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A study in Nature describes both the mechanism and the material conditions necessary for . The work could serve as a blueprint for designing materials that allow exotic quantum states—such as superconductivity, superfluidity or superfluorescence—at high temperatures, paving the way for applications such as quantum computers that don't require extremely low temperatures to operate.

The international team that did the work was led by North Carolina State University and included researchers from Duke University, Boston University and the Institut Polytechnique de Paris.

"In this work, we show both experimental and theoretical reasons behind macroscopic quantum coherence at high temperature," says Kenan Gundogdu, professor of physics at NC State and corresponding author of the study.

"In other words, we can finally explain how and why some materials will work better than others in applications that require exotic quantum states at ambient temperatures."

Picture a school of fish swimming in unison or the synchronized flashing of fireflies—examples of collective behavior in nature. When similar collective behavior happens in the quantum world—a phenomenon known as macroscopic quantum phase transition—it leads to exotic processes such as superconductivity, superfluidity, or superfluorescence.

In all these processes, a group of quantum particles forms a macroscopically coherent system that acts like a giant quantum particle.

However, quantum phase transitions normally require super-cold, or cryogenic, conditions to occur. This is because higher temperatures create thermal "noise" that disrupts the synchronization and prevents the phase transition.

In a previous study, Gundogdu and colleagues determined that the atomic structure of some hybrid perovskites protected the groups of quantum particles from the thermal noise long enough for the phase transition to occur.

In these materials, large polarons—groups of atoms bound to electrons—formed, insulating light-emitting dipoles from thermal interference and allowing superfluorescence.

In the new study, the researchers found out how the insulating effect works. When they used a laser to excite the electrons within the hybrid perovskite they studied, they saw large groups of polarons coming together. This grouping is called a soliton.

"Picture the atomic lattice as a fine cloth stretched between two points," Gundogdu says. "If you place solid balls—which represent excitons—on the cloth, each ball deforms the cloth locally. To get an exotic state like superfluorescence, you need all the excitons, or balls, to form a coherent group and interact with the lattice as a unit, but at high temperatures thermal noise prevents this.

"The ball and its local deformation together form a polaron," Gundogdu continues.

"When these polarons transition from a random distribution to an ordered formation in the lattice, they make a soliton, or coherent unit. The soliton formation process dampens the thermal disturbances, which otherwise impede quantum effects."

"A soliton only forms when there is enough density of polarons excited in the material," says Mustafa Türe, NC State Ph.D. student and co-first author of the paper.

"Our theory shows that if the density of polarons is low, the system has only free incoherent polarons, whereas beyond a threshold density, polarons evolve into solitons."

"In our experiments we directly measured the evolution of a group of polarons from an incoherent uncorrelated phase to an ordered phase," adds Melike Biliroglu, postdoctoral researcher at NC State and co-first author of the work.

"This is one of the first direct observations of macroscopic quantum state formation."

To confirm that the soliton formation suppresses the detrimental effects of temperature, the group worked with Volker Blum, the Rooney Family Associate Professor of Mechanical Engineering and Materials Science at Duke, to calculate the lattice oscillations responsible for thermal interference.

They also collaborated with Vasily Temnov, professor of physics at CNRS and Ecole Polytechnique, to simulate the recombination dynamics of the soliton in the presence of thermal noise. Their work confirmed the experimental results and verified the intrinsic coherence of the soliton.

The work represents a leap forward in understanding both how and why certain hybrid perovskites are able to exhibit exotic quantum states.

"Prior to this work it wasn't clear if there was a mechanism behind high temperature quantum effects in these materials," says Franky So, co-author of the paper and the Walter and Ida Freeman Distinguished Professor of Materials Science and Engineering at NC State.

"This work shows a quantitative theory and backs it up with experimental results," Gundogdu says.

"Macroscopic quantum effects such as superconductivity are key to all the quantum technologies we are pursuing—quantum communication, cryptology, sensing and computation—and all of them are currently limited by the need for low temperatures. But now that we understand the theory, we have guidelines for designing new quantum materials that can function at high temperatures, which is a huge step forward."