Harvard Physicists Achieve Breakthrough in Quantum Internet Using Boston-Area Fiber Optics

doi:10.5281/zenodo.11549656

Published June 10, 2024 | Version v1

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Harvard Physicists Achieve Breakthrough in Quantum Internet Using Boston-Area Fiber Optics

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Harvard physicists have made a significant leap towards realizing a quantum internet, demonstrating the world's longest fiber distance between two quantum memory nodes using existing Boston-area telecommunication fiber. This remarkable achievement, which sends hacker-proof information via photons in different quantum states, has been published in Nature.

Led by Mikhail Lukin, the Joshua and Beth Friedman University Professor in the Department of Physics, the research team included Harvard professors Marko Lončar and Hongkun Park, all members of the Harvard Quantum Initiative, along with researchers from Amazon Web Services.

The First Practical Quantum Internet

The team created a quantum internet by entangling two quantum memory nodes separated by a roughly 22-mile fiber optic loop through Cambridge, Somerville, Watertown, and Boston. The nodes, housed a floor apart in Harvard's Laboratory for Integrated Science and Engineering, enabled a simple, closed quantum internet that carries perfectly secure signals via individual light particles.

Quantum memory, akin to classical computer memory, is crucial for future interconnected quantum computing, facilitating complex network operations and information storage. While previous quantum networks have been established, Harvard's network marks the longest fiber connection between devices capable of storing, processing, and transmitting information.

Advanced Quantum Nodes

Each node consists of a miniature quantum computer made from diamond with a silicon-vacancy center defect. Inside the diamond, tiny structures enhance interactions between the silicon-vacancy center and light. This center holds two qubits: an electron spin for communication and a longer-lived nuclear spin for memory storage. Both qubits are controllable with microwave pulses. These diamond devices, only a few millimeters in size, operate at -459 degrees Fahrenheit in dilution refrigeration units.

Overcoming Signal Loss

Using silicon-vacancy centers as quantum memory devices for single photons addresses a major challenge in quantum internet theory: signal loss that can't be boosted conventionally. Unlike standard optical-fiber signal repeaters, which can't copy arbitrary quantum information, these nodes can catch, store, and entangle quantum information while compensating for signal loss. Light sent through the first node becomes entangled with it due to the silicon-vacancy center's atomic structure.

"Since the light is already entangled with the first node, it can transfer this entanglement to the second node," explained first author Can Knaut, a graduate student in Lukin's lab. This process, called photon-mediated entanglement, is fundamental to the network's operation.

Real-World Quantum Networking

Over several years, the researchers have leased optical fiber from a Boston company, overlaying their quantum network on the existing fiber infrastructure. This real-world demonstration indicates the feasibility of a quantum internet using similar network lines.

"Showing that quantum network nodes can be entangled in the real-world environment of a very busy urban area is an important step towards practical networking between quantum computers," Lukin stated.

While a two-node quantum network is a significant achievement, the researchers aim to extend their network's performance by adding more nodes and experimenting with advanced networking protocols. This breakthrough marks a pivotal step towards a future where quantum internet becomes a practical reality.

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