The future of quantum technology doesn’t depend on every home, business, or organization owning a superpowered computer. Its true potential lies in the internet — in a network of connections that enables even lower-capacity machines to tap into the advantages of quantum computing, whether from research centers or corporations, ultimately benefiting end users.
But building such a network requires the ability to transmit quantum information between machines over long distances without relying on costly infrastructure. A study published in Nature on Wednesday marks a major step in that direction, demonstrating the coherent transmission of quantum data across 155 miles (250 kilometers) of standard fiber-optic cable in Germany — without the need for cryogenic cooling.
Mirko Pittaluga, lead author of the research and a scientist at Toshiba, calls the breakthrough a “record” — not just for the distance covered, but for the infrastructure used, and the coherence achieved in communication. “It is fundamental to the phase-based architecture of the quantum internet,” the researchers state in the paper.
Antia Lamas, director of the Quantum Networks Center at Amazon Web Services (AWS), who was not involved in the study, says this next-generation internet will be effective “when all the capabilities of the quantum network are available.” According to Lamas, its implications will be critical, “first in the area of security and, later, for connecting quantum computers and expanding their potential.” “These networks will allow us to implement amazing capabilities,” she told EL PAÍS.
The recent achievement goes beyond the 155-mile communication distance between Frankfurt and Kehl in Germany. The same research group previously demonstrated quantum data transmission over more than 372 miles (600 kilometers) of cable. What sets this study apart is the maintenance of quantum coherence using a conventional underground fiber-optic network under everyday environmental conditions.
This is significant because quantum communications have historically depended on specialized equipment — such as cryogenic systems — to reach the near-absolute-zero temperatures that particles need to preserve their properties.
Qubits, the basic unit of quantum information which are exponentially more powerful than conventional bits, are extremely fragile and prone to errors due to their interactions with the environment. The expansion and contraction of optical fibers caused by changes in environmental conditions — such as temperature fluctuations — introduce errors and cause them to lose coherence.
But the research published in Nature, in line with the previous work of the same team, has succeeded in overcoming this important limitation for the future quantum internet. “Our research aligns the requirements of coherence-based quantum communication with the capabilities of existing telecommunication infrastructure,” the researchers argue.
“With the new techniques we have developed, further communication distance extensions for QKD [Quantum Key Distribution] are still possible, and our solutions can also be applied to other quantum communication protocols and applications,” Pittaluga said after the 372-mile record.
Before this experiment, the researchers simulated conditions in the laboratory, but within a controlled temperature environment. However, these previous tests showed more fluctuations. Under real-world communication conditions, the team managed to preserve the system.
“In phase-based quantum communication systems, maintaining coherence among quantum states encoded by different users is crucial for system performance and error minimization,” explains the Nature study.
Carlos Sabín, a researcher in the Department of Theoretical Physics at the Autonomous University of Madrid (UAM), who was not involved in the study, welcomed the new study. “The most innovative aspect of these new results is that they use already-existing commercial optical fiber and do not add more sophisticated technology typically found in quantum physics laboratories, such as cavities or cryogenic refrigerators,” he told Science Media Center Spain.
“The quantum bits used are photons generated with lasers, in contrast, for example, to other previous experiments, such as the one published last year in Nature, in which quantum entanglement was generated in Boston between experimentally more complex systems, including the use of cavities. These systems might be more suitable for building quantum memories, but using optical photons, on the other hand, allows for quantum communication over very long distances.”
The physicist recalls another recent study, published in Optica, which tested quantum teleportation with photons over conventional, in-use optical fibers, although at a much shorter distance of about 18.6 miles (30 kilometers) and with error rates of 10%.
“These new results,” Sabín added, referring to Wednesday’s publication, “with small error rates of around 5%, represent a step forward in the possibility of creating quantum physics-based communication networks integrated with existing optical fiber technology in our cities. Although it should be noted that we are still at a very preliminary stage of development.”
Pittaluga’s team believes they have reached a fundamental milestone for the quantum internet: “Our work demonstrates the compatibility of coherence-based quantum communications with existing network infrastructure and the practical implementation of an effective quantum repeater over commercial networks. We also achieved, to our knowledge, the longest distances for real-world QKD using non-cryogenically cooled technology.”
“Our findings confirm that environmental conditions in operational telecommunications hubs are comparable to or even better than those simulated in laboratories, encouraging further commercialization and prototyping of coherent quantum communication equipment. This achievement lays the groundwork for future practical, high-performance quantum communications and networks,” he continued.
This high performance is another challenge in quantum communication. Traditional methods, such as quantum key distribution (QKD), with which Pittaluga’s team has worked, and others, like chaotic encryption, often sacrifice speed or transmission capacity for the sake of security.
However, in a study by researchers at Shanghai Jiao Tong University, they presented an integrated encryption and communication (IEAC) framework that combines robust security with high-capacity transmission performance, based on end-to-end deep learning (E2EDL), to achieve a record-breaking secure transmission of 1 TB per second over 745 miles (1,200 kilometers) of optical fiber, a milestone in communications over this secure, high-capacity, long-distance infrastructure.
“Our work bridges the gap between security and transmission performance in optical communications. By incorporating encryption at the physical layer, IEAC paves the way for secure, high-performance networks capable of supporting the data demands driven by AI,” says Lilin Yi, co-author of the study and a professor at Shanghai University.
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