It’s one thing to dream of a quantum internet that could send unhackable information around the world via photons superimposed in different quantum states. Physically demonstrating that this is possible is a completely different thing.

That’s exactly what Harvard physicists did; They demonstrated the world’s longest fiber distance between two quantum memory nodes using Boston’s existing telecommunications fiber. Think of this as a simple, closed Internet between points A and B; transmit a signal encoded not in classical bits like the current Internet, but in completely secure individual light particles.

Innovative study titled “Entangled nanophotonic quantum memory nodes in a telecommunications network” Nature It was carried out under the leadership of Joshua University Professor of Physics Mykhailo Lukin and Beth Friedman, in collaboration with Harvard Professors Marco Lonchar and Hongkun. Park, who is a member of the Harvard Quantum Initiative with Amazon Web Services researchers.

A Harvard team has laid the groundwork for the first quantum Internet by connecting two quantum memory nodes separated by a fiber-optic link, deployed along a roughly 22-mile loop through Cambridge, Somerville, Watertown, and Boston. The two nodes were located on the same floor at the Harvard Integrated Science and Engineering Laboratory.

Similar to classical computer memory, quantum memory is a key component of the interconnected future of quantum computing as it enables complex network operations and the storage and retrieval of information. Although other quantum networks have been created in the past, the Harvard team’s is the longest fiber-optic network between devices that can store, process and transport information.

Each node is a tiny quantum computer made of a piece of diamond with a defect in its atomic structure called a silicon void center. Engraved structures inside the diamond, less than one-hundredth the size of a human hair, strengthen the interaction between the silicon cavity center and light.

A silicon cavity center contains two qubits, or bits of quantum information: one in the form of an electron spin used for communication, the other in the form of a long-lived nuclear spin used as a memory qubit for storing entanglement (a property of quantum mechanics). This allows perfect correlation of information from any distance).

Both rotations are completely controlled by microwave pulses. Measuring just a few square millimeters, these diamond devices are housed inside cooling dilution units that reach temperatures of -459°F.

Using empty silicon cavities for individual photons as quantum memory devices is a long-standing research program at Harvard. This technology solves the main problem of the theoretical quantum internet: signal loss that cannot be amplified by traditional methods.

A quantum network cannot use standard fiber optic signal repeaters because it is impossible to copy random quantum information, making the information secure, but also very difficult to transport it over long distances.

Network nodes based on silicon void centers can capture, store and scramble bits of quantum information while smoothing signal loss. After cooling the nodes to absolute zero, light passes through the first node and travels with it due to the nature of the atomic structure of the silicon void center.

“Since the light is already entangled with the first node, it can transfer that entanglement to the second node,” explained first author Kahn Knauth, a Kenneth S. Griffin Institute of Arts and Sciences student in Lukin’s laboratory. “We call this photon entanglement.”

For the past few years, researchers have rented optical fiber from a company in Boston to run their experiments, and have built demonstration networks on top of existing fiber to show that it might be possible to create a quantum internet with similar network lines.

“Demonstrating that quantum network nodes can circulate in the real-world environment of a very dense urban area is an important step towards a practical network between quantum computers,” Lukin said.

The two-node quantum network is just the beginning. Researchers are working hard to improve the performance of their networks by adding nodes and experimenting with more network protocols.