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Bose-Einstein condensates (BECs) are a kind of quantum “magnifying glass” in which atoms lose their identity and behave as a singular quantum wave function.
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Scientists have now devised a method to create a one-dimensional BEC using light, called a “photon gas.”
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A new study reveals that the restriction of one-dimensional space blurs the BECs condensation point, opening up new avenues of researching in quantum optical effects.
Bose-Einstein condensates (BECs), the fifth form of matter—the existence of which was originally predicted in the 1920s, but not observed until the 1990s—helps physicists explore the nature of the quantum world. Because BECs coalesce into a single quantum mechanical entity at temperatures approaching absolute zero, they allow experts to study quantum properties on a macro level. Because of this unique trait, some describe the matter as a quantum “magnifying glass.”
Since their first discovery in 1995, studies have altered BECs into different forms to further probe quantum mechanics in different ways. Earlier this summer, for example, scientists from Columbia University created molecular BECs as a pathway to forming more exotic forms of matter. Now, scientists from the University of Bonn and the University of Kaiserslautern-Landau (RPTU) in Germany have created a one-dimensional gas made of light called a “photon gas,” and the team tested hypothetical predictions about the transition into this exotic phase for the first time. The results were published in the journal Nature Physics.
Of course, to form a one-dimensional BEC made of photons, the scientists needed to get things cold—very, very cold. So, they implemented an ingenious method to form an ultracold gas within the microcavity of two highly reflective mirrors, placed only a micrometer apart (one thousandth of a millimeter). A tiny container of dye solution was then excited using the lasers, and when a photon collided with a dye molecule, those photons were cooled until the matter condensed. By modifying these reflective surfaces, scientists were able to alter a property of the BEC known as dimensionality.
“We were able to apply a transparent polymer to the reflective surfaces to create microscopically small protrusions,” Julian Schulz, a co-author of the study from RPTU, said in a press statement. “These protrusions allow us to trap the photons in one or two dimensions and condense them.”
Scientists know that, in two dimensions, there is a precise temperature at which condensation occurs—similar to how water freezes at a certain temperature. But when things get down to one dimension, things get less clear.
“So-called thermal fluctuations take place in photon gases but they are so small in two dimensions that they have no real impact. However, in one dimension these fluctuations can—figuratively speaking—make big waves,” Frank Vewinger, the senior author of the study from the University of Bonn, said in a press statement. “We have now been able to investigate this behavior at the transition from a two-dimensional to a one-dimensional photon gas for the first time.”
These “fluctuations,” as described Vewinger, disrupt the quantum “oneness” of these one-dimensional systems and cause different regions of the gas begin acting differently—the authors describe this process as the phase transition being “smeared out.” But this new phase of matter is still a “degenerate quantum gas,” meaning its properties are still governed by quantum mechanics. Continuing with the water metaphor, it’s forming icy water at very low temperatures without actually freezing. The condensation point is blurred because the dimensionality restricts the photons’ movements.
“These polymers act like a type of gutter, but in this case for light,” physicist Kirankumar Karkihalli Umesh, from the University of Bonn, said in a news release. “The narrower this gutter is, the more one-dimensionally the gas behaves.”
While the scientists admit that this is just basic research, these gasses could help explore new quantum applications. In other words, BECs continue to be the go-to method for scouting the unknown frontiers of the quantum world.
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