Can light be caught between two mirrors

Pause button for light particles

How do you stop something that is faster than anything else, intangible and always in motion by nature? A team led by physicists Dr. Thorsten Peters and Professor Thomas Halfmann do what seems impossible. They stop light for tiny fractions of a second. At the push of a button they end the intermediate stop so that the light pulse continues on its journey. The researchers even stop individual light particles.

What sounds like a physical gimmick could be useful for future applications. So-called quantum technology tries to use bizarre effects of quantum physics for faster computers, more precise sensors or secure communication. Photons play a crucial role in this. They are used as information carriers in quantum technology. In physics, for example, you need light sources that emit individual photons at the push of a button. In order to process the information stored on light particles, it would also be important if individual photons were to interact with one another. They usually don't. In the quantum computers of the future, light particles will transfer their information to atoms and vice versa. For this, too, the interaction between the two types of particles must be intensified. The photons stopped by the group from TU Darmstadt could make that possible.

How does the emergency stop for lights work? It has been possible to freeze photons and send them off again on command for a long time. During the stopover, however, the light particles do not exist as such. They are swallowed up by an atomic cloud, which in the process assumes a so-called excited state. This stores the photon as information, as it were. Only in response to a signal does the excitation change back into a light particle, which now flies on. The Darmstadt researchers do it in a similar way, but with one major difference: they retain the photons as such.

The light literally stands still. The Darmstadt team uses a special glass fiber. This has a hollow channel in the middle less than ten thousandths of a millimeter in diameter. Around the core, the fiber has a porous structure that keeps light away from it. As a result, a laser beam is concentrated in the center of the hollow channel. Its cross-section narrows to around a thousandth of a millimeter. The researchers use the bundle of light as a kind of trap for atoms. They bring rubidium atoms into the hollow fiber. Due to electromagnetic forces, these collect in the center of the laser beam. Now the experts send the photons that they want to stop into the channel. Roughly speaking, the full braking is achieved by two additional laser beams that are guided into the hollow fiber on both sides. Metaphorically speaking, these hold the photons trapped between them, like two soccer players kicking the ball back and forth.

Use "non-linear optics"

"It is also similar to a chamber in which light is thrown back and forth between two mirrors," explains Thorsten Peters. “Just without a mirror.” The TU team is the first to have succeeded in slowing down photons in such a narrow capillary in this way. It wasn't easy. An optical property called “birefringence” makes it very complicated. By means of an extensive analysis of the birefringence, the team was able to refine its method in such a way that it succeeded in stopping individual photons.

But they are not satisfied with stopping the light itself. “Our goal was for photons to interact more strongly with atoms than they normally would,” says Peters. In particular, it should be possible for two light particles to interact with an atom at the same time. That would create a useful phenomenon called "nonlinear optics" in physics. Photons penetrate a medium such as a special crystal. When two light particles hit one of the atoms of the crystal at the same time, they interact with each other. This changes the frequency, i.e. the color, of the light. The new frequency can, for example, be the sum of the frequencies of the incident photons.

Crystal made of light particles

There are many technical applications of such effects, for example in laser pointers. The method has one disadvantage: to guarantee that enough pairs of photons hit an atom in the medium simultaneously, you need high-intensity lasers. “With our method, on the other hand, a weak light intensity could be sufficient,” says Peters. This works because the atoms are squeezed into the same narrow area in the hollow fiber as the laser beam. The light thus has maximum contact with the atomic cloud. The probability that two photons hit an atom at the same time is therefore relatively high even with a low light intensity. The same technical trick that enables the light particles to be stopped is intended to create a new method for non-linear optics.

The Darmstadt team has more ideas for using its new process. A switchable source for single photons is one of them. Another: a crystal made of light particles. Crystals normally consist of atoms arranged absolutely regularly, comparable to spheres stacked on top of one another. A large number of stopped light particles could also form an ordered grid. “We could use it to simulate a solid,” says Peters. The physics of solid materials is an active area of ​​research. In research, theoretical models are used to understand them - often through simulation in computers. But the models are so complex that they can quickly overwhelm computers. Therefore, researchers are looking for ways to mimic crystals in other ways. A simulated solid body made of light particles would be a kind.

“We are still working intensively on it,” says Peters. Cooperation with other research groups is crucial for success, says the physicist. The team achieved the current work together with groups from Taiwan and Bulgaria as part of an EU-funded project. Partners from industry are also involved in the research project, which aims to develop innovative techniques for the interaction of light with matter. "The exchange is very lively," says Peters happily. The next successes will not be long in coming.

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