Scientists Send Unhackable Quantum Keys Across 120 Kilometers in Major Security Breakthrough
Researchers achieve record-breaking secure communication using semiconductor quantum dots that maintained stability for over six hours without manual adjustment.
Scientists have demonstrated a remarkably stable quantum encryption system capable of transmitting virtually unhackable security keys across more than 120 kilometers of optical fiber, marking a significant advance toward ultra-secure quantum communication networks. The breakthrough experiment, conducted by an international research team from universities in Germany and China, achieved one of the highest secure key rates yet recorded for quantum dot-based technology while maintaining continuous operation for over six hours without requiring manual adjustments.
The system relies on quantum key distribution (QKD), widely considered the most advanced form of quantum cryptography available today. The researchers used tiny semiconductor quantum dots as single-photon sources, combined with time-bin encoding that stores information in the precise arrival times of light particles. This approach proves especially valuable for long-distance quantum communication because it naturally resists many environmental disturbances that typically disrupt fiber optic networks.
During the proof-of-concept experiment, the team successfully generated three separate time-bin qubit states both deterministically and randomly using a self-stabilized time-bin encoder. The setup converted polarized single photons produced by a telecom C-band quantum dot into encoded quantum signals. On the receiving end, photonic qubits were decoded using an actively stabilized interferometer containing a phase shifter, enabling extended operation periods without human intervention.
The quantum dot source produced bright, highly pure single photons at an operating rate of approximately 76 MHz. Even after traveling through 120 kilometers of standard optical fiber, the system maintained average quantum bit error rates below 11 percent. Under practical finite key conditions, the setup achieved an average secure key rate of about 15 bits per second, a level considered suitable for real-world encrypted text messaging applications.
The researchers emphasized the significance of their advance, noting that "telecom-band quantum dots with Purcell enhancement can provide high-brightness photons suitable for intercity fiber communication, making them promising candidates for integration into practical QKD systems." The breakthrough could accelerate development of quantum internet infrastructure and provide unprecedented security for sensitive communications across vast distances.
