Lightwaves, which typically oscillate perpendicular to their propagation direction, have been seen oscillating longitudinally when coupled into glass fibers, suggesting that light and matter couple much more strongly than previously thought. Storing light in a bottle can be easy: Light can be coupled into an optical glass fiber in such a way that it does not travel along the fiber but rather spirals around it in a bulged, bottlelike section. In such a bottle microresonator light can be stored for about 10 ns, corresponding to 30,000 revolutions around the fiber — long enough to enable interactions between the light and single atoms, which are brought very close to the fiber surface. In atom-physics experiments conducted at the Vienna University of Technology (TU Vienna), scientists discovered that in this situation, light and matter couple more strongly than previously expected. This surprising result stems from an exceptional property of light inside such microresonators: It oscillates in the longitudinal direction. A transversal wave transfers light into a glass fiber, where it is stored in a bottle microresonator. Atoms close to the fiber couple to the lightwave. Images courtesy of TU Vienna. “Initially, we were really surprised. It was already known before that light can oscillate longitudinally, but, up to now, no one considered its importance in the context of light-matter interactions in microresonators,” said professor Arno Rauschenbeutel of the Vienna Center for Quantum Science and Technology at TU Vienna’s Institute of Atomic and Subatomic Physics. Lightwaves can oscillate in a fixed direction or twist like a corkscrew, but for plane lightwaves, this oscillation is always transversal, or perpendicular to its propagation direction. “One can picture this like the propeller of an aircraft: its rotation is always perpendicular to the direction of motion,” Rauschenbeutel said. “However, the light confined in our bottle microresonator also has a longitudinal component, oscillating along the propagation direction. Thus, the light wave rather resembles the rotor of a horizontally flying helicopter. While the tips of the aircraft propeller trace out a corkscrew-like path through space, the trajectories of the tips of the helicopter rotor describe a more complicated trajectory — a so-called cycloid.” In a glass fiber, light is captured in a bulge in the fiber. It cannot escape because the diameter of the fiber decreases on both sides. Oscillation direction is an important factor for determining the behavior of lightwaves. In the bottle microresonator, light travels clockwise and counterclockwise around the fiber. If the polarization of the two counter-propagating lightwaves is transverse, they will enhance each other at certain locations while canceling out in others. “It is this destructive interference which limits the coupling strength between the light waves and the atoms around the glass fiber,” Rauschenbeutel said. However, if the two lightwaves also oscillate along the direction of propagation, their oscillation states will inevitably differ. As a result, a complete cancellation of counter-propagating beams by destructive interference is not possible. The findings could provide a new twist to research concerned with longitudinally oscillating lightwaves in very different scientific fields: Even a focused laser beam in free space has a longitudinal component. “Most importantly for our research, we now understand that our experimental method works much better than expected,” Rauschenbeutel said. “We realize a very strong coupling between light in the glass fiber and single atoms that are situated very close to the fiber.” The polarization of the lightwave has a longitudinal component — therefore, the polarization direction resembles the tips of the rotor of a flying helicopter. This opens up the possibility of constructing sensors sensitive enough to detect single atoms with light. Additionally, the bottle microresonators could serve as the ideal tool for studying the fundamental properties of light-matter interactions. The scientists’ are now working to realize a router for light that is controlled by a single atom and switches light between two output ports. Such a quantum-mechanical router could be used for interconnecting future quantum computers in optical fiber networks. The findings were reported in Physical Review Letters (doi: 10.1103/PhysRevLett.110.213604). For more information, visit: www.tuwien.ac.at/en