Networks Achieve Quantum Data Transfer over 30km Fibre

Scientists are advancing quantum communication networks by successfully demonstrating quantum teleportation of weak coherent polarization states on a metropolitan fibre. Zofia A. Borowska from Deutsche Telekom AG, Shane Andrewski from Qunnect Inc, and Giorgio De Pascalis from the Institute for Photonic Quantum Systems (PhoQS), Center for Optoelectronics and Photonics Paderborn (CeOPP), and Department of Physics, Paderborn University, led a collaborative effort involving researchers from Deutsche Telekom AG, Qunnect Inc, and Orbit GmbH. This research represents a significant step towards practical quantum networking, as the team achieved 90% average teleportation fidelity over a 30km field-deployed fibre loop within Deutsche Telekom’s Berlin testbed, crucially utilising commercial components and coexisting with live classical data channels. The demonstration validates the potential for integrating quantum key distribution and other quantum protocols into existing telecommunications infrastructure, paving the way for secure and high-performance future networks.

Can quantum communication function alongside existing internet traffic on standard fibre optic cables. Experiments in Berlin confirm it can, successfully ‘teleporting’ information using a live telecommunications network. This achievement moves quantum networks closer to practical deployment, paving the way for genuinely secure data transmission. Scientists are increasingly focused on the potential of quantum networks to connect advanced technologies such as quantum computers, sensors, and timing devices.

Realising this potential demands a demonstration of fundamental quantum protocols, including quantum teleportation, operating within the constraints of existing telecommunications infrastructure. At the core of this demonstration lies a sophisticated system employing a local Bell-state measurement. This measurement acts upon photons at 795nm originating from both a weak coherent source and a bichromatic warm-atom entangled photon source, enabling the transfer of quantum state information onto a photon operating within the O-band, a standard in telecommunications.

Then, this O-band photon traverses a 30-kilometer field-deployed fibre loop, mimicking real-world environmental conditions encountered in operational networks. Once received, the teleported state is reconstructed using a technique called state tomography, resulting in an average teleportation fidelity of 90 percent across the deployed link. System performance was thoroughly evaluated under two distinct scenarios: a standard dark-fibre configuration and with co-propagating C-band classical traffic present within the same fibre, confirming compatibility with wavelength-division multiplexed telecom infrastructure currently carrying live data.

Researchers are addressing the challenge of bridging the gap between quantum devices, often operating at specific wavelengths, and the optimal transmission wavelengths used in deployed fibre optic cables. Teleportation, supported by a bichromatic entanglement source, offers a solution by mapping a device-compatible photonic qubit onto a fibre-compatible telecom photon via a Bell-state measurement. This result signifies a key step towards practical quantum communication networks utilising existing telecommunications infrastructure. Measurements performed using state tomography reveal this fidelity, indicating a high degree of accuracy in reconstructing the teleported quantum state.

The system successfully transferred polarization-encoded qubits, prepared with a weak coherent state source, onto photons operating within the O-band for transmission. This wavelength asymmetry is intended to support hybrid networking, with 1324nm enabling low-loss telecom transmission and 795nm compatible with rubidium-based quantum devices. Coincidence rates between channels 2 and 3, indicative of successful entanglement, measured approximately 1.6 kcps with a 64ps window.

The research also evaluated system performance under realistic conditions, including the presence of co-propagating C-band classical traffic. The introduction of a 10 Gbit/s dense wavelength-division multiplexing signal did not impede the teleportation process, confirming compatibility with existing wavelength-division multiplexed telecom infrastructure carrying live data.

Optical time-domain reflectometry revealed attenuation within the 30-km fibre loop closely matched that of standard single-mode fibre at 1324nm, with a rate of 0.34 dB/km. At the Bell-state measurement station, single-photon detectors exhibited efficiencies of approximately 82 percent, 90 percent, and 92 percent for channels 1, 2, and 3 respectively, alongside a jitter of around 20ps.

Qu-APC modules were employed to compensate for fibre-induced polarization rotations, ensuring stable operation throughout the experiment. These modules were activated at predefined intervals, maintaining consistent polarization across the 30-km loop despite potential disturbances. By utilising commercial components and a real-world testbed, this study provides valuable data for the development of future quantum networks and their integration with current telecommunications systems.

Warm-atom entanglement and weak coherent light for metropolitan fibre teleportation

A 795nm weak coherent source and a bichromatic warm-atom entangled photon source initiated the experimental procedure. These sources generated the photons necessary for establishing quantum teleportation, with the weak coherent source providing a continuous stream of photons and the warm-atom source creating entangled pairs. Employing a local Bell-state measurement, the system distinguished between the four maximally entangled states, preparing the quantum channel for teleportation.

This measurement, performed on photons from both sources, is a key component in transferring the quantum state. Then, the conditional state transfer occurred onto an O-band photon, selected for compatibility with standard telecommunications wavelengths. This deployment under real-world conditions allowed for assessment of the system’s performance against environmental disturbances and typical telecom network stresses.

Evaluating compatibility with existing infrastructure demanded more than just fibre transmission. The experiment incorporated co-propagating C-band classical traffic, simulating the data streams present in a live telecommunications network. By transmitting classical data alongside the teleported quantum state, researchers verified that the quantum protocol did not interfere with, nor was interfered with by, conventional communications.

State tomography was used for state reconstruction, a technique that fully characterizes the quantum state of a photon by performing a series of measurements. At the receiving end, the teleported state underwent reconstruction via state tomography, allowing for a detailed analysis of fidelity. By bridging device-compatible photons at 780-795nm to telecom photons, the bichromatic teleportation scheme enables integration with Rubidium-based quantum processors, memories, clocks, and sensors, expanding the potential applications of the technology.

Quantum teleportation demonstrated within a live metropolitan fibre network

Once a futuristic dream, quantum teleportation is edging closer to practical reality, as demonstrated by recent work achieving high-fidelity transfer of quantum states over a live telecommunications network. For years, the biggest obstacle hasn’t been the physics itself, but the engineering challenge of maintaining delicate quantum states while coexisting with the noisy, bandwidth-hungry world of conventional data transmission.

Previous experiments often relied on dedicated fibre, isolating quantum signals from the everyday traffic that sustains our digital lives. Achieving this compatibility demanded clever solutions, in particular a system that performs a local measurement using photons from different sources, enabling the transfer onto a wavelength compatible with standard telecom infrastructure.

Beyond the technical achievement, this work signals a shift in focus, from proving the principle of quantum teleportation to building systems that can actually function within the constraints of a real-world network. Scaling these systems remains a considerable hurdle. Maintaining fidelity over longer distances will require quantum repeaters, devices that are themselves complex and imperfect.

The field faces the task of improving the rate at which entangled states can be generated and distributed, and refining the error correction techniques needed to combat signal degradation. Unlike earlier demonstrations, this experiment co-exists with classical data, but further research must explore how to increase this capacity without compromising the quantum signal.

In the next few years, we can anticipate seeing more experiments integrating quantum systems into increasingly complex network topologies, and perhaps even the first small-scale quantum networks connecting multiple users. In the end, the goal isn’t just to teleport quantum states, but to build a secure and powerful communication infrastructure for the future.