The quantum internet is in its dial-up era, with even the most powerful quantum computers only able to transmit information at slow rates.
“Right now, the prototype quantum network has a rate that's abysmal, sometimes bits per second – at most 100 bits per second. A real, practical network that can perform real-world applications requires megabits per second,” said University of Chicago Pritzker School of Molecular Engineering (UChicago PME) Asst. Prof. Tian Zhong. “Our proposal cranks up those numbers, showing that you can reach 10, up to 50 megabits per second.”
A new paper from Zhong’s lab, published this month in Physical Review Letters, outlines a new plan for entangling light and memory, uniting photons and atoms in a single system to leapfrog quantum information at fast speeds for long distances.
“We’re excited that this could be a powerful enabling technology for quantum communication,” said UChicago PME Prof. Aashish Clerk, a co-author on the paper. “While the basic idea we describe is simple to state, it wasn’t clear at the outset whether the kind of photon-atom entanglement that is generated would have the correct form for applications. Our analysis shows it does, and moreover, that this can be achieved in realistic devices.”
This breakthrough clears a hurdle built into quantum systems, said first author Hoi-Kwan Lau, a former postdoctoral researcher in Clerk’s group. Quantum networks require transmitting entanglements, which degrade over the distances required for a true quantum internet.
“You usually send your entanglement in the form of photon, and the photon will get absorbed somewhere, and then you lose it,” said Lau, now an assistant professor at Simon Fraser University in Canada. “Therefore, if you have a network, there is a larger number of nodes, longer distance between each of the nodes, and you basically have lower and lower probability that you can share the entanglement, because they get absorbed very, very easily.”
Why a quantum internet?
Individual quantum computers are powerful, but due to quirks of quantum mechanics, the traits that make the information valuable also make it difficult to share. But sharing information is exactly what’s needed to democratize the secure, unhackable communications that quantum computing offers.
“In the future quantum age, we’re envisioning not a centralized quantum computer that very few people can have access to, but connecting together many quantum computers allowing regular users – you and me – to have access to the power of quantum computing,” Zhong said. “How fast we can transmit entanglement is the key.”
In classical, non-quantum communications, information is often sent long distances through “cloning.” A signal travels a short way down the line. It's then copied – cloned – and the clone is sent down the next leg of the journey, where that clone is cloned. The person receiving the message thousands of miles away a fraction of a second later will technically be seeing a copy of a copy of a copy several times over, but the content of the information will be identical.
Cloning is impossible with quantum information. Observing a quantum waveform in order to copy it collapses it, plus the same inherent unpredictability that makes quantum communications uncrackable and unhackable makes them unclonable.
“A quantum signal is ‘no-clone,’” Lau said.
The team instead created a “quantum repeater,” which will help the quantum information leapfrog down the line to its final destination.
“Suppose you have a long distance, say it’s 100 kilometers,” Lau said. “You cannot send an entanglement that long so therefore, maybe at every 10 kilometers, you place a quantum repeater.”
Getting light and matter to talk
The quantum repeater is a tiny device that typically contains both memory storage and a photon source for entanglement. The process starts when each repeater shares entanglement between the memory storage and the photon. The photon is then sent to the next repeater.
Once the photon from the other repeater arrives, its entanglement will also be transferred to the memory storage. After all repeaters successfully received photons from their neighbors, they will then “mix up” their stored entanglement by quantum measurement. All of a sudden, entanglement is established over a long distance.
This “entanglement swapping,” as it is called, requires linking particles of light with physical storage – a major hurdle pushing the boundaries of quantum physics.
“The quicker way to generate entanglement is using light, or photons. Memory is usually based on matter, or atoms,” Zhong said. “These two, usually they don't like to talk to each other. But if we can make them do so, they can generate entanglement a lot faster.”
By demonstrating entanglement between photons and atoms, the researchers have proven the path toward these new quantum networking techniques. The next steps will move the work from theory to experiment.
“This paper is a theory paper, so we just explore the idea whether it will work or not, whether you can keep any advantages or not, or in which range of setting that you can actually make the things happen, and what will actually guide the experiment to happen,” Lau said. “Of course, the next step is for us to realize the scheme in real world.”
Citation: “Efficient in-situ generation of photon-memory entanglement in a nonlinear cavity,” Lau et al, Physical Review Letters, February 4, 2025. DOI: 10.1103/PhysRevLett.134.053602
2025 International Year of Quantum Science and Technology
The United Nations declared 2025 the International Year of Quantum to mark a century of progress in quantum science and engineering. The University of Chicago and its partners join the celebration of the groundbreaking fields that continue to positively impact lives around the world.