Quantum physics, the science of the very smallest structures in the universe, is an exotic and “spooky” world. Yet it governs so much of the technology we use in our everyday lives, from phones and computers to fluorescent light bulbs and toasters. One area where quantum technology may one day revolutionize our world is in communications.

Providing “instantaneous transfer of information” and incredibly secure connections, quantum communication has attracted scientific research for years. Now a new study by a team of Chinese scientists have taken us a step closer to realizing this quantum dream.

Published in Nature, the researchers explain how they managed to entangle two clouds of quantum memories over distances of 22 kilometers (13.7 miles) and 50 kilometers (31.1 miles), obliterating the previous record of 1.3 kilometers (0.8 miles). Having achieved “city-sized” distances of entanglement, the researchers believe the possibility of building a prototype quantum network is slowly becoming closer to reality.

Quantum memories, which in some ways are like our current computer memory, contain quantum information (qubits), which in this case are stored on 100 million very cold Rubidium atoms in a vacuum chamber. However, unlike our current computer memory, qubits can exist in many states, known as superposition, which allows them to perform multiple calculations at the same time. If one set of quantum memory is entangled with another, the state of each memory is shared with the other.

To get to this point of entanglement is incredibly complex. Even Einstein famously described quantum entanglement as “spooky action at a distance”. When particles are brought close enough together, they interfere with and influence one another. Once these particles are separated, a manipulation to one particle heralds an instantaneous change to the other.

In 2017, entangled photons managed to be sent a distance of 1,200 kilometers (745 miles) between Earth and a satellite. However, managing to maintain entanglement of larger systems over such distances (containing more information) has proved more difficult.

Here enters the latest study. Pan Jian-Wei, often dubbed the “Father of Quantum” in China, along with his colleagues, first entangled each system of atoms (called nodes) with a single photon. Altering the frequency of the photons allowed them to be sent along fiber-optic cables, where they met at a central point. Here, the two photons were made to interfere, and when an observation called a Bell measurement was performed on them together, the two quantum memory clouds became remotely entangled.

For the 22-kilometer (13.7-mile) distance, the fibers were installed underground between two sites, whereas the 50-kilometer (31.1-mile) achievement was through coiled cables in the lab. There were several challenges for the researchers to overcome in both set-ups in order to produce a reliable entanglement system.

“The main technology advance lies in developing an efficient atom-photon entanglement source that is suitable for low-loss transmission in fibers,” study co-author Dr Xiao-Hui Bao, from University of Science and Technology of China, told ABC Science.

To achieve this, the team used cavity enhancement to generate bright (i.e. efficient) atom-photon entanglement, reducing the risk of disruption to the system over the larger distances. The conversion of the photon frequency from the near-infrared to the telecommunications O band also meant that the photons would experience low-loss transmission in the optical fibers.

“Our experiment could be extended to nodes physically separated by similar distances, which would thus form a functional segment of the atomic quantum network,” the researchers wrote in the paper, “paving the way towards establishing atomic entanglement over many nodes and over much longer distances.”

https://www.iflscience.com/physics/entanglement-of-quantum-memories-achieved-over-recordbreaking-distance/