Optical-fibre room temperature single-photon light source for quantum processing

Quantum systems hold the promise of accelerated computing and reinforced encryption capabilities for both computational and communication infrastructures and are readily being embraced across the globe. They can be integrated into fibre networks that feature nodes with qubits and single-photon generators capable of producing entangled photon pairs.

In the realm of such technologies, rare-earth (RE) atoms and ions within solid-state materials emerge as strong contenders for single-photon generation. Compatible with fibre networks, these materials emit photons across a spectrum of wavelengths. Fibres doped with RE elements could have applications ranging from free-space telecommunication and fibre-based telecommunications to quantum random number generation and high-resolution imaging. However, the development of single-photon sources using RE-doped crystalline materials has been limited to cryogenic temperatures, posing limitations for practical quantum network applications.

A pivotal study released on 16/10/2023 in Physical Review Applied, Volume 20, Issue 4, by a team from Japan, guided by Associate Professor Kaoru Sanaka from Tokyo University of Science, has marked a significant development. The research, also involving Associate Professor Mark Sadgrove, Mr. Kaito Shimizu from Tokyo University of Science, and Professor Kae Nemoto from Okinawa Institute of Science and Technology Graduate University, led to the creation of a single-photon light source at room temperature, using ytterbium ions (Yb3+) doped in an amorphous silica optical fibre. This innovation potentially renders quantum networks more economically viable and widely accessible by obviating the need for expensive cooling systems.

Dr. Sanaka stated, "Single-photon light sources are devices that control the statistical properties of photons, which represent the smallest energy units of light," and continued, “In this study, we have developed a single-photon light source using an optical fibre material doped with optically active RE elements. Our experiments also reveal that such a source can be generated directly from an optical fibre at room temperature.”

Ytterbium, an RE element with beneficial optical and electronic characteristics, is ideal for fibre doping due to its straightforward energy-level structure and a long fluorescence lifetime in its excited state, approximately one millisecond.

The ytterbium-doped fibre was produced by tapering a commercially available fibre using a heating and pulling technique, resulting in a reduction in diameter.

Within this tapered fibre, the spacing between individual RE atoms is critical to its optical properties. If the separation exceeds the optical diffraction limit, the resultant light appears to be sourced from clusters rather than individual atoms.

To verify the single-ion photon emission, the researchers applied an analytical technique known as autocorrelation, evaluating the similarity of the light signal with its delayed self. The analysis yielded non-resonant emissions, confirming the single ytterbium ion's photon emission within the doped fibre.

While there is room for improvement in the quantity and quality of the photons produced, this ytterbium-doped fibre overcomes a major barrier in quantum information technology. Dr. Sanaka concluded, "We have demonstrated a low-cost single-photon light source with selectable wavelength and without the need for a cooling system. Going ahead, it can enable various next-generation quantum information technologies such as true random number generators, quantum communication, quantum logic operations, and high-resolution image analysis beyond the diffraction limit."