A Pause Button for Light

With this experiment TU Darmstadt physicist Thorsten Peters is stopping light for tiny fractions of a second. At the push of a button he lets the light pulse continue its journey. (Source: K. Binner, TU Darmstadt)

A team led by physicists Thorsten Peters and Thomas Halfmann from the TU Darmstadt is stopping light for tiny fractions of a second. They then end the stopover at the push of a button letting the light pulse continue its journey. The researchers are even stopping indi­vidual light particles. What sounds like a physical gimmick, could be of use for future appli­cations. Quantum tech­nology attempts to use bizarre effects of quantum physics for faster computers, more precise sensors and bug-proof communi­cations. Photons, which are used in quantum tech­nology as information carriers, play a decisive role in this.

To this end, physicists, for example, require light sources that emit individual photons at the push of a button. To process the infor­mation stored on light particles, it would also be important for indi­vidual photons to interact, which they do not usually do. In future quantum computers, photons will for example have to transfer their infor­mation to atoms and vice versa. To this end too, the inter­action between the two types of particles must be intensified, which the photons stopped by the group could make possible.

For some time now it has been possible to freeze photons and re-emit them on command. However, whilst they are stopped, the photons do not exist as such. They are swallowed by an atomic cloud, which then assumes a excited state and stores the photon as infor­mation. Only upon receipt of a signal does the excitation change back into a photon, which then continues on. The researchers in Darmstadt are doing it in a similar manner, but with one crucial difference: their photons are actually preserved.

The light literally stands still. The team uses a special glass fiber with a hollow channel in the centre with a diameter of less than ten thou­sandths of a millimetre. The fiber has a porous structure round the core that keeps light at bay. This causes a laser beam to concen­trate in the centre of the hollow channel. Its cross-section narrows to around one thousandth of a millimetre. The researchers use the light beam as a kind of trap for atoms. They introduce atoms of rubidium into the hollow fibre, which concen­trate in the centre of the laser beam due to electro­magnetic forces. The researchers then send the photons they want to stop into the channel. Roughly speaking, the photon is brought to a complete stop by two addi­tional laser beams that are guided into the hollow fiber on both sides.

“It is also similar to a chamber in which light is thrown back and forth between two mirrors,” as Thorsten Peters explains. “Just without a mirror.” The team is the first to succeed in slowing down photons in such a narrow capillary in this way and it was not easy. It is made extremely complicated by bire­fringence. The team was able to refine their method through a laborious bire­fringence analysis to the point where stopping individual photons became possible. But simply stopping light itself they did not satisfy themselves. “Our objective,” says Peters, “was to make photons interact with atoms more strongly than they normally do.”

In particular, it should be possible for two light particles to interact with an atom at the same time, which would produce nonlinear optics in which photons pene­trate a medium, such as a special crystal. When two photons simul­taneously strike one of the atoms in the crystal, they interact with one another, which changes the frequency of the light. The new frequency could, for example, be the sum of the frequencies of the photons that are sent in.

There are many technical applications for such effects, for example in laser pointers. The method does have one disad­vantage: high intensity lasers are needed to guarantee that enough pairs of photons strike an atom within the medium simul­taneously. “With our method, on the other hand,” says Peters, “a weak light intensity may be sufficient.” This is possible because the atoms are confined to the same narrow area as the laser beam within the hollow fibre, thus maximising the contact between the light and the atomic cloud. Therefore the proba­bility of two photons hitting an atom simul­taneously is rela­tively high even when the light intensity is low. So the same technical trick that makes it possible to stop the photons should also create a new method for nonlinear optics.

The team has more ideas for how to apply his new process. One of these involves a switchable source for single photons. Another is to create a crystal made of photons. A large number of stopped photons could also form an ordered grid. “We could use this to simulate a solid,” says Peters. The physics of solid materials is an active field of research. Theo­retical models are used in research to gain a better under­standing of them – often through computer simu­lations. But the models are so complex that they quickly overwhelm the computers. Researchers are therefore looking for other ways to imitate crystals. A simulated solid made of photons would be one way of doing this.

“We are continuing to work intensively on this,” says Peters. According to the physicist, colla­boration with other research groups is crucial for success. The team achieved the current work in collaboration with groups from Taiwan and Bulgaria within the framework of an EU-funded project. Industrial partners are also involved in the research project, whose objective is to develop innovative techno­logies for the inter­action of light with matter. (Source: TU Darmstadt)

Reference: T. Peters et al.: Single-photon-level narrowband memory in a hollow-core photonic bandgap fiber, Opt. Exp. 28, 5340 (2020); DOI: 10.1364/OE.383999

Link: Nonlinear Optics/Quantum Optics, Technical University of Darmstadt, Darmstadt, Germany

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