An international teamof Italian and Canadian researchers claimtheyhave achieved superfluidity in light quasiparticles at room temperature for the first time. This feat was reported in the journal Nature Physicsand has importantimplications that rangefrom theoretical physics to technical applications.

There are a few things we need to understand before we can truly appreciate the importance of this milestone. First is the concept of a superfluid. At extremely low temperatures, some substances stop behaving like they are made of individual particles and instead behave like a single entity. Superfluids are not only frictionless and move around anobstacle without ripples, they can also climb the side of containers and move uphill. They have both puzzled and fascinated people for decades.

Superfluidity is an impressive effect, normally observed only at temperatures close to absolute zero such as in liquid Helium and ultracold atomic gasses,” team lead Daniele Sanvitto said in astatement. “The extraordinary observation in our work is that we have demonstrated that superfluidity can also occur at room-temperature, under ambient conditions, using light-matter particles called polaritons.”

These polaritons are the key. These objects are one of many quasiparticles, which are usually complex interactions thatbehave as a single entity like it was a real particle. Polaritons are interactions between photons and the excited state in the electrons of a material. The team discovered that this interaction can be made to act like a superfluid.

We sandwiched an ultrathin film of organic molecules between two highly reflective mirrors,” Stphane Kna-Cohen, coordinator of the Montreal team, explained. “Light interacts very strongly with the molecules as it bounces back and forth between the mirrors and this allowed us to form the hybrid light-matter fluid. In this way, we can combine the properties of photons such as their light effective mass and fast velocity, with strong interactions due to the electrons within the molecules.”

Superfluidity is related to the fifth state of matter called the Bose-Einstein condensate, whichis of extreme interest. In this state, it is possible to observe macroscopic quantum effects, whichhas allowed physicists to explore a wide-range of unsolved mysteries in fundamental physics. Bose-Einstein condensates are extremely fragile and require some of the lowest temperatures we can achieve in labs, so this observation has the possibility of being monumental.

The fact that such an effect is observed under ambient conditions, the research team added, can spark an enormous amount of future work, not only to study fundamental phenomena related to Bose-Einstein condensates with table-top experiments, but also to conceive and design future photonic superfluid-based devices where losses are completely suppressed and new unexpected phenomena can be exploited.

The observed flow of polaritons as they go around an obstacle in the supersonic (top) and superfluid (bottom) regime. Polytechnique Montreal/CNR NANOTECH

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