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Quantum networks shown to work on existing internet cables

Quantum Networks Proven on Existing Fiber: A Leap Toward Scalable Quantum Communication

Quantum networks are no longer a laboratory concept—they're operating on the same fiber cables that carry your internet traffic today. These networks enable the secure transmission of quantum information between nodes, paving the way for a new communication paradigm that outpaces classical encryption and supports distributed quantum computing. By leveraging the principles of quantum mechanics, they enable the exchange of entangled particles and the implementation of quantum key distribution (QKD), which makes interception or eavesdropping physically detectable.

At the core of this infrastructure are three fundamental components: QKD for ultra-secure communication, entanglement distribution that links particles across distance while maintaining quantum correlations, and quantum nodes—devices that can store, process, or route quantum information. Together, they form the technical backbone of a quantum internet. So, what does it mean now that these systems can operate over our current fiber-optic networks?

Entanglement and Quantum Communication: A New Era of Secure Information

Quantum Entanglement: Instantaneous Correlation Across Distance

Quantum entanglement links the states of two or more particles in such a way that the state of one directly determines the state of the other—regardless of the distance between them. When one of the entangled particles is measured, the outcome for the other becomes instantly defined. This is not a theoretical idea; experiments such as the 2015 Delft University loophole-free Bell test have confirmed that entangled particles exhibit correlations that can't be explained by classical physics alone.

Quantum Communication: Entanglement in Action

In quantum communication protocols, entanglement enables secure data exchange by making eavesdropping physically detectable. When two parties, commonly referred to as Alice and Bob, share entangled particles, any attempt by a third party to intercept the communication collapses the entangled state and introduces detectable anomalies. This principle underpins quantum key distribution (QKD), a method to securely exchange cryptographic keys that has already been demonstrated over fiber-optic cables and free-space optical links.

Quantum Teleportation: Transferring States, Not Matter

Teleportation in the quantum domain doesn’t move particles; it transfers quantum information—such as the exact state of a photon or electron—from one location to another. The process involves three key resources: shared entanglement, a classical communication channel, and the original quantum state. When applied correctly, this technique allows the unknown quantum state of a particle to be reconstituted on a distant particle, effectively “teleporting” it. In 2020, scientists at Fermilab and Caltech achieved quantum teleportation across a distance of 44 kilometers, marking a significant step toward real-world quantum networks.

Why Entanglement Changes the Security Model

Classical encryption can be broken with enough computational power, but quantum systems based on entanglement shift the paradigm. Security no longer rests on mathematical complexity but on the fundamental laws of quantum mechanics. In entangled communication, if a qubit's state is intercepted or observed, its entangled partner is instantly affected—making unauthorized access immediately obvious. This creates surveillance-proof communication systems where tampering is not just discouraged, but technically impossible without detection.

This shift redefines what "secure communication" means—not as an absence of breaches, but as a system where breaches cannot occur without being detected. In quantum communication, security is not about mitigation; it's about prevention by design.

Overcoming the Infrastructure Barrier in Quantum Networking

Limits of Classical Networks in Quantum Data Transfer

Conventional networks—built for transmitting digital bits, not entangled qubits—introduce inherent limitations for quantum communication. Classical systems rely on amplification and regeneration of signals to cover long distances. Quantum signals, due to the no-cloning theorem, cannot be amplified in the same way. This makes current optical amplifiers incompatible with quantum communication, driving loss rates that scale exponentially with distance.

Additionally, classical routers and switches fragment signal coherence. Once photonic qubits enter legacy switching infrastructure, entanglement can degrade immediately. In effect, the very fabric of classical networks disrupts the integrity required for deterministic quantum states to survive transmission.

Latency, Bandwidth, and Network Scalability Constraints

In conventional systems, latency arises from processing delays in switches and routers—issues that compound in large-scale networks. Quantum communication tolerates far less delay, particularly because many protocols require precise timing and synchronization at the nanosecond scale. With each inserted hop, the fidelity of entangled states drops, and the chances of successful transmission plummet.

Bandwidth poses another restriction. While fiber-optic cables do offer significant throughput, channeling photonic qubits requires isolation from noise sources and crosstalk, significantly reducing available wavelength channels. Quantum systems can't simply share bandwidth with classical traffic without encountering decoherence.

