Low-loss optical fiber, developed in 1970 by Corning Incorporated, quickly established itself as the most effective way to efficiently transmit data over great distances without data loss. Nowadays, traditional optical fibers are the most commonly used method of transmitting data; this method uses a single core channel to send the data. These systems are, however, exceeding their information-carrying capacity limits due to the exponential growth in data creation. Thus, by investigating their internal structure and using novel methods for signal creation and transmission, researchers are currently concentrating on finding new ways to fully use fibers. Extending this research from classical to quantum light also enables applications in quantum technology.
The concept of Anderson localization was foreseen by physicist Philip W. Anderson in the late 1950s. Anderson also made significant contributions to particle physics and superconductivity. He won the 1977 Nobel Prize in Physics for this finding. Anderson theorized the circumstances under which an electron in a disordered system can either travel freely across the system overall or be fixed in place as a “localized electron.” For instance, a semiconductor with impurities can be this disordered system.
Later, using the same theoretical framework on a variety of disordered systems, it was determined that light could also experience Anderson localization.In the past, experiments have shown how Anderson localization in optical fibers may limit or localize light—classical or conventional light—in two dimensions while propagating it in a third. While these tests with classical light yielded positive results, no one had yet tried them with quantum light, which consists of quantum linked states.So it was until recently.
Researchers from ICFO, Alexander Demuth, Robing Camphausen, and Alvaro Cuevas, under the direction of ICREA Professor at ICFO Valerio Pruneri, have successfully demonstrated the transport of two-photon quantum state information in a study published in Communications Physics. They worked with Corning researchers Nick Borrelli, Thomas Seward, Lisa Lamberson, and Karl W. Koch, as well as Alessandro Ruggeri from Micro Photon Devices (MPD) and Federica Villa (PSF).
Anderson localization fiber versus regular optical fiber
A phase-separated fiber (PSF) or phase-separated Anderson localization fiber is composed of several glass strands contained in a glass matrix with two different refractive indices. This is in contrast to ordinary single-mode optical fibers, which carry data through a single core. As borosilicate glass is heated and melted during the manufacturing process, it is pulled into a fiber, where one of the two phases with different refractive indices tends to produce elongated glass strands. Due to the presence of two different refractive indices within the material, a lateral disorder is created, which causes light to be localized transversely (2D) throughout the substance.
Corning, which is an expert in making optical fibers, used Anderson localization to produce an optical fiber that can carry numerous optical beams in a single optical fiber. This PSF, as opposed to multicore fiber bundles, proved to be very well suited for such tests since numerous parallel optical beams can pass through the fiber with a minimum amount of space between them.
The group of researchers, who are experts in quantum communications, sought to move quantum data as quickly and effectively as possible using Corning’s phase-separated optical fiber. A transmitter and a receiver are connected in the experiment by the PSF. A quantum light source serves as the transmitter (built by ICFO). Through spontaneous parametric down-conversion (SPDC), which occurs in a non-linear crystal and transforms one high-energy photon into pairs of lower-energy photons, the source produces quantum-correlated photon pairs. the 810 nm wavelength of the low-energy photon pairs fiber. Space-time anti-correlation develops as a result of momentum conservation. The receiver is a Polimi and MPD single-photon avalanche diode (SPAD) array camera. Contrary to typical CMOS cameras, the SPAD array camera has a very high time resolution and is sensitive enough to detect single photons with incredibly little noise. This allows for highly accurate timing of the single photons’ arrival.
Quantum fiber
In order to transport the quantum light through the phase-separated Anderson localization fiber and detect its arrival with the SPAD array camera, the ICFO team designed the optical setup. They were able to detect the pairs of photons using the SPAD array, as well as recognize them as pairs because they came at the same time (coincidence). Since the two photons are quantum correlated, we can determine the location of the other photon by knowing where one of the two is detected. The researchers successfully demonstrated that the photons’ spatial anti-correlation was preserved by confirming this correlation both before and after passing the quantum light through PSF.
Following this demonstration, the ICFO team then set out to demonstrate how to enhance their outcomes in subsequent work. They used a scaling analysis to determine the best size distribution of the elongated glass strands for the 810 nm quantum light wavelength. Following a detailed investigation using classical light, they were able to pinpoint the phase-separated fiber’s existing drawbacks and suggest modifications to its manufacturing process to reduce attenuation and loss of resolution during travel.
According to the study’s findings, this method may be appealing for scalable fabrication processes in practical applications of quantum imaging or quantum communications, particularly in the areas of high-resolution endoscopy, entanglement distribution, and quantum key distribution.
