In an interview Dr. Matheus Ribeiro Sena from T-Labs’ Quantum Lab explains Marcin Makowski, Senior Communication Specialist at Group Technology, why the quantum internet is more than just a security feature and how current breakthroughs are paving the way for a new era of digital infrastructure.

Marcin: Why do we need a Quantum Internet – and what does it mean for customers?

Matheus: We need the Quantum Internet because it guarantees the security of our digital communications in the long term while opening up completely new service possibilities. Customers will not only benefit from a shield against eavesdropping and manipulation – even by future quantum computers – but also from innovative applications that will emerge from the networking of quantum computers. Step by step, the quantum internet will become a foundation for the digital world of tomorrow.

Marcin: How does this protection work?

Matheus: Quantum key distribution (QKD), one of the applications of the Quantum Internet, uses the laws of physics to enable protection. If someone tries to eavesdrop on the connection, it changes the state of the photons, and we notice this intrusion immediately. Quantum communication makes eavesdropping attempts physically detectable – and therefore preventable.

Marcin: Some people say, “I have nothing to hide.”

Matheus: It’s not just about private individuals. Governments, the military, and critical infrastructures transmit highly sensitive data. And quantum communication is more than just security. It is the foundation for an entire ecosystem of new technologies such as ultra-precise sensing technologies, high-accuracy time synchronization, and, of course, connecting quantum computing. To connect these technologies, we need a Quantum Internet.

Marcin: T-Labs recently achieved a breakthrough: 99% fidelity in the transmission of entangled photons over 30 km for 17 days. Why is that so significant?

Matheus: To understand this, you need to understand two key terms: entanglement and fidelity (accuracy). Entanglement is the basis of quantum networks. It is a phenomenon in which two photons are so closely connected that their states are dependent on each other – even over long distances. Measuring one photon immediately reveals the state (e.g., polarization) of the other.
Fidelity measures how accurately we maintain the quantum state during transmission in an ideal situation. In the real world of fiber optics, environmental influences such as temperature and vibrations affect polarization – and thus the quality of entanglement.

Marcin: And that’s what makes your success so special?

Matheus: Yes. We achieved a fidelity of over 99% over a period of 17 days – over 30 km of real telecommunications fiber, not in a laboratory. This was achieved through active polarization compensation, which compensates for interference in real time. Previous attempts achieved a maximum of 15 days, mostly under more stable conditions. Entanglement is the central building block of the Quantum Internet because it forms the basis for connectivity between quantum devices.

Marcin: What was the biggest technical challenge?

Matheus: That’s the instability of polarization in real fiber optics. Our solution: a system with an injector and a compensator. The injector sends light with known polarization, and the compensator measures and corrects deviations. This keeps the polarization stable even with entangled photons.

Marcin: You have also transmitted entangled photons over 82 km. How does that work?

Matheus: That was actually a series of experiments. First, we wanted to find out over what distance entangled photons in the O-band could be reliably transmitted via existing fiber optics – and we were able to achieve 82 km. Then we worked with adaptive routing, which involves sending the photons dynamically via different fiber optic paths. Finally, we demonstrated that transmission also works in parallel with conventional data communication by multiplexing both in the same fiber optic cable.

Marcin: What is the challenge here?

Matheus: Every fiber behaves differently, depending on whether it runs through buildings or underground. We had to automatically recalibrate the polarization compensation every time we changed routes. This is the only way to maintain the high fidelity of the entanglement while adaptively routing it.
Interviewer: And how does coexistence with conventional data traffic work?
Matheus: Classic signals are much stronger than quantum photons. Without protective measures, they can interfere with the measurement of the quantum signals. We separated the signals spectrally and filtered them heavily. This allowed both to run simultaneously over the same fiber without any loss of quality.

Marcin: Why is that so important?

Matheus: Because it allows us to use existing fiber optic networks. A separate network just for quantum communication would be expensive and inefficient. Coexistence makes expansion more realistic and economical.
Interviewer: Why is the 82 km distance so remarkable?
Matheus: Because quantum signals cannot simply be amplified; that would destroy the entanglement. We have achieved such a long distance in the O band, which is compatible with today’s telecommunications infrastructure, for the first time. This is an important step toward scalable entanglement-based quantum networks – and with 82 km, metropolitan-scale quantum networks become more realistic.

Marcin: And what’s next?

Matheus: Our results show that stable transmission of entanglement is possible in a real network – and that’s exactly what opens up the next steps for us. Now it’s time to test quantum internet protocols such as teleportation and quantum repeaters and integrate quantum memory. These developments are crucial for creating connectivity between future quantum internet devices.