A research team led by Associate Professor Kaoru Sanaka from the Tokyo University of Science (TUS) has developed a highly efficient fiber-coupled single-photon source. This new method generates single photons directly inside an optical fiber by selectively exciting a single, isolated rare-earth (RE) ion, overcoming high transmission loss issues common in conventional quantum communication systems. The study, which was conducted with third-year Ph.D. candidate Kaito Shimizu and Assistant Professor Tomo Osada from TUS, was published in Optics Express.
The core technical challenge in quantum communication is achieving high coupling and channeling efficiency between an emitter and an optical fiber. Unlike conventional systems where emitters are placed outside the fiber, the TUS team prepared a tapered silica fiber doped with neodymium ions (Nd3+). The heat-and-pull tapering process allowed them to access spatially separated individual Nd3+ ions within the fiber’s guided mode.
Using the selective excitation method, the researchers experimentally validated single-photon generation at room temperature via autocorrelation measurement. The efficiency of this new selective excitation method was calculated to be significantly higher than their previous non-selective method, meaning more photons are efficiently guided from the source to the end of the fiber. The system’s use of commercially available optical fibers and room-temperature operation positions it as a low-cost candidate for next-generation all-fiber-integrated quantum communication networks.
Dr. Sanaka noted that by individually operating multiple isolated ions within the same fiber, the approach could be extended to develop a multi-qubit processing unit, potentially enabling qubit encoding protocols for future quantum computing technologies.
Read the full announcement from the Tokyo University of Science here and the research article in Optics Express here.
October 16, 2025
