Researchers at Northwestern University have successfully achieved quantum state transfer over a 30.2 km fiber carrying 400 Gbps C-band classical traffic. The ability for quantum and conventional networks to operate within the same optical fiber will aid in the large-scale deployment of quantum network technology.

The fiber optic infrastructure and telecommunications technologies that underpin the Internet have been widely adopted by researchers aiming to develop quantum networks capable of applications such as quantum-enhanced cryptography, sensing, and networked quantum computing.

However, the feasibility of quantum networking at scale remains uncertain, as much of the existing fiber infrastructure still carries traditional communications traffic, and new fiber is expensive to lease and install. It depends on its ability to propagate within the network. Uses the same fiber as high-power classical signals.

“In optical communications, all signals are converted to light,” said Prem Kumar, a professor at Northwestern University.

“Conventional signals in classical communications are typically made up of millions of particles of light, whereas quantum information uses a single photon.”

Professor Kumar and his colleagues have discovered a way to allow delicate photons to avoid crowded traffic.

“This is incredibly exciting because no one thought it was possible,” Professor Kumar said.

“Our research points the way to next-generation quantum and classical networks that share a unified fiber optic infrastructure.”

“Essentially, this opens the door to taking quantum communications to the next level.”

After studying in detail how light is scattered in fiber optic cables, researchers have discovered a less crowded wavelength of light at which to place photons.

Next, we added a special filter to reduce noise from normal internet traffic.

“We carefully studied how light scatters and placed photons at decision points where that scattering mechanism is minimized,” Professor Kumar said.

“We found that quantum communication can be performed without interference from simultaneously existing classical channels.”

To test the new method, the scientists installed a 20-mile-long fiber optic cable with photons at each end.

They then transmitted quantum information and regular internet traffic simultaneously.

Finally, we measured the quality of the quantum information at the receiving end by taking quantum measurements at intermediate points while running the teleportation protocol.

They discovered that quantum information was successfully transmitted even in the midst of busy Internet traffic.

Next, the authors plan to extend the experiment to even longer distances.

They also plan to use two pairs of entangled photons to demonstrate entanglement swapping, another important milestone leading to distributed quantum applications.

Finally, we are exploring the possibility of running experiments via underground optical cables in the real world rather than on spools in the lab.

“Quantum teleportation has the ability to securely provide quantum connectivity between geographically separated nodes,” Professor Kumar said.

“But many people have long thought that no one would build the specialized infrastructure to transmit particles of light.”

“If you choose the wavelength properly, you don't need to build new infrastructure. Classical and quantum communications can coexist.”

of the team paper Published in this month's magazine optica.

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Jordan M. Thomas others. 2024. Quantum teleportation coexists with classical communication using optical fibers. optica 11 (12): 1700-1707;doi: 10.1364/OPTICA.540362

This article is adapted from the original release by Northwestern University.