For the first time, physicists have created a one-dimensional gas made of pure light, and they want to use it to study how photons, or particles of light, behave on a quantum level.
The scientists created the new state of matter, called a photon gas, by firing a laser at a reflective container filled with dye, causing the photons in the beam to cool and eventually condense. The researchers published their findings on September 6 in the journal Physics of Nature.
“To create these types of gases, we need to concentrate many photons in a confined space and cool them simultaneously,” study senior author Frank Vewingera physicist at the University of Bonn, told a statement.
Photons are bosons, particles with integer spin, meaning they can occupy the same state and space at a given time. When a boson gas is cooled to almost zero temperature, all its particles lose energy, entering the same energy states.
Since we can only distinguish similar particles in a cloud of gas by looking at their energy levels, this equivalence has profound implications: The once-separate cloud of quivering, moving, colliding particles forming a hotter gas then became, from a quantum mechanics point of view, perfectly identical, creating an elusive form of matter called a Bose-Einstein condensate.
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Existence in a condensate form causes the positions of particles within a gas to be highly uncertain. As a result, the areas each particle can potentially occupy grows larger in area than the spaces between the particles themselves. Rather than discrete objects, then, the overlapping photons in a photon gas behave like just one giant particle.
Physicists have created photon gases in two dimensions before. But doing them in just one is trickier.
“Things are quite different when we create a one-dimensional gas instead of a two-dimensional one,” Vewinger said. “So-called thermal fluctuations occur in photon gasses but they are so small in two dimensions that they have no real effect. However, in one dimension these changes can – figuratively speaking – create large waves.”
To create a one-dimensional photon gas, the researchers filled a miniscule, reflective container with a dye solution before firing a laser at it. Photons of laser light bounce back and forth inside the container until they collide with the dye molecules, robbing them of their energy and causing them to clump together.
By coating the container’s reflective walls with a transparent polymer, the researchers were able to tweak the way they reflected light so that it was effectively condensed into one dimension — or a line.
“These polymers act like a kind of channel, but in this case for light,” the lead author Kirankumar Karkihalli Umesha doctoral student at the University of Bonn, said in the statement. “The narrower this gutter is, the more one-dimensional the gas moves.”
By studying their newly created 1D photon gas, the researchers confirmed that it behaves quite differently from its 2D form. Unlike in 2D photon gasses, the thermal fluctuations of their 1D cousins ​​prevent them from condensing completely in certain regions. This creates a partial phase transition between the laser light and its condensate form that is “smeared” throughout the gas, like frozen water that isn’t completely frozen, according to the researchers.
Investigating how the photon gas varies in size could help researchers discover previously undiscovered quantum optical effects, the researchers said.
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