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An international research team has successfully transmitted unhackable quantum keys across 120 kilometers of standard optical fiber, overcoming one of the biggest hurdles in building a secure quantum internet. By combining semiconductor quantum dots with a technique called time-bin encoding, scientists maintained a stable quantum encryption link for over six hours without manual intervention. This breakthrough proves that ultra-secure quantum communication can survive the environmental disturbances that typically disrupt long-distance fiber optic networks.
Quantum Key Distribution represents the ultimate frontier in cybersecurity, offering encryption that is theoretically impossible to crack. However, traditional systems rely on fragile quantum states that easily degrade due to temperature shifts or vibrations in the physical cables. To solve this, the researchers utilized time-bin encoding, a method that stores data in the exact arrival times of photons rather than their polarization. "Most existing QD-based QKD systems are vulnerable to changes in the practical quantum channel caused by environmental factors," the researchers explained, noting that time-bin encoding offers intrinsic stability against these fluctuations.
Achieving 120-Kilometer Stability
The experiment, detailed in the journal Light: Science & Applications, utilized an on-demand telecom semiconductor quantum dot device operating in the standard telecom C-band. The setup converted polarized single photons into encoded quantum signals using a self-stabilized time-bin encoder. On the receiving end, an actively stabilized Sagnac interferometer decoded the photonic qubits. This hardware combination allowed the system to run continuously for six hours, demonstrating unprecedented robustness for a solid-state single-photon emitter.
During the test, the quantum dot source fired highly pure single photons at a rate of 76 MHz. Even after traversing 120 kilometers of standard fiber, the system maintained an average quantum bit error rate below 11 percent. Under practical conditions, this yielded a secure key rate of approximately 15 bits per second. While this bandwidth is narrow, it is entirely sufficient for real-world encrypted text messaging and the secure exchange of cryptographic keys.
This result underscores the feasibility of integrating QD single-photon sources into stable and field-deployable time-bin QKD systems, marking an important step toward scalable, quantum-secure communication networks.
- Light: Science & Applications
The Infrastructure Advantage
The true significance of this 120-kilometer milestone lies in its compatibility with existing telecommunications infrastructure. By successfully operating in the telecom C-band over standard optical fiber, this system proves that the future quantum internet will not require a complete, ground-up replacement of global cabling. Instead, telecom providers can theoretically overlay quantum key distribution onto the fiber networks already buried under our cities.
While a secure key rate of 15 bits per second is far too slow for streaming or heavy data transfer, it perfectly serves its primary purpose: generating unbreakable encryption keys to secure traditional high-speed data channels. The next critical step will be scaling the photon generation rates and integrating quantum repeaters to push this unhackable transmission beyond the 120-kilometer limit, moving us closer to a commercially viable, intercity quantum network.
