A quantum internet, offering unhackable communication, promises to securely transmit sensitive information like financial and national security data, distinct from the memes and cat pictures of today’s traditional internet. Building and scaling such quantum communication systems is a complex task, but scientists are steadily progressing. A recent breakthrough by a Harvard team marks a significant step forward. In a study published in Nature, the researchers report sending entangled photons between two quantum memory nodes 22 miles (35 kilometers) apart using existing fiber optic infrastructure under Boston’s busy streets.

“Demonstrating that quantum network nodes can be entangled in the real-world environment of a very busy urban area is an important step toward practical networking between quantum computers,” said Mikhail Lukin, the project leader and a physics professor at Harvard, in a press release.

Quantum networks transmit information using entanglement, a phenomenon where two particles, such as photons, become linked. A change in the state of one particle instantly affects the other, regardless of distance. This property allows for secure data transmission: if both the sender and receiver each have one of an entangled photon pair, they can securely share information. For quantum communications to work, massive numbers of entangled photons must be generated and reliably transmitted over long distances.

While scientists have successfully sent entangled particles over fiber optic cables before, scaling this up to a functional quantum internet requires particles to travel hundreds or thousands of miles. Over such distances, fiber optic cables absorb photons, causing data loss unless the signal can be periodically refreshed. This is where quantum repeaters come in.

Quantum repeaters function like internet gas stations, refreshing and strengthening information passing through long stretches of fiber optic cables. Unlike regular repeaters, quantum repeaters must also preserve the entanglement of the particles. The absence of practical quantum repeaters is a major hurdle in realizing a large-scale quantum internet.

The Harvard team, collaborating with Amazon Web Services (AWS), has focused on developing quantum memory nodes. Each node contains a piece of diamond with an atom-sized hole, known as a silicon-vacancy center, housing two qubits: one for storage and one for communication. These nodes, essentially small quantum computers operating near absolute zero, can receive, store, and transmit quantum information. The Boston experiment achieved the longest distance for transmitting information between such devices, advancing toward a practical quantum repeater.

“Our experiment really put us in a position where we’re very close to working on a quantum repeater demonstration,” said Can Knaut, a Harvard graduate student in Lukin’s lab, in an interview with New Scientist.

Next steps involve expanding the system to include multiple nodes. A separate group in China recently linked three nodes 6 miles (10 kilometers) apart using a different technique for quantum memory involving rubidium atom clouds. Previously, this group, led by Xiao-Hui Bao at the University of Science and Technology of China, entangled memory nodes 13.6 miles (22 kilometers) apart.

Significant work remains to make this technology practical, including increasing the rate of photon entanglement. However, each new development brings the prospect of unhackable communications closer to reality.

By Impact Lab