Breakthrough for the quantum internet – from the laboratory to the real world

Researchers at Deutsche Telekom Innovation Laboratories (T-Labs), together with the quantum networking company Qunnect (US & NL), have reached a milestone on the path to the quantum internet: demonstration of sustained, high fidelity (99%) transmission of entangled photons across 30 kilometers of commercially deployed fiber for 17 days.

This was achieved in a fiber-optic test track in Berlin provided by the T-Labs' quantum research lab, which brings together leading partners from across the research community to test and explore the latest quantum technologies for telecommunications networks. The results of this pioneering work were presented on March 31, 2025 at the annual OFC conference in San Francisco.

In the field experiment polarization entangled photons were distributed over a 30-kilometer-long fiber-optic network. The system automatically compensated to changing environmental conditions in the network, maintaining 99% fidelity. This performance was sustained for over 17 days with only 1% network downtime, surpassing previous demonstrations in metropolitan networks. 

In a separate field experiment, polarization-entangled photons were dynamically routed over multiple paths, totaling 82 kilometers in length, while coexisting with classical data traffic. Researchers demonstrated fidelities above 92%. This is the longest demonstration of high-fidelity entanglement distribution in the O-band, multiplexed with C-band classical data, ever performed over commercially deployed optical fibers. The results are currently posted on arXiv.

For the quantum internet to support applications beyond point-to-point secure networking, it is necessary to distribute the types of entangled photons, or qubits, that are used by quantum computers, sensors or memories. Polarization qubits, like the ones used for this work, are highly compatible with numerous quantum devices, but they are difficult to stabilize in fibers. This success represents a decisive step on the way to the quantum internet and demonstrates how existing telecommunications infrastructure can support the quantum technologies of tomorrow.

Researchers expect quantum physics to be the network of the future. A major advantage lies in the high level of security it offers. Because as soon as someone tries to intercept data, the state of the quantum particles is disturbed. This is noticed immediately. Currently, however, the challenge is to keep the quantum particles in the grid stable. So far, only transmission over short distances has been successful in the laboratory.

Entanglement can already be harnessed in quantum key distribution protocols, enabling ultra-secure communication links for enterprises and government institutions. Beyond security, entanglement also paves the way for future-oriented services such as high-precision time synchronization for satellite networks and highly accurate sensing in industrial IoT environments, reinforcing our commitment to building the networks of tomorrow.

Noel Goddard, CEO of Qunnect - “We are grateful to T-Labs and Deutsche Telekom for the opportunity to showcase the performance of our products integrated with classical data traffic over commercial network infrastructure.  Such partnerships are critical for demonstrating the progress towards commercial utility of quantum networking.”   

"Our fiber optics are ready for the Quantum Internet, even today," said Claudia Nemat, Member of the Board of Management for Technology and Innovation at Deutsche Telekom. "This is the first time that a major telecommunications provider has successfully triggered and conducted experiments with entangled photons on its own infrastructure. Our customers will benefit from such leaps in innovation. For example, through highly secure communication."