Physicists observe new phase in Bose-Einstein condensate of light particles

About 10 years ago, researchers at the University of Bonn produced an extreme global photon state, a single “super-photon” made up of several thousand individual light particles, and introduced a whole new source of light. . The state is called a Bose-Einstein optical condensate and has captivated many physicists since, because this exotic world of light particles is home to its own physical phenomena. Researchers led by Professor Martin Weitz, who discovered the super photon, and theoretical physicist Professor Johann Kroha, are now reporting a new observation: a so-called overdamping phase, a previously unknown phase transition in optical condensate by Bose-Einstein. The study was published in the journal Science.

Bose-Einstein condensate is an extreme physical state that usually only occurs at very low temperatures. The particles of this system are no longer distinguishable and are mainly in the same state of quantum mechanics; in other words, they behave like a single giant “superparticle”. The state can therefore be described by a single wave function.

In 2010, researchers led by Martin Weitz succeeded for the first time in creating a Bose-Einstein condensate from light particles (photons). Their special system is still in use today: physicists trap light particles in a resonator made up of two curved mirrors spaced just over a micrometer apart that reflect a fast reciprocating beam of light. The space is filled with a liquid dye solution, which is used to cool the photons. The dye molecules “swallow” the photons and then spit them out again, bringing the light particles to the temperature of the dye solution, equivalent to room temperature. The system helps cool light particles because their natural characteristic is to dissolve when cooled.

Clear separation of two phases

A phase transition is what physicists call the transition between water and ice during freezing. But how does the particular phase transition occur in the trapped light particle system? Scientists explain it this way: somewhat translucent mirrors cause photons to be lost and replaced, creating an imbalance that causes the system to not assume a set temperature and to oscillate. This creates a transition between this oscillating phase and a damped phase. Damped means that the amplitude of the vibration decreases.

“The overdamping phase that we observed corresponds to a new state of the light field, so to speak,” says lead author Fahri Emre Öztürk, a doctoral student at the Institute of Applied Physics at the University of Bonn. The peculiarity is that the effect of the laser is generally not separated from that of the Bose-Einstein condensate by a phase transition and that there is no clearly defined border between the two states. This means that physicists can continually go back and forth between effects.

“However, in our experience, the overdamped state of the Bose-Einstein optical condensate is separated by a phase transition from both the oscillating state and from a standard laser,” says Professor Martin Weitz, director of the ‘study. “This shows that there is a Bose-Einstein condensate, which is really a different state from the standard laser.” In other words, we are dealing with two distinct phases of the Bose-Einstein optical condensate, ”he says.

The researchers plan to use their results as a basis for further studies to look for new light field states in several coupled light condensates, which may also occur in the system. “If appropriate quantum mechanically entangled states occur in coupled light condensates, this may be of interest for the transmission of quantum encrypted messages between multiple participants,” explains Fahri Emre Öztürk.