Twente develops novel photon source to ‘bring quantum computer closer’...

10 Apr 2025

…while Karlsruhe’s new encryption approach boosts defense against quantum threats.

Quantum computers are at a tipping point: the tech giants and governments are investing billions, but there are two fundamental obstacles: the quantity of qubits and the quality of these qubits. Qubit types include the polarization of a photon, in which the two spin states (left-handed and right-handed circular polarization) can also be exhibited and measured as horizontal and vertical polarizations.

Now a new invention from the University of Twente (UT), the Netherlands, is said to “improve the quality of photons to such an extent that building quantum computers based on light becomes cheaper and more practical.”

The researchers have published their research in Physical Review Applied.

The UT researchers, led by Jelmer Renema, have invented a component for a photonic quantum computer that “exchanges photon quantity for quality,” and have shown that this exchange yields more computing power. “Our discovery brings a future with powerful quantum computers a lot closer. That means improved medicines, new materials and safer communications. But also applications that we cannot yet imagine today,” said Renema.

“For a photonic quantum computer, you need extremely high-quality photons. Our technique ensures that only the best photons remain, which is crucial for reliable calculations,” he added. Because quantum computers are extremely sensitive to errors, previous generation methods required hundreds of physical photons to make a single reliable qubit. The Twente researchers’ new method can replace part of the required error correction, requiring far fewer photons.

Whereas previous methods focused on error correction afterwards, the UT researchers have tackled the problem at the source. They designed a new component that can conveniently distil a single high-quality photon from a brew of imperfect photons. To do this, they devised an optical circuit consisting of programmable light guides and detectors. By exploiting the quantum properties of light, they created a “Schrödinger’s cat”-like state in which photons are more likely to assume “good” rather than “bad” properties. As in Schrödinger’s thought experiment, measurement ultimately determines whether a photon has retained the desired properties.

Instead of accepting imperfect photons and correcting them later, the new method directly filters perfect photons. Although filtering each perfect photon requires sacrificing several imperfect photons, this reduces the total number of photons needed. This saves an enormous amount of computing power and makes quantum computers cheaper and more efficient.

By using an optical circuit with programmable switches, the researchers can filter out “bad” photons without having to know exactly what causes the error. “Normally, you have to decide in advance what you are filtering, such as a color filter that only lets red light through. We can now filter without knowing in advance what the problem is,” said UT PhD student Frank Somhorst.

Novel encryption approach to defend against quantum threats

Together with partners, researchers at the Karlsruhe Institute of Technology (KIT), Germany, have developed a method that can reliably protect what they describe as “today’s Internet communication from tomorrow’s quantum technology threats.”

“The rapid progress in the development of quantum computers is a threat to data security and encrypted communication,” commented Laurent Schmalen, Professor at KIT’s Communications Engineering Lab. “This is because the usual encryption methods are based on the fact that it is difficult to break down large numbers into their prime factors, meaning numbers that can only be divided by one and themselves.”

Schmalen added that this process is extremely cumbersome and time-consuming for conventional computers, but not for quantum computers. “And once you know the prime factors of a large number, you can break the encryption,” he said.

It is nevertheless possible to fend off quantum attacks with traditional cryptography – by using symmetric encryption. To do so, both sides need to exchange a virtual key before establishing the encrypted connection so that the transmitted data can be decrypted later on. This is a tap-proof method, but so far, it requires complex and expensive equipment.

KIT’s new method, however, works exclusively with conventional hardware. “We were able to perform the quantum key exchange with standard hardware as used in fiber-optic communication, such as the fiber-optic connections in homes and apartments, instead of relying on expensive special equipment,” said Schmalen. On this basis, universal use of the technology should be possible within five years. “This way, we can make the global telecommunications network tap-proof,” he said

Successful demonstration

At the end of March, 2025, the project team demonstrated the process in real time at LMU Munich. A video transmission protected by the quantum key exchange was achieved via an optical fiber connection on the campus. For this purpose, the KIT researchers developed innovative key matching algorithms.

“Our new algorithms for key matching are a decisive step towards ensuring tap-proof communication. They adapt dynamically to changing conditions and prevent attackers from obtaining information from the key exchange,” said Schmalen.

Tobias Fehenberger, R&D Director at Adva Network Security, commented, “Our project (DE-QOR Adva Network Security Project - High-performance transmission components for quantum-safe fiber optic communication in urban areas) is a significant milestone in the development of quantum-safe encryption. The successful validation of a modular, high-performance system proves that quantum security based on commercially available components and an open architecture is ready for use in practice.”