Researchers at Deutsche Telekom Innovation Laboratories (T-Labs), together with the quantum networking company Qunnect have reached a significant milestone on the path to the quantum Internet: demonstrating sustained, high fidelity (99%) transmission of entangled photons across 30 kilometres of commercially deployed fibre for 17 days.
This was achieved in a fibre-optic test track in Berlin provided by the T-Labs’ quantum research lab, which brings together 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 polarisation entangled photons were distributed over a 30-kilometre-long fibre-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, polarisation-entangled photons were dynamically routed over multiple paths, totaling 82 kilometres 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 fibres.
For the quantum Internet to support applications beyond point-to-point secure networking, it is essential to distribute the types of entangled photons, or qubits, that are used by quantum computers, sensors or memories. Polarisation qubits, like the ones used for this work, are highly compatible with numerous quantum devices, but they are difficult to stabilise in fibres. This success represents a notable 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. 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, facilitating 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 synchronisation for satellite networks and highly accurate sensing in industrial IoT environments, reinforcing our commitment to building the networks of tomorrow.
“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,” said Noel Goddard, CEO of Qunnect.
“Our fibre optics are ready for the quantum Internet, even today,” added Claudia Nemat, Member of the Board of Management for Technology and Innovation, 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.”
The research strategy on quantum focuses on integrating cutting-edge quantum technologies into telecommunications to drive innovation and enhance cybersecurity. Their strategy includes key areas such as quantum safe communications and efficiency improvements from utilising quantum resources in networks.
As part of this Deutsche Telekom opened its own Quantum Lab in Berlin in 2023. This research facility is equipped with an infrastructure for quantum optical experiments. It is connected to a fibre-optic network of over 2,000 kilometres that establishes connections to partners throughout Germany. A key point is the cooperation with the technical universities in Berlin, Dresden and Munich as well as the Fraunhofer Institute HHI and other partners from science and industry.
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