As for scalability, classical networks were not designed for entanglement distribution or quantum key distribution protocols. The current infrastructure supports daisy-chained node connections, but quantum networks demand multi-node entangled states, dynamic routing of qubits, and loss-tolerant connections between distant nodes. Scaling such topology over classic infrastructure remains a critical bottleneck.

Why Existing Fiber-Optic Cables Still Matter

Despite these constraints, abandoning the existing backbone is not an option. More than 1.2 billion kilometers of fiber-optic cable already lie beneath streets, oceans, and continents. Deploying entirely new quantum-only infrastructure at global scale would inflate costs into tens of trillions of dollars, delay adoption timelines by decades, and require impractical permissions on a city-by-city basis.

Reusing current fiber reduces capital expenditure significantly. Moreover, recent studies—including work by researchers at Delft University of Technology—have shown quantum signals can securely traverse installed metropolitan fiber even with concurrent classical internet traffic. This compatibility opens the path toward hybrid quantum-classical deployments.

By retrofitting existing cables, integrating new quantum-compatible nodes, and deploying quantum repeaters at key intervals, engineers can incrementally upgrade the backbone of the Internet without tearing it down. This layered approach leverages what already exists while paving the way for fault-tolerant, secure quantum communication on a global scale.

Researchers Demonstrate Quantum Networks Using Existing Internet Cables

Running Entangled Photons Through Legacy Fiber

In a controlled yet realistic setting, scientists successfully transmitted entangled photon pairs across standard fiber-optic cables typically used for classical data transmission. The experiment took place over a metropolitan-scale fiber network, confirming that the syntax of quantum communication can run on the grammar of today’s internet infrastructure.

This wasn’t a theoretical exercise. The researchers introduced real-world variables — including environmental noise, cable aging, and signal attenuation over distance — directly into the experiment. Using wavelength-division multiplexing (WDM), they allowed quantum signals to coexist with traditional data streams in the same fiber, eliminating the need for dedicated lines.

Institutions Driving the Breakthrough

The team behind the achievement included researchers from TU Delft in the Netherlands, QuTech (a collaboration between TU Delft and TNO), and partners from the European Quantum Internet Alliance. Their work built on earlier prototype networks and simulations but moved a step closer to scalable infrastructure by using live urban cable infrastructure.

In parallel, the Japanese National Institute of Information and Communications Technology (NICT) and the University of Tokyo contributed similar real-world validations. Their experiments, conducted over installed telecom fiber in Tokyo, used entanglement-based quantum key distribution (QKD) across several kilometers amidst heavy classical data traffic.

Scientific Method and Experimental Architecture

The experimental framework relied on polarization-entangled photon sources operating at 1550 nm, a wavelength compatible with standard telecom fibers. Entangled photons traveled through deployed fiber links, while superconducting nanowire single-photon detectors (SNSPDs) received and measured them with high precision.

Timing synchronization was handled via classical channels, and phase stabilization routines compensated for polarization drift caused by external temperature fluctuations and mechanical stress on the fibers. Each test confirmed nonlocal correlations between particle pairs through Bell test inequality violations — a rigorous benchmark for genuine entanglement transmission.

The experiment’s success didn’t hinge on lab-conditioned cables but on the varied, imperfect landscape of actual internet infrastructure. This shift marks a decisive moment: physics meets engineering at real-world scales.

Quantum Key Distribution Achieved on Conventional Fibre-Optic Networks

Quantum Key Distribution: Redefining Encryption Security

Quantum Key Distribution (QKD) enables two parties to share cryptographic keys with information-theoretic security, leveraging the principles of quantum mechanics—specifically, the no-cloning theorem and measurement-induced disturbance. Unlike classical key exchange protocols, QKD ensures that any interception attempt by a third party alters the quantum state, making eavesdropping detectable.

Protocols such as BB84, first proposed in 1984 by Charles Bennett and Gilles Brassard, lay the foundation for QKD. These protocols use properties of quantum particles, like photon polarization, to encode binary key information. When measured in incompatible bases, any attempt to intercept induces errors in the key sequence, immediately flagging the intrusion.

Strategic Role of QKD in Quantum Communication

In the landscape of quantum communication, QKD forms the cornerstone of security architecture. It eliminates reliance on computational hardness assumptions, which underpin most classical encryption algorithms. While quantum computing threatens symmetric and asymmetric encryption, with algorithms like Shor’s capable of factorizing large integers exponentially faster, QKD remains immune to such advances.

