Deutsche Telekom T-Labs and Qunnect Achieve 17-Day High-Fidelity Quantum Link Over Commercial Fiber

Insider Brief:

• Deutsche Telekom’s T-Labs and Qunnect sustained 99% fidelity transmission of polarization-entangled photons over 30 km of commercial fiber for 17 days.

• The system autonomously stabilized polarization qubits and maintained performance despite environmental fluctuations, while a separate test demonstrated successful coexistence of quantum and classical signals over 82 km.

• Qunnect’s devices integrated directly with Deutsche Telekom’s infrastructure, validating the compatibility of entangled qubits with existing telecom networks for future quantum internet applications.

A team of researchers from Deutsche Telekom’s Innovation Laboratories (T-Labs) and U.S.-Dutch quantum networking firm Qunnect has demonstrated continuous, high-fidelity transmission of polarization-entangled photons over 30 kilometers of commercially deployed fiber. According to a news release, the automated system maintained 99% entanglement fidelity with less than 1% downtime over a 17-day field trial in Berlin.

The results represent a notable benchmark for quantum networking–demonstrating sustained entanglement distribution over a real-world telecommunications network rather than a lab environment. Conducted at T-Labs’ quantum research lab, the experiment used standard optical fiber infrastructure to transmit entangled qubits and was first presented at the Optical Fiber Communication conference on March 31 in San Francisco.

Field Performance and Coexistence with Classical Traffic

In the experiment, polarization qubits—quantum bits encoded into the orientation of a photon’s electric field—were transmitted across 30 kilometers while the system autonomously compensated for environmental variations such as temperature-induced fiber birefringence. This stabilization is essential, as polarization qubits are sensitive to such disturbances, making them difficult to maintain over fiber. Yet, they remain widely compatible with existing quantum computing and sensing devices, which makes this work relevant to a broad class of future quantum applications.

In a separate test, polarization-entangled photons were dynamically routed across an 82-kilometer optical path composed of multiple segments, while coexisting with classical data traffic. As noted by the release, fidelities remained above 92%, setting a new record for high-fidelity entanglement distribution in the O-band when multiplexed with classical C-band signals. This experiment demonstrates that quantum and classical traffic can coexist on shared infrastructure, which is an important requirement for scalable quantum networks.

Toward Quantum Internet Compatibility

Qunnect’s hardware was integrated directly into Deutsche Telekom’s network infrastructure, providing a real-world validation of quantum devices designed for long-haul fiber. The experiment was fully automated, with software adapting to environmental drift without manual calibration.

To support a functional quantum internet, we must be able to distribute entangled qubits that are compatible with quantum processors, sensors, and memories. These efforts confirm that the existing telecommunications infrastructure can support such a capability.

The experiment builds on ongoing efforts to operationalize entanglement distribution as a backbone of the future quantum internet. Entanglement-based quantum communication is already foundational to protocols such as quantum key distribution, and its utility is expanding into time synchronization, networked sensing, and ultimately, distributed quantum computing.

Qunnect CEO Noel Goddard emphasized the importance of industry partnerships in transitioning quantum networking out of the lab. “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 infrastructure,” she said in the release.

Telekom’s Quantum Research Ecosystem

Deutsche Telekom opened its dedicated Quantum Lab in Berlin in 2023, connecting over 2,000 kilometers of fiber to partners across Germany. The lab supports quantum optical experiments and includes collaborations with institutions such as the Technical Universities of Berlin, Dresden, and Munich, and the Fraunhofer Society.

Claudia Nemat, Deutsche Telekom’s Board Member for Technology and Innovation, commented, “Our fiber optics are ready for the quantum internet, even today,” noting this as the first demonstration of entangled photon transmission conducted by a major telecom provider on its own infrastructure.

As quantum networking technologies mature, long-term integration into classical networks remains an important goal. T-Labs’ stated research strategy focuses on quantum-safe communications and the application of quantum technologies to improve efficiency and security in telecommunication systems.