Making a quantum fiber – News Physics and Quantum Computing

Invented in 1970 by Corning Incorporated, low-loss optical fiber has become the best way to efficiently transport information from one location to another over long distances without loss of information. The most common mode of data transmission these days is through conventional fiber optics – a single central channel transmits information. However, with the exponential increase in data generation, these systems are reaching the limits of information carrying capacity. Thus, research is now focused on finding new ways to use the full potential of fibers by examining their internal structure and applying new approaches to signal generation and transmission. Moreover, applications in quantum technology are made possible by extending this research from classical light to quantum light.

In the late 1950s, physicist Philip W. Anderson (who also made important contributions to particle physics and superconductivity) predicted what is now called Anderson localization. For this discovery he was awarded the Nobel Prize in Physics in 1977. Anderson theoretically showed under what conditions an electron in a disordered system can either move freely through the system as a whole or be bound to a specific position as than “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 undergo Anderson localization. Experiments in the past have demonstrated Anderson localization in optical fibers, achieving confinement or localization of light – classical or conventional light – in two dimensions while propagating it through the third dimension. While these experiments had shown positive results with classical light, until now no one had tested such systems with quantum light – light made up of quantum correlated states. That is, until recently.

In a study published in Communications Physics, ICFO researchers Alexander Demuth, Robing Camphausen, Alvaro Cuevas, led by ICFO ICREA Professor Valerio Pruneri, in collaboration with Nick Borrelli, Thomas Seward, Lisa Lamberson and Karl W. Corning’s Koch, together with Alessandro Ruggeri of Micro Photon Devices (MPD) and Federica Villa and Francesca Madonini of Politecnico di Milano, were able to successfully demonstrate the transport of quantum states of two-photon light through an Anderson localization optical fiber ( PSF) with separate phases.

A conventional optical fiber vs an Anderson location fiber

Unlike conventional single-mode optical fibers, where data is transmitted through a single core, a phase-separated fiber (PSF) or phase-separated Anderson locator fiber consists of many glass strands embedded in a glass matrix two different refractive indices. During manufacture, when borosilicate glass is heated and melted, it is stretched into a fiber, where one of two phases with different refractive indices tends to form elongated strands of glass. Since there are two indices of refraction in the material, this generates what is called lateral disorder, which leads to a transverse (2D) Anderson localization of light in the material.

Experts in optical fiber manufacturing, Corning has created an optical fiber capable of propagating multiple optical beams in a single optical fiber by exploiting Anderson localization. Unlike multi-core fiber bundles, this PSF has proven to be very suitable for such experiments because 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 emitter is a quantum light source (built by ICFO). The source generates quantum correlated photon pairs via spontaneous parametric downconversion (SPDC) in a nonlinear crystal, where a high-energy photon is converted into photon pairs, which each have a lower energy. Low energy photon pairs have a wavelength of 810 nm. Due to conservation of momentum, spatial anti-correlation arises. The receiver is a Single Photon Avalanche Diode Array (SPAD) camera, developed by Polimi and MPD. The SPAD area camera, unlike common CMOS cameras, is so sensitive that it can detect single photons with extremely low noise; it also has a very high temporal resolution, so the arrival time of single photons is known with high precision.

quantum light

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

After this demonstration, the ICFO team then set about showing how to improve their results in future work. For this, they performed a scaling analysis, to determine the optimal size distribution of elongated glass strands for the quantum light wavelength of 810 nm. After extensive analysis in conventional light, they were able to identify the current limitations of phase-separated fiber and propose improvements in its manufacture, in order to minimize attenuation and loss of resolution during transport.

The results of this study showed this approach to be potentially attractive for scalable manufacturing processes in real-world applications of quantum imaging or quantum communications, especially for the areas of high-resolution endoscopy, entanglement and quantum key distribution.

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Making a quantum fiber – News Physics and Quantum Computing

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