Quantum key distribution (QKD) is widely considered the most advanced form of quantum cryptography, offering a path toward virtually unbreakable security for the future quantum internet. One promising technology behind these secure systems involves semiconductor quantum dots (SQDs), tiny solid-state light sources capable of generating high-quality single photons for quantum communication. These devices could help boost secure key generation rates while also supporting future quantum repeaters needed for large-scale quantum networks.
Another important development is time-bin encoding, a technique that stores information in the arrival times of photons. This method is especially attractive for long-distance quantum communication because it is naturally resistant to many of the environmental disturbances that can disrupt fiber optic networks.
Stable Quantum Encryption Over 120 Kilometers
An international research team from universities in Germany and China has now demonstrated the first true time-bin QKD system powered by an on-demand telecom semiconductor quantum dot device. Their results appeared as journal cover art in Light: Science & Applications.
In the experiment, the scientists generated three separate time-bin qubit states both deterministically and randomly using a self-stabilized time-bin encoder. The setup converts polarized single photons produced by a telecom C-band quantum dot into encoded quantum signals. On the receiving end, the photonic qubits were decoded with an actively stabilized interferometer containing a phase shifter, allowing the system to operate for extended periods without manual adjustment.
The researchers successfully transmitted the quantum signals across an optical fiber link spanning more than 120 kilometers between the encoder and decoder. The system also maintained impressive stability during more than six hours of continuous operation.
High Secure Key Rates With Quantum Dots
The proof-of-concept experiment achieved the highest secure key rate yet reported for a time-bin QKD system based on a high-performance quantum dot device. 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 kept average quantum bit error rates below 11%. Under practical finite key conditions, the setup maintained an average secure key rate of about ~15 bits/s, a level considered suitable for real-world encrypted text messaging applications.
The researchers emphasized the significance of the advance:
“Telecom-band QDs with Purcell enhancement can provide high-brightness photons suitable for intercity fiber communication, making them promising candidates for integration into practical QKD systems.”
Time-Bin Encoding Improves Real-World Stability
The team also highlighted the advantages of time-bin encoding compared with many existing quantum dot based QKD systems, which can be highly sensitive to environmental disruptions.
“Most existing QD-based QKD systems are vulnerable to changes in the practical quantum channel caused by environmental factors, such as turbulence, temperature and vibrations. This necessitates active compensation. In contrast, time-bin encoding, where qubits are encoded in the temporal position of single photons, offers intrinsic stability against such channel fluctuations even without any complex compensation protocols”
According to the scientists, the system’s long uninterrupted runtime demonstrates the robustness of the approach.
“The system is operated continuously for 6 hours, highlighting the intrinsic robustness of the time-bin scheme enabled by the system including the Sagnac interferometer (SNI), active feedback control, etc.”
The researchers say the work marks an important step toward practical, scalable quantum communication systems that could eventually support secure quantum networks in real-world environments.
“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 based on solid-state single-photon emitters.”