A study published in the academic journal IEEE Journal of Selected Topics in Quantum Electronics shows how researchers used commercially available components to generate and distribute quantum entanglement across an existing city fiber network, a step toward practical quantum communication systems.
The research focused on improving how entangled photons are created and transmitted for quantum key distribution (QKD), a technology that uses the laws of quantum physics to send encrypted messages securely.
Quantum entanglement links two particles so that measuring one instantly reveals information about the other. In communication systems, this property can be used to create encryption keys that are extremely difficult to intercept.
Most current QKD systems rely on polarization-based entanglement, which encodes information in the orientation of photons. However, this method becomes unstable when signals travel through long optical fiber networks.
The research team instead used time-bin entanglement, which stores information in the arrival time of photons. This approach is more stable over long distances but has usually required complex and custom-built equipment.
Researchers from the Austrian Institute of Technology and the University of Vienna tested whether the same process could work using widely available hardware.
“With growing applications of quantum communication technologies, robust entanglement generation and distribution will be highly valuable,” said Martin Achleitner, researcher at the Austrian Institute of Technology. “To realize this goal, we implemented a sequential time-bin entangled source for quantum key distribution across Vienna’s existing fiber network.”
The experiment sent entangled photons through a 30-kilometer fiber link across Vienna. The signals traveled from the Austrian Institute of Technology to the University of Vienna, where they were measured using highly sensitive photon detectors.
“To the best of our knowledge, this is the first time a commercial MZI delay line has been used for a quantum application,” said Alessandro Trenti of the Austrian Institute of Technology.
The system produced strong entanglement signals, reaching visibility of about 93%, which is above the threshold needed for secure key generation.
“Using this type of entanglement source on photonic crystals will significantly improve scalability of quantum networks,” said Hannes Hübel of the Austrian Institute of Technology.
Researchers said the results show that large-scale quantum communication networks may be possible using existing telecom infrastructure and commercially available equipment.