Researchers from the Max Planck Institute for the Science of Light and Leibniz University Hannover have reported an achievement that could fundamentally change the approach to building quantum computing systems. For the first time in the history of experimental physics, scientists have managed to establish strong coupling between light and acoustic vibrations directly in a standard optical fiber — without using microresonators, which until now have been the only tool for realizing such interaction. According to the authors of the study, the result opens the way to simplifying the architecture of quantum processors and, most importantly, to solving one of the most pressing problems of quantum platforms: creating efficient quantum memory.
Physics of the Process: Stimulated Brillouin Scattering in Fiber
The experiment is based on the process of inelastic light scattering by acoustic phonons, known in physics as stimulated Brillouin scattering. In this process, acoustic waves change the refractive index of the medium through which both they and the light propagate simultaneously, thereby affecting the photons. In turn, photons exchange energy with phonons — and energy, as is well known, carries information. The key difficulty faced by the researchers was that light passes through ordinary optical fiber at a tremendous speed and interacts with the acoustic wave only in a limited section, effectively just once. In microresonators, by contrast, light passes through the same region millions or tens of millions of times, which ensures the strong coupling regime. The scientists managed to overcome this limitation by proving that even a single passage of light through a section of fiber where it 'collides' with a sound wave is sufficient to record strong interaction.
Experiment at 4 K: How Strong Coupling Was Recorded
The experiment was conducted by cooling the optical fiber to a temperature of 4 K, which corresponds to approximately −269 °C. The researchers passed light through the cooled fiber and recorded a characteristic gap in the spectral response of the system. The presence of such a gap is a diagnostic sign of the strong interaction regime between light and acoustic vibrations — in microresonators, a similar regime manifests in the exact same way. Thus, the obtained spectral response confirmed that it was possible to reproduce conditions in standard fiber that were previously achievable only in specially engineered resonator structures.
Why This Simplifies Quantum Computing Architecture
Abandoning microresonators in favor of standard optical fiber fundamentally changes the engineering picture. Optical fiber is already an existing telecommunications infrastructure operating at the scale of macroscopic objects and magnitudes. If a section of fiber connecting, say, two quantum computers in adjacent laboratory rooms is capable of performing the functions of a quantum processor or a memory buffer, the need for complex resonator structures disappears. The strong interaction of light and sound in such a fiber segment occurs before decoherence takes place, making it possible to use it both for processing quantum signals and for their temporary buffering. Acoustic waves propagate significantly slower than the optical or microwave signals used in quantum processors, and quantum memory remains one of the most challenging bottlenecks of modern quantum platforms.
Prospects: From Signal Conversion to Quantum Physics in the Macroscopic World
The obtained result lays the foundation for further research into quantum signal processing using the proposed method. In the future, the interaction of light with acoustic vibrations will make it possible to convert signals between optical frequencies and the microwave range, in which many quantum computing devices operate. In other words, the microwave signal at the output of a quantum processor could be converted directly into an optical one inside the fiber and then transmitted using standard telecommunication methods. In addition, the new experimental platform opens up the possibility of studying the quantum properties of mechanical vibrations in relatively massive objects such as optical fibers. This essentially brings quantum phenomena into the world of classical devices and makes quantum science accessible to a wider circle of researchers who do not require specialized resonator infrastructure.
