Glass fiber cables are indispensable for the internet—now they can also be used as a quantum physics lab. The Vienna Univ. of Technology is the only research facility in the world, where single atoms can be controllably coupled to the light in ultra-thin fiber glass. Specially prepared light waves interact with very small numbers of atoms, which makes it possible to build detectors that are extremely sensitive to tiny trace amounts of a substance. Prof. Arno Rauschenbeutel’s team, one of six research groups at the Vienna Center for Quantum Science and Technology, has presented this new method in the journal Physical Review Letters. The research project was carried out in collaboration with the Johannes Gutenberg Univ. in Mainz, Germany.

The glass fibers used for the experiment are only five hundred millionths of a millimeter thick (500 nm). In fact, they are even thinner than the wavelength of visible light. “Actually, the light wave does not really fit into the glass fiber, it sticks out a little,” Rauschenbeutel explains. And this is precisely the big advantage of the new method: the light wave touches atoms which are located outside of, but very close to, the glass fiber. “First, we trap the atoms, so that they are aligned above and below the glass fiber, like pearls on a string,” says Rauschenbeutel. The light wave sent through the glass fiber is then modified by each individual atom it passes. By measuring changes in the light waves very accurately, the number of atoms trapped near the fiber can be determined.

When scientists study the interaction of atoms and light, they usually look at rather disruptive effects—at least on a microscopic scale: atoms can, for example, absorb photons and emit them later in a different direction. This way, atoms can be accelerated and hurled away from their original position. In the glass fiber experiments at Vienna UT however, a very soft interaction between light and atoms is sufficient. “The atoms close to the glass fiber decelerate the light very slightly,” Rauschenbeutel explains. When the light wave oscillates precisely upwards and downwards in the direction of the atoms, the wave is shifted by a tiny amount. Another light wave oscillating in a different direction does not hit any atoms and is therefore hardly decelerated at all. Light waves of different polarization directions are sent through the glass fiber—and their relative shift due to their different speed is measured. This shift tells the scientists how many atoms have delayed the light wave.