid matter
A team at the School of Basic Sciences has made an unexpected discovery in quantum physics. Just as a stone creates a vortex in the wake of a river, EPFL scientists reveal a similar phenomenon at the microscopic level in a semiconductor — a solid state. The new methodology opens up a new field of research in physics with ramifications for the future of quantum computing.
Solid-state whirlpools. While turbulence and solid states make a rather a strange association, it has now been proven possible in quantum fluids. Solid matter can behave like a liquid on a small scale. For the first time, scientists have reproduced the phenomenon of whirlpools in a semiconductor, a material widely used in industry. Thanks to this device, Gaël Nardin and his colleagues at the Laboratory of Quantum Optoelectronics have been able to perform innovative measurements, publishing the results in Nature Physics.
“For the first time ever we have created vortices in a controlled flow. We can also measure the parameters that control the creation of these whirlpools. This brings real added-value compared to previous experiments where vortices appear spontaneously, and which are similar to those created when you pull the plug out of a bath full of water”, explains Gaël Nardin.
The scientists used polaritons. These microscopic elements – between matter and light – only live for a few picoseconds, that it is 0.000000000001 seconds, at a temperature of a few degrees above absolute zero. The physicists used a laser beam to inject them into the semiconductor to provoke flow. They were able to take pictures of the vortex creation and confirm predictions by measuring the threshold of the speed above which the turbulence appears. The polaritons’ phase – one of their quantum properties – has also been measured for the first time in this type of experiment; allowing for a visualization of their flow that looks like waves on water.
More intuitive physics
Small-scale physics is usually imperceptible in macroscopic experiments. However, quantum fluids can nonetheless be approached in an intuitive manner thanks to our knowledge of liquid flow. Like the pillar of a bridge in the current of the river, the obstacle in the semiconductor creates whirlpools in its wake, with a “wave” in front and an “ebb” at the back. The analogies thus revealed represent an opportunity. In the short term, the goal is to generate successions of whirlpools to confirm the intuitions that the scientists have gained thanks to hydrodynamics.
The study of quantum fluids in semiconductors opens up a new field of fundamental research, in particular within the framework of the National Centre of Competence in Research (NCCR) Quantum Photonics. For those who dream of quantum computers, new applications could be explored. Some already imagine polaritonic circuits. “We create, study and handle coherent flows composed of light and matter. At the moment it’s mainly just a curiosity, just as laser was when it was first discovered, but who knows what the future holds?”