(Science) Billions of dollars have poured into research on quantum computers and sensors, but many experts say the devices will flourish only when they are yoked to each other over long distancesThrough entanglement, a strange quantum mechanical property once derided by Albert Einstein as a “spooky distant effect,” researchers aim to create intimate, instantaneous links across long distances. A quantum internet could weld telescopes into arrays with ultrahigh resolution, precisely synchronize clocks, yield hypersecure communication networks for finance and elections, and make it possible to do quantum computing from anywhere. It could also lead to applications nobody’s yet dreamed of.
Putting these fragile links into the warm, buzzing world will not be easy, however. Most strands that exist today can send entangled photons to receivers just tens of kilometers apart. And the quantum links are fleeting, destroyed as the photons are received and measured. Researchers dream of sustaining entanglement indefinitely, using streams of photons to weave lasting quantum connections across the globe.
For that, they will need the quantum equivalent of optical repeaters, the components of today’s telecommunications networks that keep light signals strong across thousands of kilometers of optical fiber. Several teams have already demonstrated key elements of quantum repeaters and say they’re well on their way to building extended networks.
Recently, with quantum computing starting to become a reality, quantum networking has begun to muscle its way into the spotlight. To do something useful, a quantum computer will require hundreds of quantum bits, or qubits—still well beyond today’s numbers. But quantum networks can start to prove their worth as soon as a few distant nodes are reliably entangled. “We don’t need many qubits in order to do something interesting,” says Stephanie Wehner, research lead for the quantum internet division at Delft University of Technology (TU Delft).
The first networks capable of transmitting individual entangled photons have begun to take shape. A 2017 report from China was one of the most spectacular: A quantum satellite named Micius sent entangled particle pairs to ground stations 1200 kilometers apart.
Industry and government are starting to use those first links for secure communication through a method called quantum key distribution, often abbreviated QKD. QKD enables two parties to share a secret key by making simultaneous measurements on pairs of entangled photons. The quantum connection keeps the key safe from tampering or eavesdropping, because any intervening measurement would destroy the entanglement; information encrypted with the key then travels through ordinary channels. QKD is used to secure some Swiss elections, and banks have tested it.
A full-fledged quantum network aims higher. It wouldn’t just transmit entangled particles; it “distributes entanglement as a resource,” says Neil Zimmerman, a physicist at the National Institute of Standards and Technology, enabling devices to be entangled for long periods, sharing and exploiting quantum information.
Practical applications include ultrasecure elections and hack-proof communication in which the information itself—and not just a secret key for decoding it, as in QKD—is shared between entangled nodes. Entanglement could synchronize atomic clocks and prevent the delays and errors that accumulate as information is sent between them. And it could offer a way to link up quantum computers, increasing their power. Quantum computers of the near future will likely be limited to a few hundred qubits each, but if entangled together, they may be able to tackle more sophisticated computations.

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