A single super photon made up of many thousands of individual light particles: About ten years ago, researchers at the University of Bonn produced such an extreme aggregate state for the first time and presented a completely new light source. This optical Bose-Einstein condensate has captivated many physicists ever since, because this exotic world of light particles is home to its very own physical phenomena. Researchers led by Martin Weitz, who discovered the super photon, and theo­retical physicist Johann Kroha have returned from their latest expedition into the quantum world with a very special obser­vation. They report of a new, previously unknown phase transition in the optical Bose-Einstein condensate. This is an overdamped phase. The results may in the long term be relevant for encrypted quantum communi­cation. 

The Bose-Einstein condensate is an extreme physical state that usually only occurs at very low tempera­tures. The particles in this system are no longer dis­tinguishable and are pre­dominantly in the same quantum mechanical state, 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 photons. Their special system is still in use today: Physicists trap light particles in a reso­nator made of two curved mirrors spaced just over a micro­meter apart that reflect a rapidly reci­procating beam of light. The space is filled with a liquid dye solution, which serves to cool down the photons. This is done by the dye molecules swallowing the photons and then spitting them out again, which brings the light particles to the tempera­ture of the dye solution - equivalent to room temperature. The system makes it possible to cool light particles in the first place, because their natural charac­teristic is to dissolve when cooled.

The somewhat translucent mirrors cause photons to be lost and replaced, creating a non-equi­librium that results in the system not assuming a definite temperature and being set into oscillation. This creates a transi­tion between this oscillating phase and a damped phase. Damped means that the ampli­tude of the vibration decreases. “The overdamped phase we observed corresponds to a new state of the light field, so to speak,” says Fahri Emre Öztürk, a doctoral student at the Institute for Applied Physics at the Univer­sity of Bonn. The special characteristic is that the effect of the laser is usually not separated from that of Bose-Einstein conden­sate by a phase transition, and there is no sharply defined boundary between the two states. This means that physicists can conti­nually move back and forth between effects.

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

The researchers plan to use their findings as a basis for further studies to search for new states of the light field in multiple coupled light conden­sates, which can also occur in the system. “If suitable quantum mechani­cally entangled states occur in coupled light condensates, this may be interesting for trans­mitting quantum-encrypted messages between multiple parti­cipants,” says Fahri Emre Öztürk. (Source: U. Bonn)

Reference: F. E. Öztürk et al.: Observation of a non-Hermitian phase transition in an optical quantum gas, Science 372, 88 (2021); DOI: 10.1126/science.abe9869

Link: Quantum Optics, Institute of Applied Physics, University Bonn, Bonn, Germany