Following its development by a team at Corning in 1970, low-loss optical fiber became the best means to efficiently transport information from one place to another over long distances without loss of information. However, with the exponential increase in data generation, systems that use conventional optical fiber systems to transmit data are approaching information-carrying capacity limits. Research is therefore focused on methods to use the full potential of fibers by examining their inner structure and applying new approaches to signal generation and transmission. In the 1950s, Philip Anderson showed theoretically under which conditions an electron in a disordered system can either move freely through the system as a whole or be tied to a specific position as a “localized electron.” This disordered system can, for example, be a semiconductor with impurities. Later, the same theoretical approach was applied to a variety of disordered systems, and it was deduced that light could also experience Anderson localization. Experiments in the past have demonstrated Anderson localization in optical fibers, realizing the confinement or localization of light — classical or conventional light — in two dimensions while propagating it through the third dimension. Though these experiments showed results with classical light, previous demonstrations failed to test such systems with quantum light — light consisting of quantum correlated states. Schematic overview of a phase-separated Anderson localization fiber as quantum channel between a transmitter and receiver. The illustration shows that quantum correlations such as entanglement are maintained during transport from the transmitter (generation) to receiver (detection) all the way along the fiber. Courtesy of Álvaro Cuevas/ICFO. Now, a research team from ICFO, Politecnico di Milano, Corning, and Micro Photon Devices has demonstrated the transport of two-photon quantum states of light through a phase-separated Anderson localization optical fiber (PSF). Results of the study showed that its approach was attractive for scalable fabrication processes in real-world applications in quantum imaging or quantum communications, particularly for high-resolution endoscopy, entanglement distribution, and quantum key distribution. Contrary to conventional single-mode optical fibers, where data is transmitted through a single core, a PSF or phase-separated Anderson localization fiber is made of many glass strands embedded in a glass matrix of two different refractive indexes. During its fabrication, as borosilicate glass is heated and melted, it is drawn into a fiber, where one of the two phases of different refractive indexes tends to form elongated glass strands. Since there are two refractive indexes within the material, this generates a lateral disorder, which leads to transverse (2D) Anderson localization of light in the material. Corning created an optical fiber that propagated multiple optical beams in a single optical fiber by harnessing Anderson localization. Contrary to multicore fiber bundles, this PSF showed to be very suitable for such experiments since many parallel optical beams can propagate through the fiber with minimal spacing between them. The team wanted to transport quantum information as efficiently as possible through the phase-separated optical fiber. In experiment, the PSF connected a transmitter and a receiver. The transmitter — a quantum light source — generated quantum correlated photon pairs via spontaneous parametric down-conversion (SPDC) in a nonlinear crystal, where one photon of high energy is converted to pairs of photons, which have lower energy each. The low-energy photon pairs had a wavelength of 810 nm. Due to momentum conservation, spatial anti-correlation arises. The receiver — a single-photon avalanche diode (SPAD) array camera — was so sensitive that it detected single photons with extremely low noise. The camera also had very high time resolution, such that the team members detected the arrival time of the single photons with high precision. Because the pairs are quantum correlated, knowing where one of the two photons is detected allowed the researchers to calculate the other’s location. The team verified this correlation immediately before and after sending the quantum light through the PSF, and showed that the spatial anti-correlation of the photons was maintained. Next, the team conducted a scaling analysis to determine the optimal size distribution of the elongated glass strands for the quantum light wavelength of 810 nm. After a thorough analysis of classical light, it identified the current limitations of phase-separated fiber and proposed improvements to its fabrication to limit attenuation and loss of resolution during transport. The research was published in Communications Physics (www.doi.org/10.1038/s42005-022-01036-5).
