A groundbreaking advancement in quantum internet technology has been achieved by an international research team led by QuTech, marking a pivotal shift from laboratory experiments to real-world applications. On November 1, 2024, the team announced a successful quantum connection between processors located 25 kilometers apart in the Dutch cities of Delft and The Hague.
This achievement, detailed in the journal Science Advances, involved the development of fully independent working nodes integrated with existing optical fiber internet infrastructure. Ronald Hanson, the group leader, stated, "The distance at which we create quantum entanglement in this project, over 25 km of deployed underground fiber, is a record for quantum processors. This is the first time such quantum processors in different cities have been linked together."
The implications of a functional quantum internet are vast. It enables the transmission of quantum information (qubits), which can exist in superpositions of 0 and 1 simultaneously, and can be entangled, allowing for instant correlations regardless of distance. Such capabilities promise revolutionary advancements in communication and computation, including secure encryption keys for safe financial and medical data exchanges, as well as the potential to connect remote quantum computers to enhance processing power while ensuring user privacy.
Transitioning from laboratory settings to urban quantum communication posed numerous challenges. The team devised a flexible system enabling nodes to operate independently over long distances, mitigating photon loss impacts on connection speed, and ensuring reliable confirmation whenever entangled connections were successfully established. Co-author Arian Stolk explained, "The connection had to remain stable within the wavelength of photons (less than a micrometer) over 25 kilometers of fiber. This task can be likened to maintaining a constant distance between the Earth and the Moon with an accuracy of just a few millimeters."
Utilizing a photon-efficient protocol, the researchers achieved precise stabilization of the optical fiber connection. They successfully demonstrated entanglement between two nodes of the quantum network containing diamond spin qubits, establishing a predetermined entangled state between the nodes.
The team’s extensive experience and collaboration with various organizations, including Fraunhofer ILT, OPNT, Element Six, Toptica, and Dutch telecommunications provider KPN, contributed to the project's success. The architecture and methods developed are applicable to other qubit platforms, paving the way for scalable next-generation qubits.