Record-breaking quantum memory brings quantum internet one step closer

By Leah Crane

Federico Caputo / Alamy

Two clouds of atoms that store quantum information, called quantum memories, have been connected across a longer distance than ever before. They could prove useful for building a quantum version of the internet

Quantum communication relies on a phenomenon called entanglement. When a pair of particles or systems are quantum entangled, measuring one of them instantly influences the measured state of the other, regardless of the distance between them.

These connections can’t directly transfer information, because that would mean information is travelling faster than light, but entanglement can be used to create encrypted communications channels, secured against hacking by the laws of quantum physics.

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Individual photons have been entangled across distances exceeding 1000 kilometres, but for larger systems of particles, which hold more information, maintaining this entanglement is harder. The maximum distance between a pair of entangled quantum memories so far is just 1.3 kilometres.

Xiao-Hui Bao at the University of Science and Technology of China and his colleagues have now smashed that record, entangling two quantum memories over 22 kilometres of fibre-optic cable installed underground.

Their quantum memories were each made of about 100 million extremely cold rubidium atoms in a vacuum chamber. The quantum state of each system of atoms was entangled with the state of a single photon, and the researchers sent those photons through the fibre-optic cables.

When a particular observation called a Bell measurement was performed on the two photons simultaneously, the quantum memories with which the photons were paired before the measurement became entangled to one another.

In a slightly different experiment using cables that weren’t installed underground but just coiled up in the lab, Bao and his team entangled quantum memories across 50 kilometres.

The end goal of this work is to create a quantum repeater that can receive and then retransmit quantum information so that it can be sent over long distances, eventually building up a secure internet of quantum information.

“Honestly, there is still a long way to go in order to see the quantum repeater working in real long-distance situations,” says Bao, but he thinks that building a small-scale prototype quantum network using quantum memories will be possible in the next few years.

Journal reference: Nature, DOI: 10.1038/s41586-020-1976-7