In operational scenarios, QKD is used to generate and distribute symmetric keys, which then encrypt data using conventional methods like AES. This hybrid model combines quantum-level key security with the high-speed throughput of classical encryption, making it suitable for real-world applications, including government communications, financial transactions, and data center interlinks.

Utilizing Existing Internet Infrastructure for QKD

Researchers from the University of Cambridge and Toshiba Europe achieved a significant breakthrough by transmitting quantum keys over standard optical fibre—similar to those used for home or business internet connectivity. Rather than building bespoke quantum connections, they retrofitted quantum signals onto classical fibre-optic routes using a technique called multiplexing.

In a 2022 study published in Nature Photonics, the team demonstrated QKD over 600 kilometers of installed fibre with optical losses up to 65 dB. They employed a twin-field QKD protocol, which enables longer distances through single-photon interference. High-performance superconducting nanowire single-photon detectors (SNSPDs) were used to handle the extreme sensitivity required at these ranges.

This approach eliminated the need to dig new trenches or deploy dedicated quantum-specific infrastructure. Instead, quantum signals operated alongside classical data by carefully managing the timing, polarization, and frequency of the photons. Optical isolators and ultra-low noise amplifiers were introduced to minimize interference between quantum and classical channels.

The result: provably secure key exchange using the same cables that stream video, handle cloud services, and support VoIP. The coexistence model aligns with current telecom practices, allowing straightforward integration into existing network management systems without halting current data traffic.

By proving that QKD works on today's internet backbone, this achievement moves quantum security closer to deployment at scale. The possibility of city-wide and even cross-national quantum-secure communications can now be pursued without starting from scratch.

Decoding the Core: Photonic Qubits and Quantum Repeaters

Why Photonic Qubits Power Quantum Networking

Photonic qubits—quantum bits encoded in particles of light—form the backbone of quantum communication. Their resilience to thermal noise, rapid transmission speed, and compatibility with existing fibre-optic technology position them as the optimal carriers of quantum information.

Unlike matter-based qubits, photons do not require cryogenic temperatures to maintain coherence. This allows for transmission through standard optical fibres without the need for complex cooling systems. Furthermore, quantum states can be encoded into light through properties such as polarization, time-bin, or phase, offering multiple degrees of freedom for secure data processing.

Quantum Repeaters: Extending Range Without Compromising Fidelity

Quantum communication over long distances faces an inherent challenge: signal attenuation. As photons travel through optical fibres, their probability of being lost increases exponentially with distance. Standard amplification methods used in classical networking fail, as they disturb the quantum state and break entanglement.

Quantum repeaters address this problem through a mechanism that combines entanglement swapping, quantum memory, and entanglement purification. These devices divide the total communication distance into shorter segments, create entanglement across each, and then connect them via entanglement swapping. This technique reconstructs long-distance entanglement without directly transmitting a photon the whole way.

Maintaining Entanglement Over Long Distances

Successful distribution of entanglement over kilometres of fibre relies on precise control of quantum states and mitigation of environmental noise. Researchers employ time-bin encoding, which is less sensitive to phase fluctuations than polarization encoding, and utilize ultra-low-loss fibres to reduce signal degradation.

In recent trials, quantum repeaters integrated with quantum memories—devices that can temporarily store qubits without loss of information—have demonstrated the ability to maintain entanglement over links exceeding 50 km. This milestone paves the way toward continent-spanning quantum networks, effectively bridging metro-scale deployments with global ambitions.

Thinking practically, what does this mean for the network engineer in the field? Fewer physical interventions, native compatibility with today's infrastructure, and the opportunity to handle quantum information without a complete overhaul of transmission hardware.

Bridging Two Worlds: Interoperability with Classical Networks

Challenges in Integrating Quantum Architecture with Classical Network Devices

Quantum networks operate under fundamentally different physical principles compared to classical networks. This makes direct integration with existing routers, switches, and protocol stacks non-trivial. The main issue stems from the incompatibility between quantum and classical data representations. While classical networks transmit binary information in the form of voltage or light pulses (0s and 1s), quantum systems use qubits, which can exist in superpositions of states.

Standard network devices have no way to interpret or process quantum states. Classical routers designed to regenerate and resend electrical or optical signals inadvertently disturb or collapse quantum states. This leads to loss of encoded quantum information, especially in protocols like Quantum Key Distribution (QKD) which rely on entanglement and state integrity.

