A single “super photon” made up of several thousand individual particles of light: About ten years ago, researchers at the University of Bonn first produced such an extreme state of aggregation and presented a whole new light source. The state is called Bose-Einstein optical condensate and has captivated many physicists since, because this exotic world of light particles harbors its own physical phenomena.
The researchers led by Professor Martin Weitz, who discovered the super photon, and theoretical physicist Professor Johann Kroha, returned from their last “expedition” into the quantum world with a very special observation. They report a new previously unknown phase transition in the Bose-Einstein optical condensate. This is a so-called overdamping phase. The results may in the long run be relevant for encrypted quantum communication. The study was published in the journal Science.
Bose-Einstein condensate is an extreme physical state that usually only occurs at very low temperatures. What is special: the particles of this system are no longer distinguishable and are mostly in the same state of quantum mechanics, ie 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. This is done by the dye molecules “swallowing” the photons and then spitting them out again, bringing the light particles to the temperature of the dye solution – equivalent to room temperature. Background: The system primarily cools light particles, as their natural characteristic is to dissolve when cooled.
Clear separation of two phases
The 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 Prof. 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 points out.
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 suitable quantum mechanically entangled states occur in coupled light condensates, this may be of interest for the transmission of quantum encrypted messages between multiple participants,” says Fahri Emre Öztürk.
The study received funding from the Collaborative Research Center TR 185 “OSCAR – Control of Atomic and Photonic Quantum Matter by Tailored Coupling to Reservoirs” at the Universities of Kaiserslautern and Bonn and from the Cluster of Excellence ML4Q at the Universities of Cologne, Aix-la- Chapelle, Bonn and the Jülich Research Center, funded by the German Research Foundation. The pole of excellence is integrated into the transdisciplinary research field (TRA) “Building Blocks of Matter and Fundamental Interactions” of the University of Bonn. In addition, the study was funded by the European Union under the project “PhoQuS – Photons for Quantum Simulation” and the German Aerospace Center with funding from the Federal Ministry of Economic Affairs and Energy.
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