Physicists Get Light and Sound to Strongly Couple in Ordinary Optical Fiber for First Time
A German team saw the signature gap of strong coupling in a single pass of light through fiber cooled to 4 kelvin, with no cavity needed.
Physicists have achieved strong coupling between light and long-lived sound vibrations in an optical fiber for the first time, a result that could turn the cables that carry the internet into platforms for quantum devices. The work, from the Max Planck Institute for the Science of Light and Leibniz University Hannover, appears in Nature Physics.
Strong coupling means two systems swap information faster than noise can destroy it. It is a prerequisite for using mechanical vibrations to process quantum information, for example to link quantum computers that operate at gigahertz frequencies through optical connections at terahertz frequencies. Acoustic waves travel far slower than light, so they can also act as a buffer, temporarily storing information as it moves.
Until now, researchers reached strong coupling only in optomechanical cavities, which the team likens to racetracks for light. Light circles such a cavity up to a million times, giving it many chances to interact with vibrations along the way. An optical fiber is more like a highway: signals speed in one direction and pass each vibration just once. Reaching strong coupling in a single pass has been extremely difficult.
The team, led by Birgit Stiller, did it by cooling a fiber to 4 kelvin, a moderate cryogenic temperature, and using stimulated Brillouin scattering, an effect in which light interacts with sound waves in the glass. In one pass, the light-sound interaction came out much stronger than the losses from optical and acoustic noise. No optical resonator or mechanical resonator was involved.
The evidence is a gap in the measured spectrum. Known as avoided crossing, it normally shows up only in resonator setups. "We have introduced a new measurement technique that allows us to observe the clearest sign of strong coupling: precisely this gap in the spectral response, which is otherwise only visible in resonators," said first author Laura Blázquez Martínez.
Stiller called the result "a crucial step toward utilizing optical fibers as a powerful platform for quantum interactions," and said it opens the way to quantum measurements on the same type of platform that underpins most of the telecommunications system. Fibers are also far more massive than the tiny resonators typically used in these experiments, which gives researchers a new way to probe where the line between classical and quantum behavior lies for large objects.
The paper, "Cavity-less Brillouin strong coupling in a solid-state continuous system," is published in Nature Physics (DOI 10.1038/s41567-026-03458-2). The experiment ran at 4 K, so a practical device would still need cooling, but the demonstration shows the effect exists without building a cavity around the fiber.
