Harvard and MIT researchers have found a way to correct for signal loss in quantum computing with a prototype quantum node that can catch, store, and entangle bits of quantum information. A quantum internet could be used to send unhackable messages, improve the accuracy of GPS, and enable cloud-based quantum computing. The research was published in Nature.

According to the Harvard and MIT team, the breakthrough is the missing link toward a practical quantum internet and a major step forward in the development of long-distance quantum networks.

“This demonstration is a conceptual breakthrough that could extend the longest possible range of quantum networks and potentially enable many new applications in a manner that is impossible with any existing technologies,” said Mikhail Lukin, professor of physics and co-director of Harvard Quantum Initiative. “This is the realization of a goal that has been pursued by our quantum science and engineering community for more than two decades.”

Quantum communication over long distances is affected by conventional photon losses, which is one of the major obstacles for realizing large-scale quantum internet. Still, the same physical principle that makes quantum communication ultrasecure also makes it impossible to use existing, classical repeaters to fix information loss.

According to the researchers, the solution to this involves a quantum repeater. Unlike classical repeaters, which amplify a signal through an existing network, quantum repeaters create a network of entangled particles through which a message can be transmitted.

A quantum repeater is a small, specialized quantum computer. At each stage, the quantum repeater must be able to catch and process bits of information to correct errors and store them long enough for the rest of the network to be ready. Until now, that has been impossible for two reasons: the difficulty of catching single photons, and the fragility of quantum information, making it challenging to process and store for long periods of time.

Lukin’s lab has been working to harness a system that can perform both of these tasks effectively: silicon-vacancy color-centers in diamonds. These centers are tiny defects in a diamond’s atomic structure that can absorb and radiate light, giving rise to a diamond’s brilliant colors.

“Over the past several years, our labs have been working to understand and control individual silicon-vacancy color-centers, particularly around how to use them as quantum memory devices for single photons,” said Mihir Bhaskar, a graduate student in the Lukin group.

The researchers integrated an individual color-center into a nanofabricated diamond cavity, which confines the information-bearing photons and forces them to interact with the single color-center. They then placed the device in a dilution refrigerator, which reaches temperatures close to absolute zero, and sent individual photons through fiber optic cables into the refrigerator, where they were efficiently caught and trapped by the color-center.

The device can store the quantum information for milliseconds — long enough for information to be transported over thousands of kilometers. Electrodes embedded around the cavity were used to deliver control signals to process and preserve the information stored in the memory.

According to the researchers, the device combines three important elements of a quantum repeater: a long memory, the ability to efficiently catch information off photons, and a way to process it locally.

“Currently, we are working to extend this research by deploying our quantum memories in real, urban fiber optic links,” said Ralf Riedinger, a postdoctoral candidate in the Lukin group. “We plan to create large networks of entangled quantum memories and explore the first applications of the quantum internet.”

“This is the first system-level demonstration, combining major advances in nanofabrication, photonics, and quantum control, that shows clear quantum advantage to communicating information using quantum repeater nodes. We look forward to starting to explore new, unique applications using these techniques,” Lukin said.