Beyond 1,000 kilometers, additional technological devices—quantum repeaters, potentially challenging to implement—are required. The research by a group of physicists from the Scuola Normale Superiore has just been published in Nature Photonics.
PISA, January 20, 2025. In Pisa, a team of researchers from the Scuola Normale Superiore has determined the maximum distance within which quantum computing can securely transmit passwords and other confidential data with absolute security: 1,000 kilometers. Beyond this threshold, the use of quantum repeaters, complex devices that are difficult to implement, becomes necessary.
In their article, published in Nature Photonics, researchers Francesco Mele, Ludovico Lami, and Vittorio Giovannetti tackled the issue of cybersecurity from the perspective of quantum physics. Quantum computing leverages the principles of quantum physics—the theory describing atoms and fundamental particles of nature—to enhance technological performance. While quantum information theory is highly developed, its experimental applications remain limited to small scales (e.g., quantum computers built with a few qubits in highly specialised laboratories).
The researchers utilised Quantum Key Distribution (QKD), a technology already integrated into specialised security systems, which makes it impossible for external agents (such as hackers) to intercept or decrypt data. QKD enables the generation of a “secret key” shared exclusively between the sender and the recipient, which can be used to encrypt and decrypt information. But what is the maximum distance QKD can achieve using the current Internet infrastructure, based on fiber optics and satellites?
“Currently,” explains Francesco Mele, “the most advanced QKD experiments allow for distances of up to about 200 kilometers. In our article, we address this issue quantitatively, demonstrating that under standard conditions, the maximum theoretically achievable distance is approximately 1,000 kilometers. In other words, it is not possible to establish QKD between two cities more than 1,000 kilometers apart without additional devices along the way. To surpass this limit and transmit secret keys over longer distances, quantum repeaters—complex and potentially expensive devices—are required at intermediate points along the communication line.”
The work by the Scuola Normale team addresses this challenge by linking it precisely to the study of long-distance transmission of “entanglement.” Entanglement is a fundamental resource for many quantum algorithms, and its transmission and manipulation are considered crucial steps toward the development of a quantum Internet.