Physicists create a supersolid from light and matter at room temperature
An international team has observed a supersolid, a state that is both crystal-like and frictionless, without extreme cooling, using hybrid light-matter particles inside a specially built optical chamber.
Physicists have observed a supersolid, a strange state of matter that is part crystal and part frictionless liquid, at room temperature. Until now, such experiments have meant chilling atoms to within a whisker of absolute zero. This time, an international team used hybrid particles of light and matter instead.
The researchers came from the University of Warsaw’s Faculty of Physics, the Military University of Technology in Poland, IBM Research Zurich and France’s CNRS. Their results are published in Science Advances.
A supersolid has the repeating internal pattern of a crystal, yet it also flows as a superfluid, a liquid with no internal resistance. The idea was put forward in the 1960s and first confirmed in 2017, using ultracold atomic condensates. Those experiments called for sophisticated equipment to get close to the lowest temperature possible.
Working as the TopoLight consortium, the team turned to exciton polaritons. These quasiparticles form when light and matter couple strongly inside a tiny optical chamber known as a microcavity. They survive for only a few picoseconds, but can still gather into a condensate and behave as a quantum superfluid. The microcavity held an engineered liquid crystal covered with an organic semiconductor. The liquid-crystal structures, developed by Professor Wiktor Piecek’s group, can operate at temperatures approaching 60°C.
The evidence lay in the light the sample gave off. Once it entered the supersolid phase, evenly spaced bright fringes appeared, revealing a repeating variation in density, a defining mark of crystal order. After each laser pulse the fringes turned up in a different place, as the system picked from several equally possible arrangements. Physicists call this spontaneous symmetry breaking.
The condensate also showed a persistent spin helix, which is a repeating variation in the polarisation of the emitted light. In other words, two kinds of order were present together: one in the light’s intensity and one in its polarisation.
It was no mean feat. The semiconductor slowly reacted with the liquid crystal, so each sample offered only a few hours of useful measuring time, and a repeat attempt meant building a fresh one. Every step had to be planned beforehand.
The key measurements took place at IBM Research Zurich, using photonic structures manufactured at the Military University of Technology. For a project spread across several institutions, it is a cracking result.
Why it matters
Many exotic quantum experiments depend on expensive cooling systems, which complicate the research and can make it harder to turn laboratory findings into practical devices. Showing that a supersolid can be studied in a photonic device at far more accessible temperatures lowers that barrier. It could help bring light-based computing and highly sensitive sensors a step closer.
