Researchers have taken two separate steps toward a functioning quantum internet, with a US team successfully transmitting entangled photons across ordinary aerial-optic cable in a Washington suburb, and a European consortium launching a three-year project to build quantum memory devices for future networks.
Physicists at the US National Institute of Standards and Technology (NIST) said they transmitted entangled photos across 62 kilometers – about 38.5 miles – of existing telecommunications cabling strung between utility poles connecting NIST’s campus in Gaithersburg, Maryland, and the University of Maryland, without breaking the photons’ entanglement, according to the Journal of Optical Communications and Networking.
Together, the research points to one of the biggest moves now taking place in quantum communications. Scientists are no longer asking only whether quantum information can travel between laboratory systems.
They are trying to understand how it can survive existing telecom infrastructure, move between different types of networks and eventually operate over distances large enough for a practical quantum internet.
Quantum Memories Could Extend the Network
A European research consortium led by the University of Strathclyde in Glasgow has begun a three-year project to develop quantum memory device that can temporarily store quantum states, tackling a current hurdle in extending quantum networks over long distance.
Researchers’s three-year Atom-Light Free-Space Quantum Optics Networking, or AL FreSQO, project will develop cold-atom quantum memories that could become part of future quantum repeaters.
“Quantum signals are fragile and easily lost over long distances,” said Daniel Oi, a professor in Strathclyde’s Department of Physics and AL FreSQO’s lead coordinator.
“Because they can’t be copied or amplified, we need new ways to extend their range,” Prodessor Oi added.
That limitation separates quantum networks from conventional telecom systems. A normal optical signal can be amplified as it weakens.
Also, quantum information cannot simply be copied because doing so would destroy the quantum state carrying the information. Therefore, quantum repeaters offer another approach.
A long connection can be divided into smaller sections, with entanglement created separately across each segment.
Quantum memories then hold those states temporarily until the other parts of the connection are ready. The segments can eventually be joined through a process known as entanglement swapping.
Atom-Light Free-Space Quantum Optics Networking (AL FreSQO) project, is also designed to connect fiber networks with free-space optical systems, where photons travel through the air. That could become important for networks involving satellites, aircraft, ships, trains or vehicles where continuous fiber connections are difficult or impossible.
“Frequency conversion allows us to connect free-space and fiber networks by matching different wavelengths, giving greater flexibility in how quantum networks are built,” said professor Oi.
The project will develop wavelength-conversion technology because telecom fiber and cold-atom memories may operate using different wavelengths of light. According to researchers, cold-atom systems could reduce dependence on some of the large, energy-intensive cryogenic equipment required by alternative quantum-memory technologies.
Aidan Arnold, Strathclyde researcher will carry out quantum non-demolition experiments aimed at detecting photons without destroying them.
Taking Quantum Communication into Real Networks
While Strathclyde is working on how quantum networks could be extended, researchers at the NIST have tested whether entanglement can survive the infrastructure telecom operators already use.
The led team successfully distributed entangled photons across 62 kilometers of existing fiber between NIST and the University of Maryland. Much of that cable runs above ground on poles, exposing it to wind, temperature changes, traffic vibrations and other disturbances.
“I would call this a stress test of quantum networking systems,” said Yicheng Shi, NIST physicist, adding, “We put this to an extreme test in an environment that’s really noisy. Amazingly, it turned out it still worked.”
The problem is that environmental changes can alter the polarization of photons, disturbing the quantum information they carry.
“It’s about as bad a connection as you can possibly have,” said Oliver Slattery, NIST physicist.
Researchers addressed the challenge by sending a laser reference signal through fiber and measuring how much the light changed during transmission. That information allowed the system to correct the entangled photons.
Only 7.2% of operating time was required for the reference process, leaving 92.8% available for stabilized quantum transmission.
“Our results demonstrate the feasibility of distributing polarization-entangled photons over challenging fiber conditions, which is an important step toward the practical deployment of quantum networks,” researchers wrote.
The current transmission rate of roughly 200 to 1,500 entangled photons per second still needs improvement, and faster hardware and software will be needed before such systems can support large quantum networks. However, the direction is becoming clearer.
Quantum networking may develop as another communications layer running beside existing fiber and wireless infrastructure rather than replacing today’s internet.
“It’s a demonstration that quantum networking protocols can work in real-world environments,” Shi said.
The upcoming challenge is connecting these advances: memories that can hold quantum states, repeaters that extend their range, wavelength converters that bridge different systems, and telecom links stable enough to carry them. If those technologies can work together, quantum networking could move from isolated experiments toward infrastructure supporting secure communications, distributed quantum computing, precision sensing, and eventually a functioning quantum internet.
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