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CHICAGO — Picture trying to float a soap bubble through a sandstorm without popping it. That’s roughly equivalent to what Northwestern University researchers have accomplished in the world of quantum communications; except, instead of a soap bubble, they’re protecting individual particles of light carrying quantum information through a torrent of conventional internet traffic.
For years, experts believed quantum communications would require its own private highway system, separate from the bustling freeways of classical internet traffic. Now, scientists have proven those experts wrong, demonstrating that quantum and classical signals can share the same fiber optic roads without crashing into each other, a discovery that could dramatically accelerate the development of quantum networks.
‘Nobody thought it was possible’
This achievement represents a crucial milestone in making quantum networks practical and cost-effective – and challenges long-held assumptions about quantum networking infrastructure. Before this breakthrough, many experts believed this coexistence was impossible – that quantum signals would be overwhelmed by conventional internet traffic.
“This is incredibly exciting because nobody thought it was possible,” says Prem Kumar, who led the study at Northwestern’s McCormick School of Engineering, in a statement. “Our work shows a path towards next-generation quantum and classical networks sharing a unified fiberoptic infrastructure. Basically, it opens the door to pushing quantum communications to the next level.”
Think of it like trying to hear a whisper while a rock concert is playing in the same room. Quantum signals are incredibly delicate — just a single particle of light (photon) carrying information — while classical internet signals are comparatively like blasting music at full volume. Or as Kumar explains: “While conventional signals for classical communications typically comprise millions of particles of light, quantum information uses single photons.”
Kumar’s team at the Center for Photonic Communication and Computing found an ingenious solution: they identified a less congested “lane” of light waves and installed specialized filters to protect the quantum signals from the noise of regular internet traffic.
“We carefully studied how light is scattered and placed our photons at a judicial point where that scattering mechanism is minimized,” says Kumar. “We found we could perform quantum communication without interference from the classical channels that are simultaneously present.”
Quantum teleportation breakthrough
The research team demonstrated quantum teleportation, a process fundamental to quantum networks, over a 30.2-kilometer (about 19-mile) fiber optic cable while simultaneously transmitting conventional internet data at 400 gigabits per second. Quantum teleportation works by harnessing quantum entanglement, where two particles are linked regardless of the distance between them. Instead of particles physically traveling to deliver information, entangled particles exchange information over great distances — without physically carrying it.
The setup involved three main players: Alice (the sender), Bob (the receiver), and Charlie (the middleman). Alice prepared special quantum states that she wanted to transmit to Bob. Meanwhile, Bob created pairs of entangled photons, keeping one and sending the other to Charlie. When Charlie performed a special measurement involving both Alice’s photon and Bob’s entangled photon, it caused Bob’s remaining photon to instantly take on the properties of Alice’s original state, hence the term “teleportation.”
The team achieved impressive results, maintaining high-quality quantum teleportation even with classical signals approximately 150 times stronger than necessary for error-free 400-Gbps communications. This suggests that quantum and classical networks could potentially share the same fiber infrastructure, dramatically reducing the complexity of building quantum networks.
Communications coexistence
The research, published in the journal Optica, could be significant for the future of quantum networking. According to the research paper, potential applications include quantum-enhanced cryptography, sensing capabilities, and networked quantum computing. A quantum network infrastructure that can coexist with classical internet traffic could make these applications more feasible to implement using existing fiber optic infrastructure. However, considerable research and development will still be needed to realize these possibilities.
“Many people have long assumed that nobody would build specialized infrastructure to send particles of light,” Kumar notes. “If we choose the wavelengths properly, we won’t have to build new infrastructure. Classical communications and quantum communications can coexist.”
The team isn’t stopping here. Their next steps include extending the experiments over longer distances and attempting to demonstrate entanglement swapping using two pairs of entangled photons instead of one. They’re also planning to move beyond laboratory conditions to test their approach in real-world underground optical cables.
While this research represents a significant step forward, we’re still years away from a full-fledged quantum internet. However, just as the classical internet evolved from simple connections between a few computers to the global network we have today, quantum networking is following a similar path. This demonstration of quantum-classical coexistence might just be remembered as one of the key moments that helped make the quantum internet a reality.
Paper Summary
Methodology
The researchers built a sophisticated optical system using specialized equipment including fiber optic cables, laser sources, and ultra-sensitive photon detectors. The setup had three main stations (Alice, Bob, and Charlie) connected by fiber optic cable. They used a technique called wavelength division multiplexing to separate quantum and classical signals, similar to how different radio stations can broadcast on different frequencies without interfering with each other. The quantum signals were generated using special crystals that create pairs of entangled photons, while the classical signals were generated using standard telecommunications equipment.
Results
The team successfully demonstrated quantum teleportation over a 30.2km fiber while simultaneously transmitting 400Gbps classical data. They achieved quantum state fidelity (a measure of how well the quantum information was preserved) of around 90%, well above the classical limit of 67%. The system worked even with classical signals 150 times stronger than needed for normal operations, suggesting significant headroom for practical applications.
Limitations
The system still requires extremely sensitive and expensive equipment, including superconducting detectors that must be cooled to very low temperatures. The quantum data rates are also relatively low compared to classical communications. The distance over which the system works (30.2km) is still relatively short compared to classical fiber optic networks that can span thousands of kilometers.
Discussion and Takeaways
This research demonstrates that quantum and classical networks can coexist in the same fiber infrastructure, potentially making quantum networks much more practical to deploy. The techniques developed could be applied to other quantum networking applications beyond teleportation. The work suggests that building a quantum internet might be possible using existing fiber optic infrastructure rather than requiring an entirely new network.
Funding and Disclosures
The work was funded by the U.S. Department of Energy through a subcontract from Fermi Research Alliance, LLC to Northwestern University. The research was supported by the DOE’s Advanced Scientific Computing Research Transparent Optical Quantum Networks for Distributed Science program. The authors declared no conflicts of interest, and data from the study is available upon reasonable request.
