Led by University of Bonn professor Martin Weitz, researchers have observed a previously unknown phase transition in the optical Bose-Einstein condensate. The state is known as an overdamped phase, and it may be relevant for encrypted quantum communication. “If suitable quantum mechanically entangled states occur in coupled light condensates, this may be interesting for transmitting quantum-encrypted messages between multiple participants,” said Fahri Emre Öztürk, a doctoral student at the Institute of Applied Physics at the University of Bonn and lead author on a study describing the development. The optical Bose-Einstein condensate, originally discovered by Weitz, is a “superphoton” composed of several thousand individual light particles. Weitz and his team produced the extreme aggregate state approximately 10 years ago. Particles in this system are no longer distinguishable and are predominantly in the same quantum mechanical state; they behave as a single “superparticle,” which can be described as a single wave function. This system is typically only formed at very low temperatures. On the right is a microscope objective used to observe and analyze the light emerging from the resonator. Courtesy of Gregor Hübl, University of Bonn. The method is still in use today: Light particles are trapped in a resonator composed of two curved mirrors spaced just over a micrometer apart. The mirrors reflect a rapidly reciprocating beam of light within a space filled with a liquid dye solution used to cool down the photons. This happens as a result of the dye molecules “swallowing” the photons and then spitting them out, matching them to the temperature of the dye solution. The somewhat translucent mirrors used in the method causes photons to be lost and replaced, which creates a state of nonequilibrium. With that nonequilibrium, the system isn’t able to maintain a definite temperature, which sets it into oscillation. This creates a transition between the oscillating phase and a damped (decreased amplitude vibration) phase. “The overdamped phase we observed corresponds to a new state of the light field, so to speak,” Öztürk said. The defining 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, meaning 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,” Weitz said. “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.” In future work, the researchers plan to use their findings as a foundation to search for new states of the light field in multiple coupled light condensates, which can also occur in the system. The research was published in Science (www.doi.org/10.1126/science.abe9869).