Researchers have created a network that they say demonstrates the real-world feasibility of a quantum internet that's physically impossible to hack, at least without detection.

Working with quantum startup Qunnect and networking company Cisco, the team connected a trio of nodes across New York's existing fiber-optic cables with quantum signals in the form of photons (packets of light) where quantum states are used to carry information through entangled qubits. By distributing and swapping entanglement between the signals, the scientists effectively connected them into a small quantum network.

This third node acts as an intermediate hub where the team could perform entanglement swapping and routing, turning two links into a small multi‑node quantum network that can distribute entanglement across different pairs on demand. This would act more like a true network rather than a single line.

"Manhattan is a very compact place," said Javad Shabani, director of NYU's Center for Quantum Information Physics and the NYU Quantum Institute. "Everything is within five or six miles, and you can find hundreds of financial institutions in a very small radius. That density — of infrastructure, institutions, and potential users — may make the city one of the first places where a quantum internet begins to take shape. Having this network right now is important. It's a huge investment that will pay off probably in the next decade or so."

A blueprint for future quantum networks

A quantum internet is deemed "unhackable" due to device-independent quantum key distribution (DI-QKD), a method by which cryptography keys are encoded in the quantum state of particles such as photons. It's not possible to copy quantum states, and measuring them disturbs them — meaning that eavesdropping is difficult and simple to detect.

Information travels via photons, but they can get easily lost in fiber. In addition, "noise" — disturbances caused by the environment or other stimuli — scrambles their states, thus limiting data transfers to very short distances.

To extend this range, the team created a "hub-and-spoke" network — an intermediary hub for swapping and routing with two outlying spokes. To accomplish this, they created simple nodes at Qunnect's Brooklyn facility and generated pairs of photons that are entangled — meaning their quantum states are linked so they share information over space and time. These flowed across 5 to 6 miles (8 to 10 kilometers) of deployed commercial fiber to a central hub at a QTD Systems facility, a commercial data center and network facility in Lower Manhattan.

One vital advance came in the form of "entanglement swapping" — a process by which particles that have never previously interacted can become entangled. This is key for building short connections into a larger network, the scientists said.

This relies on measurements that "transfer" entanglement from initial pairs to distant ones. It relies on quantum teleportation — the phenomenon where two or more particles share linked quantum states — so measuring one instantaneously determines correlated properties of the others. However, instead of teleporting data between two entangled qubits, it teleports the state of entanglement itself.

The swapping happened at the QTD center, where cryogenic detectors ‪—‬ ultra-sensitive photon detectors cooled to extremely low temperatures to reliably detect single photons carrying quantum information ‪—‬ measured the photons and linked pairs that had never interacted. The result was city-spanning entanglement between the original outer sources.

Addressing the internet's Achilles' heel

Conventional data transfers are highly susceptible to eavesdropping. Scientists say the quantum internet would solve this issue because any interception disturbs the photons, making the tampering immediately apparent.

This experiment proves metropolitan-scale quantum links work with live telecom fibers, solving the issues of weakening or loss of photons as they travel through optical fiber cables, alongside temperature extremes and vibration that can wreck fragile entanglement.

The hub-and-spoke design addresses scalability by centralizing complex cryogenic gear at one hub. This sidesteps the issue of every node requiring pricey, power-hungry cooling, meaning the network can be expanded without ballooning costs.

In the short term, this demonstration paves the way for QKD, the sharing of unhackable encryption keys to protect sensitive data from sources like banks, the government or the healthcare industry.

In the longer term, it's a step toward true distributed quantum computing, which could link multiple devices to address highly sophisticated problems, like drug discovery or climate modeling, that no single operator could handle.

Entangled networks could also be deployed to boost quantum sensing, which could lead to ultraprecise clocks, navigation without GPS and other high-precision sensor arrays.

Among the key challenges are that fiber-optic cables absorb and scatter photons exponentially with length — about 0.2 decibels per kilometer at telecom wavelengths — dropping entanglement success to near zero beyond 62 miles (100 km) without boosting. The new experiment transmitted information over a mere 5 to 6 miles (8 to 10 km) per leg; spanning longer distances will require quantum repeaters, which lack the quantum memories required to function effectively.

However, the experiment was important in proving the viability of quantum networks outside a strictly controlled laboratory environment. The scientists showed that the effects of noise and loss can be adequately managed to sustain entanglement across a dense metropolis like New York.

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