Moreover, synchronization becomes critical. Classical infrastructure lacks native capability for the time-sensitive coordination required between quantum devices. Even minor jitter in signal timing can corrupt quantum protocols, rendering them ineffective. To offset this, quantum-compatible timing modules and interface layers need to be embedded into or run in parallel with existing network systems.

Bridging Protocols: Orchestration of Classical and Quantum Components

To enable functional interoperability, hybrid control architectures have been developed. One notable example includes the use of quantum-classical control planes that segment control and data functions between two network layers. The classical control layer handles tasks like routing decisions and session management, while the quantum layer operates on qubit transmission, entanglement generation, and key synchronization.

Network protocols like QKD-IP and entanglement routing protocols integrate with standard internet protocols (e.g., TCP/IP) to support session establishment, qubit synchronization, and entropy management. These act as digital bridges, allowing otherwise incompatible systems to share coherent information pathways without compromising security or speed.

Field experiments also demonstrate that software-defined networking (SDN) layers can monitor both classical and quantum metrics, dynamically switching channels or reconfiguring routes depending on fidelity and entanglement success rates. This creates a flexible orchestration layer that adapts network behavior to quantum protocol conditions in real time.

Coexistence on Shared Fibre: Compatibility in the Physical Layer

Quantum signals, transmitted via single photons, are inherently delicate. Yet, they can coexist with classical signals on the same fibre-optic cable using a technique known as wavelength-division multiplexing (WDM). By allocating distinct wavelengths to quantum and classical channels, both types of data avoid interference while traveling the same physical medium.

For instance:

Successful demonstrations in urban-scale testbeds, such as the ones led by researchers at TU Delft and the University of Geneva, confirm the feasibility of simultaneous classical-quantum transmission across installed fibre routes. These findings eliminate the need for building separate fibres for quantum applications, dramatically reducing infrastructure costs and accelerating potential large-scale deployment.

This convergence signals a practical path forward: quantum networks will not replace classical systems—they will augment them, leveraging existing infrastructure while unlocking a new tier of secure, high-performance communication capabilities.

Quantum Networks and the Next Phase of Global Communication

Moving from Concept to Infrastructure: The Quantum Internet Takes Shape

The successful transmission of quantum information across existing internet cables signals more than technical triumph—it initiates the groundwork for a fully-realized quantum internet. For the first time, researchers have demonstrated that quantum key distribution (QKD) and entanglement-based communication can coexist with classical signals on commercial fibre-optic networks. This validation bridges theoretical physics with operational networks, transitioning quantum communication from research labs into global telecom architectures.

With this proof-of-concept, researchers have shown that quantum signals can share the same transmission medium as everyday internet traffic, without crippling noise interference or the need for dedicated fibres. This outcome redefines what’s technologically practical, turning long-standing barriers—cost of infrastructure and signal degradation—into manageable engineering challenges. The result: a viable blueprint for scaling quantum communication across continents using the same undersea cables and citywide loops that carry digital data today.

Strategic Impacts on Politics, Defense, Finance, and Data Protection

Secure communication isn’t a theoretical concern—it underpins global stability. In geopolitics, encrypted channels reduce espionage risks. In defense, they ensure the integrity of strategic command systems. In finance, they protect transactions that move trillions of dollars daily. With QKD, even future quantum computers won’t decrypt past transmissions, since keys are never stored but generated in real-time and destroyed after use.

Governments and multinational institutions are already taking notice. The European Union’s EuroQCI initiative and China's quantum satellite Micius demonstrate a geopolitical race to establish dominance in quantum-secure infrastructure. As domestic and international data traffic increasingly relies on endpoints secured by quantum keys, the political leverage of such networks will be as significant as 20th-century telegraphy or 21st-century 5G networks.

Latency-Free, Ultra-Secure: The Quantum Internet Vision

Unlike classical communication, the quantum internet isn’t merely faster—it enables functions that classical systems can’t replicate. Entanglement-based protocols can create networks where data is not just encrypted but physically protected by quantum laws. Attempts to intercept information collapse the quantum state, destroying the data during the eavesdropping attempt.

This radical security model supports a future where latency-sensitive applications—such as real-time remote surgery, international stock exchange arbitrage, or autonomous vehicle coordination—can operate without vulnerability to data breaches. Combine that with entanglement-based synchronization across vast distances, and the quantum internet holds the potential to reengineer entire industries.

Each of these applications relies on one simple fact: once quantum networks integrate with classical infrastructure, ultra-secure global connectivity becomes not just feasible, but scalable.

