Fiber optic cable giant Corning develops quantum fiber
In condensed matter physics, "Anderson localization" refers to the absence of wave diffusion in a disordered medium.
In the recent study "Quantum optical transport in phase-separated Anderson-localised fibres"[1] published in Physics of Communication, Professor Valerio Pruneri's team at the Spanish Institute of Photonic Sciences (ICFO), in collaboration with the fibre optic cable giant Corning Incorporated, has successfully demonstrated the transport of two-photon quantum states through phase-separated Anderson-localised fibres (PSFs). of Anderson Positioning Fibre (PSF).
01Quantum fibre and Anderson Positioning
Invented by Corning in 1970, low-loss optical fibre has become the best means of efficiently transmitting information from one place to another over long distances without loss of information. The most common way of transmitting data today is through conventional optical fibre - a single core channel through which information is transmitted. However, with the exponential growth in data generation, these systems are reaching the limits of their information carrying capacity.
Research is therefore now focusing on finding new ways to exploit the full potential of optical fibres by studying their internal structure and applying new methods to generate and transmit signals; in addition, by extending this research from classical light to quantum light, applications of quantum technology are being realised.
In the late 1950s, physicist Philip W. Anderson (who also made important contributions to particle physics and superconductivity) predicted what is now known as Anderson localisation; for this discovery he was awarded the 1977 Nobel Prize in Physics. Anderson showed theoretically the conditions under which an electron in a disordered system can move freely throughout the system, or be bound to a specific position as a 'localised electron'. This disordered system could be, for example, a semiconductor with impurities.
Later, the same theoretical approach was applied to various disordered systems and it was deduced that light could also undergo Anderson localisation. Past experiments have demonstrated Anderson localisation in optical fibres, achieving confinement or localisation of classical or conventional light in two dimensions, while propagating it in a third dimension. While these experiments have shown successful results for classical light, until now no one has tested such systems with quantum light (light consisting of quantum correlated states).
Only recently has the ICFO team succeeded in demonstrating the transmission of two-photon quantum states through a phase-separated Anderson Positioning Fibre (PSF).
02Conventional fibre versus Anderson Positioning Fibre
In contrast to conventional single-mode fibres (where data is transmitted through a single core), a phase-separated fibre (PSF) or phase-separated Anderson-located fibre consists of a number of glasses embedded in a glass matrix with two different refractive indices.
Phase-separated fibres: comparison image and propagation schematic through Anderson's local pattern
Schematic diagram of a phase-separated Anderson localisation fibre as a quantum channel between the emitter and the receiver. The diagram shows that quantum correlations such as entanglement are maintained along the fibre all the way from the emitter (generation) to the receiver (detection).
During its fabrication, as the borosilicate glass is heated and melted, it is drawn into fibres in which one of the two phases with different refractive indices tends to form elongated strands of glass. As there are two refractive indices within the material, this creates what is known as transverse disorder, which leads to the transverse (2D) Anderson localisation of light in the material.
Corning, a specialist in the manufacture of optical fibres, has created an optical fibre that allows multiple light beams to be propagated in a single fibre by using Anderson localisation. In contrast to multi-core fibre bundles, this PSF shows to be very suitable for such experiments, as many parallel beams can propagate through the fibre with minimal spacing between them.
The team of scientists, who are experts in quantum communication, wanted to transmit quantum information as efficiently as possible through Corning's phase-separated fibres. In the experiment, the PSF is connected to a transmitter and a receiver. The transmitter is a quantum light source (built by ICFO). The source generates quantum correlated photon pairs by spontaneous parametric down-conversion (SPDC) in a non-linear crystal, where a high energy photon is converted to a photon pair, each with a lower energy.
The low-energy photon pair has a wavelength of 810 nm. Due to momentum conservation, spatial incoherence occurs. The receiver is a single photon avalanche diode (SPAD) array camera. Unlike ordinary CMOS cameras, the SPAD array camera is very sensitive and can detect single photons with very low noise; it also has a very high temporal resolution so that the arrival time of a single photon is known with high precision.
Schematic diagram of the experiment
03Quantum phase-separated fibres for quantum imaging, quantum communication applications
The ICFO team designed an optical device that sends quantum light through a phase-separated Anderson localisation fibre and detects its arrival with a SPAD array camera. the SPAD array allows them not only to detect pairs of photons, but also to identify them as pairs because they arrive at the same time (overlap).
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 both before and after sending the quantum light through the PSF, successfully showing that the spatial anti-correlation of the photons was indeed maintained.
After this demonstration, the ICFO team will then start to show how their results can be improved in future work. To this end, they performed a scaling analysis to find the optimal size distribution of the elongated glass chains at the quantum light wavelength of 810 nm. After a thorough analysis with classical light, they were able to identify the current limitations of phase-separated fibres and suggest improvements to their fabrication to minimise attenuation and loss of resolution during transmission.
The results of this study suggest [2] that this approach is potentially attractive for scalable fabrication processes for practical applications in quantum imaging or quantum communications, particularly for areas such as high-resolution endoscopy, entanglement distribution and quantum key distribution.
Reference links：
[1]https://www.nature.com/articles/s42005-022-01036-5
[2]https://phys.org/news/2022-11-two-photon-quantum-states-phase-separated-anderson.html
