A One-Way Street for Light

The researchers in the laboratory of the Institute of Applied Physics at the University of Bonn. (Source: V. Lannert, U. Bonn)

Light can be directed in different direc­tions, usually also back the same way. Physicists from the University of Bonn and the Univer­sity of Cologne have however succeeded in creating a new one-way street for light. They cool photons down to a Bose-Einstein conden­sate, which causes the light to collect in optical valleys from which it can no longer return. The findings from basic research could also be of interest for the quantum communi­cation of the future.

A light beam is usually divided by being directed onto a partially reflecting mirror: Part of the light is then reflected back to create the mirror image. The rest passes through the mirror. “However, this process can be turned around if the experi­mental set-up is reversed,” says Martin Weitz from the Institute of Applied Physics at the Univer­sity of Bonn. If the reflected light and the part of the light passing through the mirror are sent in the opposite direction, the original light beam can be recon­structed.

Together with his team and Achim Rosch from the Institute for Theo­retical Physics at the University of Cologne, Weitz was looking for a new method to generate optical one-way streets by cooling the photons: As a result of the smaller energy of the photons, the light should collect in various valleys and thereby be irreversibly divided. The physicists used a Bose-Einstein conden­sate made of photons for this purpose, with which Weitz made a name for himself in 2010 because he was the first to create such a super-photon.

A beam of light is thrown back and forth between two mirrors. During this process, the photons collide with dye molecules located between the reflecting surfaces. The dye molecules swallow the photons and then spit them out again. “The photons acquire the tempera­ture of the dye solution,” says Weitz. “In the course of this, they cool down to room tempera­ture without getting lost.” By irradiating the dye solution with a laser, the physicists increase the number of photons between the mirrors. The strong concen­tration of the light particles combined with simul­taneous cooling causes the individual photons to fuse to form a Bose-Einstein conde­nsate.

The current experiment by the team of physicists from Bonn and Cologne worked in accordance with this principle. However, one of the two mirrors was not completely flat, but had two small optical valleys. When the light beam enters one of the indents, the distance, and therefore the wavelength, becomes slightly longer. The photons then have a lower energy. These light particles are cooled by the dye molecules and then pass into a low-energy state in the valleys.

However, the photons in the indents do not behave like marbles rolling over a corrugated sheet. Marbles roll into the valleys of the corru­gated sheet and remain there, separated by the peaks. “In our experiment, the two valleys are so close together that a tunnel coupling occurs,” reports Christian Kurtscheid from the Weitz team. It is therefore no longer possible to dete­rmine which photons are in which valley. “The photons are held in the two valleys and enter the lowest energy state of the system,” explains Weitz. “This irre­versibly splits the light as if it were passing through an intersection at the end of a one-way street, while the light waves remain in lockstep in different indents.”

The scientists hope that this experi­mental arrange­ment will make it possible to produce even more complex quantum states that allow the gene­ration of inter­laced photonic multi-particle states. “Perhaps quantum computers might one day use this method to communi­cate with each other and form a kind of quantum Internet,” says Weitz with a view towards the future. (Source: U. Bonn)

Reference: C. Kurtscheid et al.: Thermally condensing photons into a coherently split state of light, Science 366, 894 (2019); DOI: 10.1126/science.aay1334

Link: Quantum Optics (M. Weitz), University of Bonn, Bonn, Germany

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