Harvard physicists have demonstrated the world’s longest fiber distance between two quantum memory nodes, a significant step towards a next-generation quantum internet. The team, led by Mikhail Lukin, Marko Lončar, and Hongkun Park, used existing Boston-area telecommunication fiber for the demonstration. The quantum network was established by entangling two quantum memory nodes separated by a 22-mile loop. Each node is a small quantum computer made from a diamond with a silicon-vacancy center. The work, published in Nature, was carried out with researchers at Amazon Web Services. The team is now working to extend the performance of their network by adding nodes.
Quantum Internet: A Reality in the Making
Harvard physicists have made a significant stride towards the realization of a quantum internet, demonstrating the world’s longest fiber distance between two quantum memory nodes. This groundbreaking work, published in Nature, was conducted using existing telecommunication fiber in the Boston area. The team successfully established the practical foundations of the first quantum internet by entangling two quantum memory nodes separated by an optical fiber link deployed over a roughly 22-mile loop through Cambridge, Somerville, Watertown, and Boston.
Quantum memory, akin to classical computer memory, is a crucial component of a quantum computing future as it enables complex network operations and information storage and retrieval. The Harvard team’s quantum network is the longest fiber network between devices capable of storing, processing, and moving information. Each node is a miniature quantum computer, made from a diamond sliver with a defect in its atomic structure known as a silicon-vacancy center. This center contains two qubits, or bits of quantum information: one in the form of an electron spin used for communication, and the other in a longer-lived nuclear spin used as a memory qubit to store entanglement.
Quantum Entanglement: The Key to Quantum Internet
Quantum entanglement, a quantum-mechanical property, allows information to be perfectly correlated across any distance. Unlike classical computing, where information is stored and transmitted as a series of discrete binary signals, quantum computing is more fluid. Information can exist in stages between on and off and is stored and transferred as shifting patterns of particle movement across two entangled points.
The use of silicon-vacancy centers as quantum memory devices for single photons has been a multiyear research program at Harvard. This technology addresses a significant challenge in the theorized quantum internet: signal loss that can’t be boosted in traditional ways. A quantum network cannot use standard optical-fiber signal repeaters because simple copying of quantum information as discrete bits is impossible. This makes the information secure but also very hard to transport over long distances.
Overcoming Signal Loss in Quantum Networks
Silicon-vacancy-center-based network nodes can catch, store, and entangle bits of quantum information while correcting for signal loss. After cooling the nodes to close to absolute zero, light is sent through the first node and, by nature of the silicon vacancy center’s atomic structure, becomes entangled with it, so able to carry the information. Since the light is already entangled with the first node, it can transfer this entanglement to the second node, a process referred to as photon-mediated entanglement.
Quantum Internet: A Practical Possibility
The researchers have leased optical fiber from a company in Boston to run their experiments, fitting their demonstration network on top of the existing fiber to indicate that creating a quantum internet with similar network lines would be possible. The successful entanglement of quantum network nodes in the real-world environment of a busy urban area is a significant step towards practical networking between quantum computers.
Future of Quantum Networking
A two-node quantum network is only the beginning. The researchers are working diligently to extend the performance of their network by adding nodes and experimenting with more networking protocols. The work was supported by the AWS Center for Quantum Networking’s research alliance with the Harvard Quantum Initiative, the National Science Foundation, the Center for Ultracold Atoms, the Center for Quantum Networks, the Air Force Office of Scientific Research, and other sources.
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