October 23, 2025• Physics 18, s136
Data protected by quantum physics have been sent alongside classical data through 120 km of optical fiber.
I. Derkach/Palacký University Olomouc
I. Derkach/Palacký University Olomouc
How can we safeguard the transmission of confidential information when facing adversaries equipped with powerful quantum computers? One promising strategy is known as quantum key distribution, in which the data are protected by harnessing the laws of quantum physics. For this approach to be practical and affordable on a large scale, it would need to be implemented in current optical-fiber networks that already carry classical information. However, detrimental interactions between the quantum-secured data and the classical data have limited such joint transmission to distances of a few tens of kilometers. Now researchers in Denmark and the Czech Republic have pushed that distance record to 120 km using so-called continuous-variable quantum key distribution (CVQKD) [1].
In CVQKD, the sender encodes random numbers in the amplitude and phase of light waves, which are transmitted to the receiver. The receiver measures the waves and uses the results to create a secret key, which the two parties use to securely encrypt and decrypt their information. Quantum physics dictates that any eavesdropping disturbs the light waves in a detectable way, alerting the parties that their communication may no longer be secure.
The researchers extended the joint-transmission range of such quantum-secured data and classical data by suppressing the interactions between the two data types. Unlike in previous experiments, this suppression did not require extra optical filters or other changes to an existing fiber network. Instead, the researchers exploited a previously overlooked built-in filter provided by the CVQKD setup and optimized the transmission of the quantum-secured data. The team demonstrated CVQKD across 120 km of optical fiber, even when the fiber was fully loaded with classical data traffic.
–Ryan Wilkinson
Ryan Wilkinson is a Corresponding Editor for Physics Magazine based in Durham, UK.
ReferencesA. A. E. Hajomer et al., “Coexistence of continuous-variable quantum key distribution and classical data over 120 km fiber,” Phys. Rev. Lett. 135, 170804 (2025).Subject AreasQuantum InformationQuantum PhysicsRelated Articles
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