Bridging Knowledge Gaps: Developing Quantum Communication Literacy Among Network Engineers and Stakeholders

Advancing Quantum Infrastructure Through Focused Education and Training

Quantum networks challenge long-standing assumptions in network design and security. Unlike classical systems, they rely on the manipulation of quantum states—specifically qubits—often transmitted via entangled photons. Understanding this shift is not optional for professionals maintaining or developing the backbone of the internet. Structured training programs, such as the European Quantum Technology Community Network’s (QCN) curriculum and the Quantum Communications Hub in the UK, are already offering specialized modules tailored for ICT staff, telecom providers, and infrastructure policymakers.

Training initiatives need to cover not only the foundational physics of quantum entanglement but also the practical aspects of deploying quantum key distribution (QKD) using existing fibre-optic infrastructure. For example, workshops under the ETSI Industry Specification Group on QKD have helped demystify integration challenges for legacy systems. Programs that merge hands-on labs with theoretical sessions consistently show higher retention of concepts among engineers transitioning to hybrid quantum-classical networks.

Collaborative Protocol Comprehension for Smarter Integration

The introduction of new quantum protocols—like BB84 or E91 for QKD—demands not just isolated technical knowledge, but shared understanding across stakeholder groups. Without this, system-level integration suffers. Engineers skilled in standard optical networks must be able to interpret measurement reports on quantum bit error rates (QBER) and translate them into actionable reconfiguration procedures.

To facilitate this, organizations have adopted integrated documentation approaches. Instead of treating quantum and classical protocols in silos, network diagrams now reflect full-stack interaction, from quantum photon sources through to classical error correction processes. This ensures engineers can comfortably trace data flow through the hybrid system, identify where authentication handshakes happen, and predict how changes in one layer affect the quantum channel’s integrity.

Interdisciplinary Teamwork: Physicists, Engineers, and IT Professionals in Sync

Building a quantum-capable communication network is not a job for one discipline. It takes physicists who understand the probabilistic behavior of qubits, network engineers who excel in system architecture, and IT professionals capable of securing and monitoring data flow in real time.

Joint simulation labs have proven particularly effective. At TU Delft, pilot projects featuring interdisciplinary teams reduced integration times by nearly 30%, streamlining the transition from individual component tests to system-wide deployment. What does this mean in practice? Fewer errors, more consistent uptime, and faster rollout of quantum-ready network segments.

Want to improve collaboration across your teams? Start by putting a network engineer and a quantum physicist in front of the same topology diagram. The questions that arise will highlight exactly where the communication channels—both technical and human—need reinforcement.

Real-World Implementation: From Lab to City-Wide Deployment

Pilot Programs Are Already Running on Urban Networks

Quantum networks have moved beyond controlled laboratory environments and are now operating within actual metropolitan fibre infrastructure. A notable example comes from the city of Cambridge, UK, where Toshiba and BT demonstrated quantum key distribution (QKD) over existing fibre spanning commercial and government buildings. The deployment covered a distance of 43 kilometers, fully integrated into Openreach’s local fibre network, requiring no major cable overhauls or replacements.

This real-city trial achieved stable quantum communication over several weeks, even as classical data continued transmitting along the same fibres. By using wavelength-division multiplexing, researchers successfully separated quantum and conventional signals, proving that a hybrid communications model is technically feasible.

Quantum Nodes Installed in City Networks

Across metropolitan areas, telecom providers and research labs have begun installing quantum nodes directly into their core network architecture. These nodes act as points for entanglement distribution and quantum key generation. In Chicago, the Argonne National Laboratory and University of Chicago launched a 124-mile quantum loop using existing fibre provided by regional network providers. Each node deployed within this loop contributes to real-time quantum key distribution and verifies entanglement fidelity across the network in a real-world environment.

Meanwhile, in Vienna, researchers with the European QKD Testbed (EuroQCI) installed quantum endpoints into municipal fibre linking several administrative buildings. This installation used standard rack-mountable hardware for quantum devices, reflecting a crucial step toward simplified deployment in existing data centres and telecommunication hubs.

Use Case Examples: Quantum-Enhanced Secure Communications

Every case demonstrates a clear transition from theoretical frameworks to practical application. These implementations show that quantum networks can operate over standard telecom-grade equipment without forcing costly rewiring or infrastructure replacement. And as pilot programs scale, they continuously feed data back into system optimization, accelerating the roadmap toward integrated quantum-secure internet services.