New phase observed in Bose-Einstein condensate of light particles

A new state of light.


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Almost a decade ago, scientists from the University of Born had created a single super photon, an extreme aggregate photon state. This super photon is consists of many thousands of individual light particles. 

That aggregate state is known as optical Bose-Einstein condensate. It has gathered so much interest as this exotic world is home to its very own physical phenomena. 

Prof. Dr. Martin Weitz, who discovered the super photon, and theoretical physicist Prof. Dr. Johann Kroha have reported a new observation: a so-called overdamped phase, a previously unknown phase transition within the optical Bose-Einstein condensate.

In condensed matter physics, a Bose-Einstein condensate (BEC) is the fifth state of matter. The particles in this state behave like a single giant superparticle; hence the state can also be described as a single wave function.

In 2010, scientists were able to create Bose-Einstein condensate using photons. They have had developed a system that is still in use: By trapping photons in a resonator made from curved mirrors. The mirrors were placed, so that was a tiny space between them: about a micrometer. 

Scientists later filled the space with a liquid dye solution, which serves to cool down the photons. These photons are then absorbed by the dye molecules, spitting them out again to bring the photons’ temperature to the same level as the dye solution. This system made it possible to cool down the photons as their natural characteristic is to dissolve when cooled.

In the phase transition process, physicists call the transition between water and ice during freezing. But how does the particular phase transition occur within the system of trapped light particles?

Scientists explained, “The somewhat translucent mirrors cause photons to be lost and replaced, creating a non-equilibrium that results in the system not assuming an actual temperature and being set into oscillation. This creates a transition between this oscillating phase and a damped phase. Damped means that the amplitude of the vibration decreases.”

Lead author Fahri Emre Öztürk, a doctoral student at the Institute for Applied Physics at the University of Bonn, said, “The overdamped phase we observed corresponds to a new state of the light field. The special characteristic is that the laser’s effect 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.”

In this study, the phase transition separates the optical Bose-Einstein’s overdamped state condensate from both the oscillating state and a standard laser.

Prof. Dr. Martin Weitz said, “This shows that there is a Bose-Einstein condensate, which is a different state than the standard laser. In other words, we are dealing with two separate phases of the optical Bose-Einstein condensate.”

Scientists are now looking forward to using this study to explore new states of the light field in multiple coupled light condensates, which can also occur in the system.

Fahri Emre Öztürk said, “If suitable quantum mechanically entangled states occur in coupled light condensates, this may be interesting for transmitting quantum-encrypted messages between multiple participants.”

Journal Reference:

  1. Fahri Emre Öztürk et al. Observation of a non-Hermitian phase transition in an optical quantum gas. DOI: 10.1126/science.abe9869