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Physicists Observe New Phase in Quantum Condensate of Light | Physics

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Physicists at the University of Bonn have experimentally observed a new, previously unknown phase in the photon Bose-Einstein condensate.

The optical microresonator filled with dye solution (yellow); on the right is a microscope objective used to observe and analyze the light emerging from the resonator. Image credit: Gregor Hübl / University of Bonn.

The optical microresonator filled with dye solution (yellow); on the right is a microscope objective used to observe and analyze the light emerging from the resonator. Image credit: Gregor Hübl / University of Bonn.

The Bose-Einstein condensate is a gas of atoms so dense and cold that their matter waves lose their individuality and condense into a ‘superatom wave.’

It was predicted in the 1920s by Satyendra Nath Bose and Albert Einstein and was eventually created in the lab in the 1990s at the University of Colorado Boulder, MIT and Rice University using laser cooling and evaporative cooling techniques.

In 2010, Professor Martin Weitz from the Institute of Applied Physics at the University of Bonn and colleagues created for the first time a Bose-Einstein condensate from photons.

“Our special system is still in use today: we 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,” they said.

“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.”

In their new experiments, the physicists observed a so-called overdamped phase in the photon Bose-Einstein condensate coupled to the environment.

“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,” they explained.

“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,” added team member Fahri Emre Öztürk, a doctoral student at the Institute of 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 we 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,” Professor 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.”

The team’s work was published in the journal Science.

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Fahri Emre Öztürk et al. 2021. Observation of a non-Hermitian phase transition in an optical quantum gas. Science 372 (6537): 88-91; doi: 10.1126/science.abe9869

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