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

Three teams – in Boston, in China, and the Netherlands – have simultaneously announced that they’ve figured out ways to store entangled photons without breaking the entanglement, a critical step in building quantum repeaters, and, thus, scalable quantum networks.

The Boston team used silicon atoms embedded in diamond chips. A team in the Netherlands also used diamond crystals, but with nitrogen atoms instead of silicon. And the Chinese team used clouds of rubidium atoms. The American and Chinese teams both published their papers in this month’s Nature magazine, while the Netherlands research is available as a pre-print.

This is a big deal for two reasons. First, because it brings us closer to actually having secure quantum networks. And, second, because China is finally getting some competition in the quantum networking space, where it’s long held a substantial lead.

According to a McKinsey report released last month, China is far ahead in investment in quantum technology, with public investments totaling over $15 billion. The United States, at just $3.8 billion, is far behind.

China is also in the lead when it comes to the size of its quantum networks. In 2021, China built a 4,600-kilometer network between Beijing, Hefei, and Shanghai, an order of magnitude larger than networks elsewhere.

And, at the end of 2023, it demonstrated a secure quantum connection with Moscow, for a distance of 3,800 kilometers, according to a report in the South China Morning Post.

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.

One problem is that each quantum repeater needs to be supercooled and is about the size of a large refrigerator. That’s not practical for commercial deployments.

Second, the repeater can currently handle just one photon at a time. For practical deployments, each repeater will need to handle multiple entangled photons at once. “The goal is to scale it up so we have lots of memories living on that diamond chip,” says Bhaskar. “Then you can support lots of engineering bandwidth.”

Finally, there’s the problem of business demand.

There are two main near-term use cases for quantum networks. The first use case is to transmit encryption keys. The idea is that public key encryption – the type currently used to secure Internet traffic – could soon be broken by quantum computers. Symmetrical encryption – where the same key is used to both encrypt and decrypt messages – is more future proof, but you need a way to get that key to the other party. Quantum key distribution uses the secure quantum network to send the key, then transmits the body of the communication via traditional methods. Quantum networks aren’t expected to have enough capacity to carry significant amounts of traffic anytime in the foreseeable future, but using them to send just the key is a practical application, especially for particularly sensitive government or financial communications.

Today, however, the encryption we currently have is good enough, and there’s no immediate need for companies to look for secure quantum networks. Plus, there’s progress already being made on creating quantum-proof encryption algorithms.

The other use for quantum networks is to connect quantum computers. Since quantum networks transmit entangled photons, the computers so connected would also be entangled, theoretically allowing for the creation of clustered quantum computers that act as a single machine.

“There are ideas for how to take quantum repeaters and parallelize them to provide very high connectivity between quantum computers,” says Oskar Painter, director of quantum hardware at AWS. “Or to provide private access to calculations performed by a quantum computer in the cloud.”

But, again, we don’t yet have commercially useful quantum computers on the market, so there’s not much demand in connecting them together, or connecting them to customers.

“We have our own internal efforts to build quantum computing hardware,” says Painter. “And we’re working on quantum networking solutions like these quantum repeaters. But both of those are research and development projects at this point – we foresee a number of years before these technologies become practical and we can bring them to customers.”

But when that day comes, being able to leverage existing fiber networks will be crucial, he says.

“People have built all kinds of quantum networks before, but we’ve never shown that those are compatible with real-world infrastructure,” adds Bhaskar. “What’s new here is that yes, we can do this. That’s a step forward in thinking about how we’re going to do this stuff over long distances, on a global scale, and in real data centers.”