Physicists gain control over transitions between different states of matter – Scienmag: Latest Science and Health News\n"Polaritons are quasiparticles formed due to the hybridisation between matter and light. Once they are supplied with additional energy and densities, they form collective excitations, solitons. A soliton has an ability to propagate in space preserving its shape. In other words, despite being a collective state consisting of many particles, a soliton behaves like a single particle. At the same time, a Bose-Einstein condensate is a quantum state of matter where all particles, in our case polaritons, populate the ground state of the system with minimal energy. Usually, the ground state is extended through all the area of the system under study. The soliton and Bose-Einstein condensate are two widely different regimes, and we managed to observe the transition between them," explains Ivan Shelykh, head of the International Laboratory of Photoprocesses in Mesoscopic Systems at ITMO University in St Petersburg.\nThe researchers observed that an increase in the laser pumping power triggered nonlinear effects in the system. "Increasing the laser power, we create more and more particles, which start to inter?ct with each other. Therefore, the whole system goes into a nonlinear regime. Separate polaritons form solitons, which then transfer into a Bose-Einstein condensate. Although it was clear we had obtained some interesting results, without a good theory we would have never understood what they actually meant," Skolnick continues.\nM. Sich et al.\nPhys. Rev. Lett. 19 April 2018