Polish-Led Team Makes a Room-Temperature Supersolid From Light and Liquid Crystal

Physicists have observed a supersolid, a quantum state of matter that flows without friction while also forming a crystal-like pattern, at room temperature. Nearly every supersolid made so far has needed atoms chilled close to absolute zero, and only one other group, at Rensselaer Polytechnic Institute, has reported a light-based version that works at room temperature. The new result, from the University of Warsaw, the Military University of Technology in Poland, IBM Research Zurich and France's CNRS, was published in Science Advances.

A supersolid sounds like a contradiction. A crystal is rigid and ordered, while a superfluid flows with no viscosity at all. Quantum mechanics allows both at once. Physicists first predicted in the 1960s that a superfluid could spontaneously develop a periodic, crystal-like structure while keeping its frictionless flow. The idea stayed theoretical for more than half a century, until 2017, when supersolidity was first seen in ultracold atomic Bose-Einstein condensates.

The TopoLight consortium took a different route. Instead of atoms, the team used exciton polaritons, hybrid particles formed when light and matter couple strongly inside an optical microcavity. These particles last only a few picoseconds, but that is enough for them to condense into a Bose-Einstein condensate and behave like a quantum superfluid. The microcavity was filled with a specially designed liquid crystal and coated with an organic semiconductor, a combination that allowed strong light-matter coupling at room temperature.

"Our liquid-crystal microcavities can operate even at temperatures approaching 60°C," said Wiktor Piecek of the Military University of Technology, whose group built the liquid-crystal structures. The sign of the transition was regularly spaced bright fringes in the light coming off the sample. Those fringes mean the condensate spontaneously formed a periodic density pattern, which is the hallmark of crystallization.

The experiment showed something else. With each laser pulse, the fringes appeared at a different position, so the system independently chose one of many equivalent configurations every time. "This is a textbook example of spontaneous symmetry breaking," said Guillaume Malpuech of Institut Pascal at CNRS. "Theory predicted that the supersolid could choose among several equivalent states, and that is exactly what we observed." The condensate also showed a persistent spin helix, a periodic modulation in the polarization of the emitted light, so two kinds of order appeared at once, one in polarization and one in intensity.

The work was hard to carry out. The organic semiconductor slowly reacted with the liquid crystal, so every measurement had to be finished within a few hours of building the sample. "We had to plan the entire sequence of experiments in advance," said first author Marcin Muszyński, now at The City College of New York. "Repeating a measurement meant fabricating an entirely new sample." The most important measurements were made at IBM Research Zurich, where thin films of the semiconductor MeLPPP were deposited on photonic structures made in Poland. IBM's Thilo Stöferle said his group had studied the material for years and seen Bose-Einstein condensation in it before, but this was the first time it was combined with a liquid-crystal microcavity.

Most studies of exotic quantum phases have required expensive cryogenic systems operating near absolute zero. The researchers say the new results show that similar phenomena can be explored in photonic systems at room temperature. They point to possible uses in ultrafast photonic devices, neuromorphic computing accelerators and highly sensitive optical sensors that take advantage of the polarization behavior of the condensate.