A highly sensitive method that requires specially prepared light waves coupled to ultrathin glass fibers can be used to count and interact with a very small number of atoms, making extremely sensitive detectors possible. The glass nanofibers used for the experiment are only 500 nm thick – thinner than the wavelength of visible light. The light wave does not really fit into the glass fiber but, instead, sticks out a little, which is the big advantage in the new method. The light wave actually touches atoms that are located out 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,” said professor Arno Rauschenbeutel of the Vienna Center for Quantum Science and Technology. The center is a joint initiative of the University of Vienna, the Vienna University of Technology and the Austrian Academy of Sciences. The light wave sent through the glass fiber is then modified by each individual atom it passes. By measuring changes in the light waves, the number of atoms trapped near the fiber can be determined. Typically, when scientists study the interaction of atoms and light, they look at 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. The fiber protrudes from the inside of the fiber and is influenced by the atoms that are attached top and bottom just outside the fiber. Courtesy of ©TU Vienna. In the glass fiber experiments, however, a very soft interaction between light and atoms is sufficient. “The atoms close to the glass fiber decelerate the light very slightly,” Rauschenbeutel said. When the light wave oscillates precisely upward and downward 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 the relative shift from their different speed is measured. This shift tells the scientists how many atoms have delayed the light wave. Hundreds or thousands of atoms can be trapped less than 1000th of a millimeter away from the glass fiber; their number can be determined with an accuracy of several atoms. The new method enables the researchers to detect as few as 10 or 20 atoms. We are working on a few more technical tricks, such as the reduction of the distance between the atoms and the glass fiber,” Rauschenbeutel said. “If we can do this, we should even be able to reliably detect single atoms.” The new glass fiber measuring method is important not only for new detectors but also for basic quantum physical research efforts. “Usually, the quantum physical state of a system is destroyed when we measure it,” Rauschenbeutel said. “Our glass fibers make it possible to control quantum states without destroying them.” The research project was carried out in collaboration with Johannes Gutenberg University in Mainz, Germany. The work was published in Physical Review Letters (doi: 10.1103/PhysRevLett.107.243601).