Invented in 1970 by Corning Incorporated, low-loss optical fiber became the best means to effi­ciently transport information from one place to another over long distances without loss of information. The most common way of data tran­smission nowadays is through conventional optical fibers – one single core channel transmits the information. However, with the exponential increase of data generation, these systems are reaching information-carrying capacity limits. Thus, research now focuses on finding new ways to utilize the full potential of fibers by examining their inner structure and applying new approaches to signal generation and trans­mission. Moreover, appli­cations in quantum tech­nology are enabled by extending this research from classical to quantum light.

In the late 50s, the physicist Philip W. Anderson predicted what is now called Anderson localization. For this discovery, he received the 1977 Nobel Prize in Physics. 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 also light could 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. While these experiments had shown success­ful results with classical light, so far no one had tested such systems with quantum light – light consisting of quantum correlated states. That is, until recently.

Now, ICFO researchers Alexander Demuth, Robing Camphausen, Alvaro Cuevas, led by Valerio Pruneri, in collaboration with Nick Borrelli Thomas Seward, Lisa Lamberson and Karl W. Koch from Corning, together with Alessandro Ruggeri from Micro Photon Devices (MPD) and Federica Villa and Francesca Madonini from Politecnico di Milano, have been able to success­fully demonstrate the transport of two-photon quantum states of light through a phase-separated Anderson locali­zation optical fiber (PSF). Contrary to conventional single mode optical fibers, where data is transmitted through a single core, a phase separated fiber (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. Experts in optical fiber fabri­cation, Corning created an optical fiber that can propagate 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 of scientists, experts in quantum communi­cations, wanted to transport quantum information as efficiently as possible through Corning’s phase-separated optical fiber. In experiment, the PSF connects a trans­mitter and a receiver. The transmitter is a quantum light source (built by ICFO). The source generates quantum correlated photon pairs via spontaneous parametric down-conversion (SPDC) in a non-linear crystal, where one photon of high energy is converted to pairs of photons, which have lower energy each. The low-energy photon pairs have a wavelength of 810 nanometers. Due to momentum conser­vation, spatial anti-corre­lation arises. The receiver is a single-photon avalanche diode (SPAD) array camera, developed by Polimi and MPD. The SPAD array camera, unlike common CMOS cameras, is so sensitive that it can detect single photons with extremely low noise; it also has very high time resolution, such that the arrival time of the single photons is known with high precision.

The ICFO team engineered the optical setup to send the quantum light through the phase-separated Anderson locali­zation fiber and detected its arrival with the SPAD array camera. The SPAD array enabled them not only to detect the pairs of photons but also to identify them as pairs, as they arrive at the same time. As the pairs are quantum correlated, knowing where one of the two photons is detected tells us the other photon’s location. The team verified this correlation right before and after sending the quantum light through PSF, success­fully showing that the spatial anti-corre­lation of the photons was indeed maintained.

After this demons­tration, the team then set out to show how to improve their results in future work. For this, they conducted a scaling analysis, in order to find out the optimal size distri­bution of the elongated glass strands for the quantum light wavelength of 810 nanometers. After a thorough analysis with classical light they were able to identify the current limi­tations of phase-separated fiber and propose improvements of its fabri­cation, in order to minimize attenuation and loss of resolution during transport. The results of this study have shown this approach to be potentially attractive for scalable fabri­cation processes in real-world applications in quantum imaging or quantum communi­cations, especially for the fields of high-resolution endoscopy, ent­anglement distri­bution and quantum key distribution. (Source: ICFO)

Reference: A. Demuth et al.: Quantum light transport in phase-separated Anderson localization fiber, Commun. Phys. 5, 261 (2022): DOI: 10.1038/s42005-022-01036-5

Link: Optoelectronics, ICFO-Institut de Ciències Fotòniques, The Barcelona Institute of Science and Technology, Castelldefels, Spain