In a new paper published in Light: Science & Applications, a team of scientists from the Technical University of Denmark (DTU), the Austrian Institute of Technology (AIT), Fragmentix, University of Waterloo (UW), and Technische Universität Wien (TU) have successfully demonstrated a more practical and robust method for quantum key distribution, a breakthrough that could soon lead to more secure and cost-effective communication networks worldwide.
Imagine sharing secrets today that will stay safe – even from the most powerful quantum computers of tomorrow. That's the promise of quantum key distribution (QKD), a method that uses the principles of quantum physics to securely distribute keys that can be used to encrypt secret messages, banking transactions and sensitive health data. Thanks to its design based on the quirks of quantum mechanics, QKD offers forward security: since an attacker cannot copy the quantum states used to exchange the keys, even if an attacker gets access to powerful technologies like quantum computers in the future, they still won't be able to retroactively decode the messages.
Among QKD systems, continuous-variable QKD (CV-QKD) stands out for its compatibility with standard components found in today's optical telecommunication networks. However, the most common version of CV-QKD – based on a complex encoding scheme knows as Gaussian modulation – faces significant practical challenges. It represents a theoretical idealization that can never be perfectly realized in practice and requires highly precise hardware and vast amounts of random data. These demands pose serious obstacles to scaling CV-QKD for real-world applications.
To address these challenges, the research team has turned to a simpler variant of CV-QKD known as discrete-modulated (DM) CV-QKD. Instead of encoding the key information in a continuous range of values, they only use four distinct quantum states, which is known as quadrature phase-shift keying (QPSK) modulation. This simplification reduces hardware complexity, lowers the demand for random data, and aligns naturally with existing telecom infrastructure. However, this comes at a price: the reduced symmetry makes the security analysis more complex.
But there's also another catch: real-world systems face noise, errors, and other imperfections – so they never behave exactly as idealized with pen and paper. That's where composable security comes in. Unlike claims limited to controlled lab settings, composable security guarantees that the generated keys remain secure even when used within larger cryptographic systems, such as secure messaging apps or digital payment platforms. It's a tough standard - and until now, no one had experimentally shown that DM CV-QKD could meet it.
In a groundbreaking experiment, the researchers demonstrated the first-ever generation of composable secure keys using DM CV-QKD. "Bringing together theory, experiment and classical postprocessing in a clean and rigorous way is crucial ", says Florian Kanitschar, Junior Scientist at AIT, who worked on the security analysis. "We focused on bridging these aspects, to ensure the theoretical security claims hold up in the real world." The team transmitted signals over 20 km of standard telecom fiber and proved that the resulting keys could withstand real-world noise and hacking attempts. The system achieved a secure key rate of about 0.011 bits per symbol, using simple telecom hardware combined with clever digital postprocessing.
"This work is a major step forward because it shows that practical quantum security can integrate with today's telecom networks.", says Adnan Hajomer, tenure track researcher at DTU, who carried out the experiment. "And because it's based on composable security, it meets the strict requirements needed to protect real-world applications." Associate Professor Tobias Gehring, who leads CVQKD activities at DTU emphasizes the broader significance of the work " This demonstration is not just a technical milestone – it's a proof that quantum-secure communication can be both practical and scalable. It brings us closer to integrating quantum technologies into everyday digital infrastructure."
