Bose-Einstein condensation of photons: New states of light in open environments
A short video on how a Bose-Einstein condensate (BEC) of light is created by Physicists of the University of Bonn, and on how its environment causes new states of light [by Julian Schmitt, University of Bonn]

== Deutsche Pressemitteilung ==
https://www.uni-bonn.de/news/074-2021

== Video Content ==
The transition between water and ice is a phenomenon that is called a phase transition in physics. In our work we also observe a phase transition but here between two different states of light. We observe an oscillating and an overdamped state of a Bose-Einstein condensate (BEC) of photons, which is revealed only because of the openness of the system. For the experiment we use two highly reflecting mirrors, which are spaced by only a micrometer. The space is filled by a liquid dye solution that absorbs and emits light, which leads to a thermalization for the photons at room temperature. At large photon numbers, the Bose-Einstein condensate of photons emerges. The basis for our new observation is the openness of the system, which we can control extremely well. First, photons are lost from the cavity, which we compensate by pumping the dye. Secondly, we couple the photons to a large reservoir. This leads to strong fluctuations back and forth between condensate and reservoir. This is a very crucial ingredient to observe the phases. Essentially, we probe the response of the system to these fluctuations. What we find is exciting - the BEC does not always behave in the same way: While for large condensates, we find an oscillatory response dynamics, for very small condensates, the dynamics behaves like a biexponential decay. The transition between the dynamical phases occurs exactly at an exceptional point, which was also predicted by the theoretical model developed within this theory-experiment-collaboration. The observation of a non-Hermitian phase transition opens up new research directions, as the investigation of multiple coupled photon condensates in novel phases that emerge only because the system is open.


== Press release ==
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. The state is called optical Bose-Einstein condensate and has captivated many physicists ever since, because this exotic world of light particles is home to its very own physical phenomena. Researchers led by Prof. Dr. Martin Weitz, who discovered the super photon, and theoretical physicist Prof. Dr. 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 a so-called overdamped phase. The results may in the long term be relevant for encrypted quantum communication. The study has been published in the journal Science.

More: https://www.uni-bonn.de/news/074-2021

== Publication ==
Fahri Emre Öztürk, Tim Lappe, Göran Hellmann, Julian Schmitt, Jan Klaers, Frank Vewinger, Johann Kroha & Martin Weitz: Observation of a Non-Hermitian Phase Transition in an Optical Quantum Gas. Science, DOI: 10.1126/science.abe9869

== University of Bonn ==
Video: University of Bonn / Sebastian Eckert