A single ‘superphoton’ made up of thousands of individual light particles: About ten years ago, researchers at the University of Bonn first produced such an extreme total state and presented a completely new light source. The state is called optical Bose-Einstein condensate and has since fascinated many physicists because this exotic world of light particles is home to its own physical phenomena.
Researchers led by prof. Dr. Martin Weitz, who discovered the superphoton, and the theoretical physicist prof. Dr. Johann Kroha, returned to the quantum world with a very special observation of their latest “expedition”. They report a new, previously unknown phase transition in the optical Bose-Einstein condensate. This is a so-called excessive phase. The results may be relevant to encrypted quantum communications in the long run. The study was published in the journal Science.
The Bose-Einstein condensate is an extreme physical condition that usually occurs only at very low temperatures. What is special: the particles in this system can no longer be distinguished and are essentially in the same quantum mechanical state, in other words, they behave like a single ‘superparticle’. The condition 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 used today: Physicists capture light particles in a resonator made of two curved mirrors that are just over a micrometer apart, reflecting a fast and recurring beam of light. The space is filled with a liquid dye solution that cools 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 – equal to room temperature. Background: The system makes it possible to cool light particles in the first place, because their natural property is to dissolve when cooled.
To the right is a microscope objective used to observe and analyze the light emanating from the resonator. Credit: © Gregor Hübl / Uni Bonn
Clear separation of two phases
Phase transition Physicists call the transition between water and ice during freezing. But how does the specific phase transition take place within the system of trapped light particles? The scientists explain it this way: The somewhat translucent mirrors cause photons to be lost and replaced, resulting in a non-equilibrium, which results in the system not assuming a definite temperature and being set in oscillation. This creates a transition between this oscillating phase and a damped phase. Attenuated means that the amplitude of the vibration decreases.
“The too-attenuated phase corresponds, so to speak, to a new state of the light field,” says lead author Fahri Emre Öztürk, a doctoral student at the Institute of Applied Physics at the University of Bonn. The special feature is that the effect of the laser is usually not separated by the phase transition of the Bose-Einstein condensate, and that there is no sharply defined boundary between the two conditions. This means that physicists can constantly move back and forth between effects.
with the optical setup at the measuring table at the Institute of Applied Physics at the University of Bonn. Credit: © Gregor Hübl / Uni Bonn
“In our experiment, however, the attenuated state of the Bose-Einstein condensate optical is separated by a phase transition from both the oscillating state and a standard laser,” says study leader, prof. Dr. Martin Weitz. ‘It shows that there is a Bose-Einstein condensate, which is actually a different condition 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 look for new conditions of the light field in multiple coupled light condensates, which may also occur in the system. “If suitable quantum mechanically entangled conditions occur in coupled light condensates, it may be interesting to transmit quantum encrypted messages between multiple participants,” says Fahri Emre Öztürk.
Prof. dr. Martin Weitz, dr. Julian Schmitt, dr. Frank Vewinger, prof. Dr. Johann Kroha and Göran Hellmann from the Institute of Applied Physics at the University of Bonn. Credit: © Gregor Hübl / Uni Bonn
Reference: “Observation of a non-Hermetic phase transition in an optical quantum gas’ by Fahri Emre Öztürk, Tim Lappe, Göran Hellmann, Julian Schmitt, Jan Klaers, Frank Vewinger, Johann Kroha and Martin Weitz, 2 April 2021, Science.
DOI: 10.1126 / science.abe9869
The study received funding from the Collaborative Research Center TR 185 “OSCAR – Control of Atomic and Photonic Quantum Matter at Tailored Coupling to Reservoirs” from the universities of Kaiserslautern and Bonn and the Cluster of Excellence ML4Q from the universities of Cologne, Aachen, Bonn and the Jülich Research Center, funded by the German Research Foundation. The cluster of excellence is embedded in the Transdisciplinary Research Area (TRA) “Building Blocks of Matter and Fundamental Interactions” of the University of Bonn. In addition, the study was funded by the European Union within the “PhoQuS – Photons for Quantum Simulation” project and the German Aviation Center with funding from the Federal Ministry of Economic Affairs and Energy.