Longest Quantum Network Tested On Existing Fiber Optics In Boston

Quantum internet would allow for sending hacker-proof information around the world. Its secret is quantum mechanics and it is seen as an important development for the future of telecommunications. There are challenges to overcome, but progress has also been made – such as a new record set by researchers at Harvard University.

The team was able to demonstrate the longest transmission between two nodes using already existing fiber optics within the city of Boston. The fiber path was 35 kilometers (22 miles) in total, stretching around the city. The two nodes that connected to the close path were actually located one floor apart, so the fiber was not the shortest route but rather the more interesting one.

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.

Professor Mikhail Lukin

Quantum information has been sent over longer distances, but there are some innovations in this experiment that deliver the practical makings of a realistic quantum internet. Beyond the use of existing optical fibers, the innovation is in the nodes.

A standard network uses optical fiber signal repeaters. These devices combine optical receivers, electrical amplifiers, and optical transmitters. The signal is taken in, converted to an electrical version, and then turned back into light and sent out. They are key to extending the reach of the original signal. And in its current form, this doesn’t work for quantum internet.

Map showing the path of the two-node quantum network through Boston and Cambridge.
Image credit: Can Knaut via OpenStreetMap

The problem is not with the tech but with physics itself. Quantum information cannot be copied like that. Quantum information is secure because it is in an entangled state. The Harvard system is set up so that each node is a small quantum computer that stores, processes, and moves information. Even though it’s just two nodes, this is the longest quantum network to date with nodes that can do that.

“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,” senior author Professor Mikhail Lukin said in a statement.

           

The small quantum computer at each node is made of a sliver of diamond with a defect in its atomic structure called a silicon vacancy center. When brought to temperatures near absolute zero, the silicon vacancy can catch, store, and entangle bits of information, making it perfect for a node.  

“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 researcher in Lukin’s lab. “We call this photon-mediated entanglement.”

The study is published in the journal Nature.

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