Proof-of-concept quantum repeaters bring quantum networks a big step closer

However, if traditional fiber is used to transmit the protons, there’s a limit to how far they can go. At long distances, the loss of photons due to noise in the fiber is exponential, says Mihir Bhaskar, senior research scientist and head of the AWS Center for Quantum Networking. Even with ideal fiber under laboratory conditions, single-hop trips top out at 40 to 50 kilometers, he says.

China’s 4,600 kilometer network uses trusted nodes to bridge longer distances, he says, meaning the entanglement gets broken after every leg of the trip. China also uses satellite communications, since photons can go very far in empty space.

But, until now, scalable, secure quantum networks that use existing infrastructure were impossible, he says. “We want to show that this technology is robust and can work over any kind of fiber.”

The ideal situation is to be able to transmit the photons along standard commercial fiber, with repeaters along the way that pass along the photons without breaking the entanglement. Now, it looks like this problem has been cracked.

In the Boston project, researchers from Harvard University and the AWS Center for Quantum Networking were able to use 35 kilometers of commercial fiber running under and above the streets of Boston to transmit entangled protons.

The researchers used a single silicon atom embedded in a diamond chip to store an entangled photon for up to a second before passing it along on the next leg of its journey. The chip doesn’t amplify the photon – doing so would break the entanglement – but it breaks up a long trip into short, more doable hops. “We store it in memory, and it’s just a buffer that waits until it’s able to establish the connection,” says Bhaskar. But we’re still five to ten years away, before the technology is commercially viable, he adds.