Pulses of Light Could Behave Like Strange Gases – ScienceDaily

In our modern society, huge amounts of data are transmitted every day, primarily in the form of short pulses of light propagated through glass fibers. With the steadily increasing intensity of these optical signals, their interaction grows, which can lead to data loss. Physicists at Friedrich Schiller University Jena and the School of Optics and Photonics in Orlando, Florida, are studying how to control large numbers of light pulses as precisely as possible to minimize the impact of such interactions. To this end, they observed a collection of light pulses as they propagated through an optical fiber and found that they followed immutable rules—albeit mainly those of thermodynamics.

In a recent issue of the magazine SciencesA team led by Professor Dr Ulf Peschel reported measurements of a series of pulses traveling thousands of kilometers through glass fibers no more than a few microns thick. The researchers were surprised by the results. “We found that the pulses of light organize themselves after about a hundred kilometers, and then behave like molecules of a conventional gas, like air, for example,” says Professor Ulf Peschel, head of the group in Jena. In a gas, particles move back and forth at different speeds, but they still have an average speed that is determined by their temperature. Although pulses of light propagate through glass fibers at an average speed of about 200,000 kilometers per second, not all are the same speed. “The statistical distribution of their velocities is exactly the same as that of a conventional gas at constant temperature,” Peschel says.

As the researchers have now shown for the first time in their latest publication, this photon gas can be cooled, for example, through a process known as thermal expansion. As in a real gas, the velocity differences of the molecules decrease during cooling and the order in the signal sequence spontaneously increases. When the absolute temperature of zero K is reached, all the pulses propagate at exactly the same speed.

The reverse process is also possible. “When the optical gas is heated, the velocities increase,” Peschel explains. If all pulse velocities occur equally, the disorder is maximum and the temperature is infinite—a state that cannot be reached in a real gas because it requires an infinite amount of energy. “By contrast, periodic modulation of the refractive index can limit the range of pulse velocities allowed in glass fibres. In this way, all available velocity states can be excited equally, resulting in a photon gas of infinite temperature. In addition, the Preferentially moderating extreme velocity states – photon gas gets hotter than infinite heat.

“For this state, which has been described only theoretically for light, a temperature less than absolute zero is mathematically assumed,” Peschel says. He and his colleagues have now managed to create such a photonic gas with negative temperatures, and have shown for the first time that it obeys the classical laws of thermodynamics. “Our results will contribute to a better understanding of the collective behavior of large ensembles of optical signals. If we take the laws of thermodynamics into account, we can make optical data transmission more robust and reliable, for example by structuring the distributions of pulses to best match the thermal distributions.”

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