Superfluidity – the flow of liquid without any friction – was first observed in ultracold liquid helium in 1938. Such a superfluid liquid can creep up along the walls of a container, boil without bubbles and even flow around obstacles. Shortly afterwards, the physicist Fritz London suggested that there might be some sort of link between superfluids and Bose-Einstein condensates, or BECs (a state of matter in which all the particles in a system condense into a single state). He was proved right when researchers indeed observed superfluidity in ultracold atomic BECs in 1995. Something similar also occurs in superconductors in which electron pairs condense giving rise to supercurrents able to conduct electricity without any losses.

To date, however, superfluidity has only been observed at very low temperatures because most of the particles that condense into a BEC cannot survive at room temperature. A team of researchers led by Daniele Sanvitto of the CNR NANOTEC in Leece, Italy, and Stéphane Kéna-Cohen of the Polytechnqiue Montréal in Quebec, Canada, have now shown that a condensate of quasiparticles made from excitons (electron-hole pairs) and photons can behave as a superfluid at room temperature.

Fluid can flow without friction around obstacles

The sample studied by the team consists of a Fabry-Pérot microcavity made up of two highly reflective dielectric (Bragg) mirrors surrounding a thin film of organic material (2,7-Bis[9,9-di (4-methylphenyl)-fluoren-2-yl]-9,9-di(4-methylphenyl)fluorene or TDAF) that confines photons in a small volume. Electrons and their vacancies (known as holes) can form bound states (called excitons) even at high temperatures in this organic material and the photons interact with the excitons so strongly in the microcavity that they form hybrid light–matter modes (the polaritons).

By making use of a very fast detection technique, like an ultra-slow motion camera able to take images every tens of femtoseconds (10–14 seconds), the researchers were able to observe the dynamics of the polariton fluid. They found that the fluid can flow without friction around obstacles (formed in this case by a laser burning small holes in the organic material). This behaviour is interpreted as being a signature of the superfluid regime.

Photonic counterparts to electronic devices

“The fact that such an effect has been observed under ambient conditions means that such condensate fluids can now be studied with table-top experiments in a simple device no bigger than a finger nail,” Sanvitto tells nanotechweb.org. “The specific dynamics of these fluids (also called quantum fluids) could now be revealed and observed without the need for special equipment and experimental conditions.

“In the future, devices made of such quasiparticles composed of half-light and half-matter, which have a very low mass, might be used to transport information with zero losses and zero heating,” adds Kéna-Cohen. “In fact, one of the most interesting applications of polaritons is their use as possible photonic counterparts to electronic devices (switches, gates and transistors). Daniele and I recently wrote a review on this subject ('The road towards polariton devices').”

The team says that it will now be looking at making devices in which photons can be better confined and stay confined for longer periods of time. “This will be essential for making future viable and realistic technologies based on polariton condensates,” says Sanvitto.

The present work is detailed in Nature Physics doi:10.1038/nphys4147.